Journal o f Occupational Rehabilitation, VoL 1, No. 1, 1991
The Reliability and Validity of Two New Tests of Maximum Lifting Capacity Joanette Alpert, 1 Leonard Matheson, 2,5 William Beam, 3 and Vert Mooney 4
This study investigated the test-retest reliability and validity of the isokinetic and gravity~inertia modes of the Lido Lift TM and the Progressive Lifting Capacity H test (PLC II). M a x i m u m lifting capacity tests were performed in the isokinetic and gravity~inertia modes on the Lido Lift and with the P L C II with 29 healthy male subjects. Pearson product-moment correlations were calculated to determine the test-retest reliabilities and to compare P L C H test values with isokinetic and gravity~inertia test values. The correlations for the isokinetic, gravity inertia, and the P L C II were r = .90, .82, and .91, respectively. The correlations between the isokinetic and gravity~inertia tests and the P L C H were r = .64 and .74 during the test and r = .72 and .81 during the retest. The results of this study support the reliability of both Lido Lift TM tests and of the P L C II, and the validity of the isokinetic and gravity~inertia tests when compared to the P L C II. KEY WORDS: lift capacity; isokinetic; gravity inertia.
INTRODUCTION Several studies have documented the prevalence of low back pain in industry (1-3). Bigos et al. (1) found that back injuries were more commonly associated with material handling and lifting than with slips and falls. Chaffin (2) reported that, in industry and medicine, it is important that objective evaluative tools be developed to accurately assess a worker's functional capacity. In particular, there is a need for a standardized test or group of tests that can accurately assess lifting capacity. Although various tests have been shown to be reliable, there is no lifting test which has been shown to be more valid than another. Controversy exists in the literature with respect to which type of testing is the most valid and practical. IOccucare Industrial Health Clinics, Pomona, California. Employment and Rehabdltatlon Institute of Cahforma, Santa Ana, California. 3California State University, Fullerton, California. 4Irvine Medical Center, Irvine, California. 5Correspondence should be directed to Leonard Matheson, Ph.D., Employment and Rehabilitation Institute of California, Suite 101, 600 South Grand Avenue, Santa Ana, California 92705. .
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1053-0487/91/0300-0013506.50/09 1991PlenumPublishingCorporation
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Alpert, Matheson, Beam, and Mooney
Historically, the evaluation of lifting capacity has centered around isometric, free weight, and more recently, isokinetic testing. Isometric (static) tests involve contracting a muscle or group of muscles with no shortening of the muscle and allowing no motion to occur. These tests have frequently been the tests of choice because they have been shown to be reliable, easy to administer, and the equipment needed is not too costly. Chaffin et al. (4) proposed a set of isometric lifting tests. Acceptable test-retest reliability has been demonstrated with these tests. However, since lifting is a dynamic activity, it has also been said that the testing of lifting should be dynamic (5). Because most activities are dynamic, isometric testing may not provide the most accurate assessment of an individual's capabilities. Menon and Freivalds (6) concluded from their study, which compared isometric tests with isokinetic tests, that since the correlation between dynamic and static strengths was significant, dynamic tests should be the tests of choice in that dynamic tests were better indicators of work capacity in "real world" settings. The validity of using dynamic tests in which a free weight is used, i.e., using a crate or box with a weight in it, lies in the fact that this is indeed what the individual actually does in a work setting. This simulates more closely the actual activity and may give a better evaluation of the individual's lifting capacity, instead of "predicting" it from the results of an isometric test. Kroemer (7) reported that it was at best very difficult, if not impossible, for dynamic strength to be predicted from a test of static strength, mainly because the mechanical and physiological processes that occur during each test are so different. A free weight lifting test involves a dynamic movement of a fixed load. Different protocols have been utilized with this fixed load concept. A "psychophysical" approach has been utilized by Karwowski and Yates (8) and Mital (9). As described by Mital (9), the psychophysical approach requires a subject to randomly adjust a load during a period of 20-25 minutes until an acceptable load is found that the subject estimates can be lifted repetitively during a regular 8-hour work shift. The weight that the subject chooses is considered the maximum weight that the subject can comfortably lift for the 8-hour period. Karwowski and Yates (8) found this approach reliable only for low and moderate lifting frequencies (not greater that six lifts per minute). In a study by Mital (9), the psychophysical method tended to overestimate the maximum acceptable weight of lifting. He compared the results of the psychophysical test to an actual 12-hour session of lifting, and found that none of the ten subjects were able to work with the weight they had chosen during the psychophysical test. Given these findings, the validity of the psychophysical approach is questionable. Another free weight lifting approach is the "isoinertial" technique. Kroemer (5) described this technique as one in which the individual lifts a constant mass which is progressively increased to the maximum that the individual can lift. The study reported a coefficient of variation of 3.2-3.9% in an overhead lift and 5.27.8% in a knuckle-height lift. The same subjects showed much higher variability with static tests. The coefficient of variation with the static tests ranged from 11.6 to 15.4%.
Reliability and Validity of Lifting Tests
15
Test-retest reliability of a test which is based on an adaptation of this technique, the progressive isoinertial lifting evaluation, "PILE" (10), showed a correlation of r = .87 for the lift of a progressively loaded milk crate with a frequency of four lifts every 20 seconds from floor to 76 cm and r = .93 for the lift from 76 cm to 137 cm. Similar to the PILE is the progressive lifting capacity test "PLC I" (11). The PLC I utilizes "masked weights" with a similar weighting schema and a frequency of four lifts every 30 seconds. An evaluation of the test-retest reliability of this lifting capacity test demonstrated a correlation coefficient of r = .77 for the tift from floor to 76 cm and r = .81 for the lift from 76 cm to 122 cm with a frequency of four lifts every 30 seconds. Isokinetic testing has been shown to be a reliable means of evaluating strength in the peripheral joints, in the spine, and more recently in lifting. The concept of isokinetic exercise was introduced in 1967 (12). Isokinetic performance allows motion to occur against accommodating resistance at a fixed speed. Isoldnetic testing has been found to be reliable in several studies (13-18). Isokinetic lifting simulation was shown to be reliable in studies done by Frykman et al. (19) and Porterfield et al. (20). Some critical issues have been raised regarding the use of isokinetic testing of functional activities such as lifting (21, 22). Although isokinetic testing is dynamic, it does not mimic functional activities since work is not usually performed at a fixed speed. Kishino et al. (23) felt that although isokinetic equipment can be a useful tool in industry and rehabilitation, it had important equipment limitations that limited its ability to predict performance in an actual work setting. Timm (22) also felt that since lifting is a three-dimensional activity and is composed of various combinations of isometric, isotonic, and isokinetic components, isokinetic technology may not be totally effective as an evaluative or rehabilitative tool. Mayer et al. (10) compared isokinetic lifting (Cybex Liftask) with the PILE and found the correlations between the two tests to be low. It was concluded that the tests do not evaluate the same parameters and cannot be substituted for each other. Although both tests are dynamic, they are quite different. The PILE is an unconstrained lifting test allowing the individual to move at any speed and utilize various postures. The isokinetic test limits acceleration and velocity. The Lido Lift (Loredan Biomedical, Davis, California) has been designed to evaluate dynamic lifting capacity in isokinetic and gravity-inertia modes. The object to be lifted is attached to an articulating arm that measures movement in three dimensions. The gravity-inertia mode involves moving a fixed load designed to reproduce a real lifting situation. In this mode, the force of gravity and the inertia of the weight being lifted is simulated by the machine. This feature currently is unique to this piece of equipment. While this is a potentially useful piece of equipment, no data are available on its reliability or validity. The purposes of the current study were: (1) to investigate the reliability in a test-retest design of the isokinetic and gravity-inertia modes of the Lido Lift, (2) to investigate the validity of the Lido Lift in comparison to the PLC II, and (3) to investigate the reliability of the progressive lifting capacity test (PLC II).
16
Alpert, Matheson, Beam, and Mooney
METHODS Subjects Twenty-nine male volunteers participated in this study. Ages ranged from 20 to 59 years with a mean of 32 years. Height ranged from 173 to 196 cm with a mean of 180 cm, and weight ranged from 68 to 109 kg with a mean of 83 kg (Table I). In response to questioning, subjects reported no limitation of activity due to back pain, no history of back surgery, and no reported back injury within the previous 6 months.
Procedures All subjects underwent an orientation and signed a consent document. Subjects completed a general health questionnaire and the experimenter performed a brief musculoskeletal evaluation. A submaximal cardiovascular test, to screen for abnormal heart rate and/or blood pressure response to exercise, was performed if the subject was "at higher risk" for cardiovascular disease (as described by The American College of Sports Medicine, 1986). Only one subject required this test. A 9 minute graded submaximal exercise test was performed. This subject had normal heart rate and blood pressure responses and was admitted to the subject pool. The health questionnaire, musculoskeletal evaluation, and the cardiovascular screening were reviewed for any abnormalities which would have placed the subject at risk during the lifting tests. No subjects were excluded from the study based on these screenings. Upon completion of the screening procedures, subjects were guided through 5 minutes of stretching exercises and 5 minutes on a stationary bicycle at low resistance to serve as a warm-up prior to the initiation of testing. A Fitron bicycle ergometer was used for the graded submaximal exercise test and for the warm-up prior to testing. Subjects participated in the three types of maximal lifting tests on two separate days. The test-retest interval ranged from 4 to 12 days, for all but one subject who had a 22-day test-retest interval. All subjects were given the same instructions and performed the same tests in the same order during the test and the retest. The order of testing and inter-test intervals were standardized for all subjects: (1) isokinetic practice and test, (2) 5-minute rest, (3) progressive gravity-inertia test
Table I. Physical Characteristics of the Subjectsa Variable (units) Age (yr)
Mean
(SD)
Minimum
Maximum
32.0
(9.4)
20.0
59.0
Height (cm)
180.0
(5.4)
173.0
196.0
Weight (kg)
83.0
(10.4)
68.0
109.0
an = 29.
Reliability and Validity of Lifting Tests
17
beginning at 50% of the average isokinetic maximum and progressing in 10-pound increments, (4) 20-minute rest period, and (5) progressive PLC II test beginning at 50% of the average isokinetic maximum and progressing in 10-pound increments. The following instructions (italicized words were verbally emphasized) were read to each subject during the test procedures: Isokinetic: In this test, you will use the "T" handle (demonstrate) placed at 6 inches above the floor. I want you to bend at your hips and knees, and keep your head up (demonstrate). In this test, the speed is fixed at 18 inches per second, and I want you to pull up as hard as you can until you hit the "stop" (there is a cushioned stop at 42 inches), and then let go of the handles (demonstrate). You will do six repetitions for practice. I want you to take these fairly easy, giving about 50% of your maximum effort. The purpose of the practice trials is to get you comfortable with the machine. Then, you will do four test trials. I will tell you when to start each time. I will say "get ready, 5, 4, 3, Z 1 go.t" In the test trials, I'm looking for a 100% maximum effort. It's extremely important than you give me your full effort for each test trial. Remember, I'm looking for a 100% maximum effort. Do you have any questions? OK, let's begin now! Gravity-inertia: In this test you will be using the small box attachment, and you will use these grips (demonstrate). You will lift each weight once, and you will do one lift every 30 seconds. I will time you. You will lift the box from the floor, set it on the shelf, and then step back. I will tell you when to initiate each lift. I will say, "get ready, 5, 4, 3, 2, 1 go.t" You will be lifting weights that will progressively get heavier. After each lift, I will ask you how you feel, and I will ask you if you want to add weight. If you say yes, I will add weight, and we will proceed.
I want a maximum safe lift, that you can dependably reproduce 10-12 times per day. Do you have any questions? Now remember, I want a maximum safe lift, that you can dependably reproduce ]0-12 times per day. OK, let's begin.
PLC II: In this test, I'm going to ask you to pick up this milk crate from the floor and put it on this bench right here and then put it back on the floor. I want you to do this same thing one time each 30 seconds, and then rest, and then if you're doing OK, FII ask you to do it again with more weight. When you finish each lift, I'll ask you if you can lift the crate with more weight. If you can, we'll keep going, but if you can't that's OK, just let me know. Remember, it's O K to stop the test. I don't mind, I don't want you to hurt yourself!
I want a maximum safe lift, that you can dependably reproduce 10-12 times per day. Do you have any questions? OK? Watch me. This is what l'd like you to do. See?
Remember, I want a maximum safe lift, that you can dependably reproduce 10-12 times per day. OK, let's start the test now!
All subjects wore leather gloves and comfortable non-binding work or athletic clothes. Subjects wore a Tunturi heart rate monitor which was attached to the earlobe for the duration of the testing session. Blood pressure was monitored periodically throughout the testing with a Baumanometer Calibrate V-lok blood pressure cuff. The Lido Lift was used to evaluate isokinetic and gravity-inertia lifting capacity (Fig. 1). Operated by an IBM PS 30/286 computer, the device has a fixed arm mechanism on which various "T" handles or boxes can be attached. All force
18
Alpert, Matheson, Beam, and Mooney
Fig. 1. Lido Lift (Loredan Biomedical,Davis, California).
and velocity variables were controlled through the computer. The computer printed out the result of any test on command. Before each test, the machine zeroed itself out for height of the fixed arm and for weight. The strain gauge was calibrated at the factory. The isokinetic test was performed on the Lido Lift through the use of a "T" handle with rotating padded hand grips and a 12-inch span between the hand grips. Vertical velocity was fixed at .46 m/sec (18 inches per second). The subject stood facing the machine in a squatting position and grasped the handgrips, one in each hand, placed at 15 cm (6 inches) above the floor. The subject lifted the handle to a height of 107 cm (42 inches). Six submaximal practice trials (subjects were instructed to give 50% effort) were performed to allow the subject to become comfortable with the machine, followed
Reliability and Validity of Lifting Tests
19
by four test trials. For the test trials, subjects were asked to give a maximal effort. Peak force for each trial was measured, and the average peak force using the best three out of four repetitions was calculated. The gravity-inertia test was performed on the Lido Lift. In this mode, the load was fixed in pounds, and was programmed into the computer. Subjects lifted from the floor a box which was attached to the machine, and set it on a shelf placed at 76 cm (30 inches) above the floor. The dimensions of the box were 29 cm across by 24 cm deep and 23 cm high. The padded hand holds were 18 cm above the bottom of the box, centered 12 cm from the front and rear edge of the box. After placing the box on the shelf, the subject stepped back and the experimenter pushed the box off the shelf to allow it to fall slowly to the floor. The initial load for each subject was determined by averaging the peak force values obtained during the best time out of four test trials in the isokinetic test, and taking 50% of the number. There were no practice trials preceding this test. One lift was performed with each load at 30-second intervals. The load was increased in 4.5-kg (10-pound) increments until the subject reached his maximum lift. The third test performed was the progressive lifting capacity test (PLC II). This involved lifting and lowering a load in a plastic milk crate, using gloves and hand-holds. Each subject's starting load was equal to the starting load used during the gravity-inertia test on the Lido Lift. As with gravity-inertia, the subject was not informed of the starting weight. Subjects lifted the crate from the floor and placed it on a workbench that was 76 cm (30 inches) high. The crate was then lowered back to the floor. Similar to the gravity-inertia test, one lift was performed every 30 seconds, and one lift was performed with each load. However, this was a liftlower task. The load was increased in 4.5-kg increments until the subject reached maximum lift. A standard steel reinforced milk crate which weighed 2.5 kilograms was used for the PLC tests. The crate measured 48 cm wide by 32 cm deep, and was 28 cm tall. The hand holds were 25 cm from the bottom of the crate, centered 16 cm from the front and rear edge of the box. Color coded weight canisters were used as the free weights. The canisters weighted either 2.3 kg, 4.5 kg, or 9.1 kg, and were all the same size, 28 cm tall and 10 cm in diameter. Subjects were not informed of their performance, starting loads, or increments of additional loads during and between tests and testing days. In the isokinetic and gravity-inertia modes, subjects were not able to view the computer screen. When testing was completed, subjects were contacted by telephone to perform a debriefing. The purpose of the debriefing was to determine whether or not there were any problems resulting from the test experience, and to provide the subject with feedback about his performance.
Variables
In the isokinetic mode, the average peak force was calculated from the best three out of four test trials. In this study for both the gravity-inertia and the PCL
Alpert, Matheson, Beam, and Mooney
20
II tests "maximum safe lift" was defined as a safe and reproducible maximum (GIMAX and PLCMAX, respectively). Subjects were instructed to continue until a self-selected maximum safe lift was achieved, one that could be dependably reproduced 10-12 times per day. This was similar to an "infrequent occasional" lifting requirement and was meant to be different from a one-time maximum lift that might not be able to be reproduced. If postural changes were observed which would put the subject at risk for injury, the experimenter would discontinue the test and call the previous lift the subject's maximum. These postures included using the whole body to stabilize or move the box, coming up onto the toes with either heel off the floor, or shrugging the shoulders in an effort to reach the shelf. A maximum weight of 113 kilograms was possible on the Lido Lift in the isokinetic mode. Four out of 29 subjects were unable to be evaluated in this mode because their maximum was beyond the limits of the machine. Because the starting load for the gravity-inertia and PLC II tests used 50% of the average peak force values from the isokinetic test, starting load for the four subjects unable to be tested to the maximum in the isokinetic mode was determined by starting at 50% of body weight. The gravity-inertia mode had a limit of 90.7 kg, which was reached by seven of the 29 subjects. Two of these stopped because it was their maximum, but the other five were stopped by the tester because the machine was unable to evaluate them at a higher weight. These subjects were unable to elicit their maximum in the gravity-inertia test, so the sample size in this aspect of the study was decreased to 24 (from 29).
RESULTS Physical characteristics of the subjects (age, height, and weight) were not related to test performance except with regard to a weak but statistically significant inverse relationship between age and isokinetic test performance in both the test and the retest (r = -.499 and r -- -.403, respectively).
Reliability Minimum, maximum, mean, and standard deviation for test and retest force values are presented in Table II. Coefficients of variation for each subject's set of isokinetic test trials ranged from 2.0 to 15.5% (mean = 7.8%) for the test and from 2.2 to 13.9% (mean = 6.8%) for the retest. Test-retest correlation coefficients are presented in Table III. All correlations are statistically significant at p ___.05.
Validity Pearson product-moment correlation coefficients were computed to determine the relationship between the isokinetic test and the PLC II and the gravity-inertia test and the PLC II. Results are presented in Table IV. Values were also normalized
Reliability and Validity of Lifting Tests
21
Table II. Mean Force Values for Isokinetic and Gravity-Inertia Modes of Lido Lift and for PLC II at Test and Retest Mean
(SD)
Minimum
Maximum
Isoldnetic mode (kg) Test trial Retest trial
81.2 82.7
(20.4) (22.3)
47.4 42.6
125.2 130.5
Gravity-inertia mode (kg) Test trial Retest trial
74.8 74.9
(11.9) (11.3)
46.4 44.1
90.9 90.5
PLC II (kg) Test trial Retest trial
71.9 73.6
(12.5) (14.1)
42.3 40.0
99.1 109.3
Type of test (units)
Table III. Correlation Coefficients Describing Reliability of Lifting Capacity Tests Type of test
Test-retest reliability (r)
Lido Lift (isokinetic)
0.90
Lido Lift (gravity-inertia)
0.81
PLC II
0.90
by dividing each variable by the subject's body weight. The correlations between these normalized values are presented in Table V. All correlations are statistically significant at p _< .05. Absolute lifting capacity values are classified by percentile for the test and retest in the isokinetic gravity-inertia, and PLC II tests in Table VI. The average of the best three of the four peak force values that were obtained in the isokinetic test ranged from 47.3 to 125.0 kg, and at the retest ranged from 42.5 to 130.2 kg. The normalized (body weight) values are presented in Table VII. The predictive validity of the isokinetic and gravity-inertia tests can be evaluated through the use of regression equations which, along with scatter graphs of the retest data, are shown in Figs. 2 and 3. Although the isokinetic test does not account for as much of the variance of lifting capacity as does the gravity-inertia test, Fisher's-z comparison demonstrated no statistically significant difference between the two.
DISCUSSION
Reliability of Isokinetic Test Performance. A correlation coefficient of r = .90 was reported for isokinetic test performance. This is comparable to other studies,
22
Alpert, Matheson, Beam, and Mooney
Table IV. Correlation Coefficients Describing Validity of Lifting Capacity Tests Validity compared with PLC II (r) Type of test
Test
Retest
Lido Lift (isokinetic)
0.64
0.72
Lido Lift (gravity-inertia)
0.72
0.81
Table V. Correlation Coefficients Describing Validity of Lifting Capacity Tests Normalized by Body Weight Validity compared with PLC II (r) Type of test
Test
Retest
Lido Lift (isokinetic)
0.68
0.73
Lido Lift (gravity-inertia)
0.80
0.87
y = .44x |
.3
+ .445,
|
R-squared: !
.532 !
.2
|
0
.*
.1
0
1 .g
0
.8
.....
.4
.6
~
.i
0
2
1' IKMAX/BW
~176176176176
o
1:2
1:4
1.6
Fig. 2. Regression of isokinetic test on PLC II; 95% confidence intervals for the true mean of PLC II. PLCMAX/BW = maximum weight lifted in PLC test divided by body weight; IKMAX/BW = maximum weight lifted in Lido isokinetic test divided by body weight.
Reliability and Validity of Lifting Tests
y
=
23
.738x
+
.169,
R-squared:
.755
.1 1,
0
.g,
o0"
"'"
L.
.8,
.7 .6
.5 .4 .4 GIMAX/BW
Fig. 3. Regression of gravity-inertia on PLC II; 95% confidence intervals for the true mean of PLC II. PLCMAX/BW = maximum weight lifted in PLC test divided by body weight; GIMAX/BW = maximum weight lifted in Lido gravity-inertia test divided by body weight.
including Burdent and Van Swearingen (14) and Rose et at (17). Although this was the first study to investigate the test-retest reliability of the isokinetic mode on the Lido Lift, studies investigating other isokinetic devices have reported correlation coefficients greater than r = .80. Frykman et aL (19) found test-retest reliability of peak force with the Cybex Liftask to be r = .82. Studies evaluating isokinetic knee performance have reported correlation coefficients ranging from r = .47 to .98 (24-26). Rose et al. (17) reported acceptable test-retest reliability (greater than r = .80) for average peak torque on the Lido back isokinetic dynamometer. Reliability of Gravity-Inertia and P L C H Test Performance. The present study was the first to examine the reliability of the gravity-inertia and PLC II tests. Both tests were found to be reliable. The test-retest correlation coefficients for the PLC II were comparable to that reported by Matheson and his colleagues (11) for the original PLC and by Mayer et al. (10) for the PILE. The main difference in testing protocols for those tests and the tests used in the present study were the end-point criteria. There were three defined end-point criteria for the PLC I and for the PILE. These were: (1) a psychophysical end-point, (2) a heart rate end-point, and (3) a proportion of ideal body weight end-point. Subjects in the present study were instructed to continue until a "maximum safe lift" was reached which could be dependably reproduced 10-12 times per day. Each subject used his own perception of the load as a psychophysical end-point. There were no heart rate or ideal body weight end-points. The test-retest interval for the PILE was only 1 day, while the interval in the present study averaged 8 days. Also, as with the PLC I, in the PLC II the weights in the crate were masked and subjects were unable to visually evaluate the amount of each lift. It would seem unlikely that
24
Alpert, Matheson, Beam, and Mooney Table VI. Normative Values for Lifting Capacity Tests in Healthy Adult Males
Percentile rank
Lido Lift
Lido Lift
(Isokinetic mode)
(Gravity-inertia)
PLC II
Test
Retest
Test
Retest
Test
Retest
90th
112.20
111.90
90.50
88.50
92.30
88.90
75th
95.00
98.50
86.10
83.80
78.90
80.10
50th
77.60
80.30
72.70
76.40
70.70
74.10
25th
66.50
66.60
66.60
68.30
61.60
65.30
lOth
56.30
52.50
62.00
63.10
61.30
62.70
Table VII. Normative Values for Relative Force (kg/kg) of Lifting Capacity Tests in Healthy Adult Males
Percentile rank
Lido Lift
Lido Lift
(Isokinetic mode)
(Gravity-inertia)
PLC II
Test
Retest
Test
Retest
Test
Retest
90th
1.36
1.31
1.13
1.08
1.10
1.05
75th
1.14
1.20
1.06
1.04
0.96
0.98
50th
1.03
1.06
0.87
0.96
0.88
0.91
25th
0.80
0.86
0.83
0.84
0.73
0.75
lOth
0.62
0.61
0.71
0.68
0.67
0.71
s u b j e c t s w e r e a b l e to r e m e m b e r t h e a m o u n t o f w e i g h t t h a t was lifted d u r i n g t h e initial test. I n this s t u d y , t h e e n d - p o i n t c r i t e r i a f o r t h e g r a v i t y - i n e r t i a t e s t a n d t h e P L C II w e r e t h e s a m e . S u b j e c t s w e r e i n s t r u c t e d to c o n t i n u e t h e t e s t w i t h a d d i t i o n a l w e i g h t u n t i l a " m a x i m u m s a f e lift" w a s a c h i e v e d , o n e t h a t c o u l d b e " d e p e n d a b l y reproduced 10-12 times per day." In the present study, a "reproducible" maxim u m w a s s o u g h t . T h i s was t h o u g h t to b e m o r e a p p l i c a b l e to an i n d u s t r i a l s e t t i n g t h a n w o u l d b e a " o n e r e p e t i t i o n m a x i m u m . " K r o e m e r (7) d i s c u s s e d t h e d i f f e r e n c e b e t w e e n m a x i m u m a n d o p t i m u m a n d t h a t m a x i m u m r e f e r s to a n " a m o u n t " a n d o p t i m u m r e f e r s to a " c o n d i t i o n . " C o n s i d e r i n g t h e s e d e f i n i t i o n s , t h e p r e s e n t s t u d y w a s s e e k i n g an " o p t i m u m " lift in t h e g r a v i t y - i n e r t i a a n d P L C II tests, o n e w h i c h w o u l d b e f u n c t i o n a l for u s e in i n d u s t r y . K r o e m e r also felt t h a t i n c o n s i s t e n c i e s in t e r m i n o l o g y , l a c k o f c a l i b r a t i o n o f d e v i c e s , t h e i n a b i l i t y to c o n t r o l f o r m o t i v a t i o n , a n d t h e l a c k o f s t a n d a r d i z a t i o n o f i n s t r u c t i o n s all h a v e c o n t r i b u t e d to i n c o n s i s t e n t i n t e r p r e t a t i o n s o f t h e r e s u l t s o f m a n y s t r e n g t h s t u d i e s . A n e f f o r t
Reliability and Validity of Lifting Tests
25
was made in the present study to define "maximum" and to use standardized instructions. In addition to the instructions given to the subjects regarding when to tell the tester to stop increasing the load, the tester observed the subjects' body mechanics and postural changes. Only if the subject was using body mechanics or assumed a position which would put him at risk for injury, was the test halted. Although subjects were encouraged to use the legs for power, and not the back, each used the posture of choice. It was interesting to note that postural changes were observed as subjects approached what was defined in this study as maximum. These changes included: (1) rising onto toes and/or shrugging the shoulders to clear the 76 cm shelf, and (2) going from a "leg lift" (using the legs for power and keeping the lumbar curve in a neutral position) to a "back lift" (knee extension preceding back extension). Mayer et aL (10) have discussed the "weak link" phenomenon. A whole body motion, such as that required by a lift of this nature, will be limited by the weakest part of the whole, whether it is upper body, lower body, or trunk strength. The weak link may be revealed through compensatory postural changes observed as the individual nears his maximum capacity. It appeared in this study as if the weak link provides consistent limitations on performance in that whole body lifting in all three modes demonstrated high degrees of reliability. In summary, all three of the tests evaluated in this study appear to be reliable tools for evaluating lifting capacity. Validity of Isokinetic and Gravity-Inertia Test Performance. Lifting results on the gravity-inertia mode of the Lido Lift accounted for a higher percentage of the variance on the PLC II than the lifting data from the isokinetic test mode. This may have been due to the similarity in lifting tasks between the gravity-inertia mode and the PLC II. The isokinetic test in this study was done at .46 m/sec (18 inches per second). Pytel and Kamon's study (27) showed that a simulated lift using an isokinetic motion at .73 m/sec (29 inches per second) was highly correlated with the maximal dynamic lift (fixed load) in that study. Correlation coefficients were r = .87 for men and r = .92 for women. Mayer et al. (10) compared a simulated isokinetic lift on the Cybex Liftask at .46 m/sec (18 inches per second) and .76 m/sec (30 inches per second) with the PILE, a functional dynamic lift described previously. Subjects in that study were patients who had chronic low back pain and underwent several weeks of rehabilitation between initial evaluation and discharge. Correlation coefficients for the male subjects ranged from r = .61 to .63 at the initial test, and from r = .38 to .45 at discharge. Pytel and Kamon (27) also evaluated an isokinetic lift at .97 m/sec (38 inches per second) that did not correlate as well with the maximal dynamic lift. It may be that there is a "velocity window" within which isokinetic test performance is able to be adequately predictive of lifting capacity. That such would be the case is suggested by Kim and Marras (28) in which extension velocities of 15-30 degrees per second were found to be typical of men lifting with weights that were substantial but did not overload. This may be an age-related phenomenon, although the effect does not appear to be robust, at least in the range of 20-59 years. The correlation coefficients found in the present study were close to that reported by Mayer et al. (10) for the male subjects at .46 m/sec (18 inches per second) at the initial test. In summary, the
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Alpert, Matheson, Beam, and Mooney
ability of an isokinetic lift to predict performance in a functional dynamic lift was demonstrated in the present study but there appears to be a relationship between age and isokinetic test performance. By design, the gravity-inertia and PLC II tests were very similar. Both tests involved lifting a box up from the floor to a shelf placed at 76 cm (30 inches) above the floor. Starting loads were the same and were determined individually by each subject's performance in the isokinetic test. One lift was done at each weight at 30-second intervals, and loads were increased in 4.5-kg (10-pound) increments. The gravity-inertia mode differed from the PLC II in that the box was smaller. The grip span across the milk crate was nearly twice that of the box used for the gravity-inertia test. The milk crate needed to be large in order to safely accommodate the mass of the masked weight canisters. Several of the subjects reported that lifting the crate was difficult because of the wide stance that was necessary. Additionally, the gravity-inertia box was lifted but not lowered. Lifting in these tests involved unrestricted motion. Subjects lifted the box, took a step, and set it on a shelf using the posture and movement pattern of choice. In the gravity-inertia test, however, since the box was attached to the machine, less motion occurred during the lift. It was not literally a "free weight" lift, as was the PLC II. Garg et al. (29) studied box size in an investigation which compared isometric strength with dynamic lifting capacity. One of the conclusions of that study was that the length of the box did not impact lifting capacity, but the increase in distance away from the body in the sagittal plane did decrease lifting capacity significantly. In the current study, this difference (12 cm for the gravity-inertia box and 16 cm for the PLC II crate) may have been functionally significant. The box in the gravity-inertia test was lifted but not lowered. This was done because it was difficult to tell if the subject was lowering the box independently, or if the machine was assisting. Subjects lifted and lowered in the PLC II because the investigator was unable to lower the loads for the subjects due to the large amount of weight being lifted. Although the weights lifted in the PLC II were on average 4% lower than those lifted in the gravity inertia test, this may have happened because more work was done in the PLC II. More energy was required when both lifting and lowering occurred. The correlation coefficients of r = .72 in test and r = .81 in retest shows that maximal lifting capacity as demonstrated by the PLC II can be predicted by testing in the gravity-inertia mode on the Lido Lift. This can be improved by normalizing values in terms of the subject's body weight (r = .80 in test and r = .87 in retest). Either of these approaches is reasonable and defensible. Fatigue may have played a role in differences that occurred between the two tests although subjects reported that the 20-minute rest period between the gravity-inertia and PLC II tests was adequate and there were no subjective reports of fatigue. Of all three tests (isokinetic, gravity-inertia, and PLC II), the PLC II was reported by all subjects to be the most demanding and was completed last in order. In addition, learning may have had an effect on test performance. The correlations at the retest, for both isokinetic/PLC II and gravity-inertia/PLC II were higher,
Reliability and Validity of Lifting Tests
27
although not to a statistically significant degree. This may be due to a learning effect. The correlation at the initial test is most important however, because in an actual testing situation the individual may be tested only once. In summary, the correlation between the gravity-inertia and PLC II was acceptable and accounts for 52% of the variance (66% in retest). If these tests are normalized for body weight, substantially more of the variance (64% in test and 76% in retest) in PLC II performance can be accounted for by gravity-inertia performance. Mean values in the gravity-inertia tests were higher than those in the PLC II, but they were highly correlated. This was also the case with the isokinetic tests. The gravity-inertia mode and the isokinetic mode on the Lido Lift appear to be valid ways to evaluate lifting capacity. If a more functional test is desired, the gravity-inertia mode would be the test of choice, although it utilizes isokinetic test results to set the starting weight. This approach appears to be quite effective in producing a reliable and valid result. The following conclusions were drawn as a result of this investigation: (1) The three maximal lifting capacity tests that were evaluated in a test-retest design were found to be reliable. Test-retest correlation coefficients for the isokinetic, gravity-inertia, and P L C II tests were r = .90, r = .81, and r = .90, respectively. (2) Although the isokinetic and gravity-inertia tests demonstrated higher means than those in the PLC II, both tests can be useful tools in evaluating maximal lifting capacity. The correlation between the gravity-inertia test and the PLC II was higher although not significantly different from that between the isokinetic test and the PLC II (r = .72 vs. r = .64 in test and r = .72 vs. r = .81 in retest). The ability of isokinetic testing at different speeds to predict lifting capacity needs further exploration so that an optimal range of testing speeds may be ascertained. There appears to be a weak but statistically significant relationship between age and isokinetic test performance that may dictate a slower velocity for older evaluees. The velocity of isokinetic lift in the current study (.46 m/sec or 18 inches per second) may be too fast for some evaluees, especially those with spinal injuries. Additional research is needed to investigate the behavioral attributes of lifting. The observation and description of postural changes as an individual nears maximal lifting capacity may help clinicians determine what is a safe and reproducible maximum. End-point criteria also need to be clearly defined, and terminology must be standardized. Lastly, the evaluation of new tests and computerized equipment needs to be an ongoing process. The Lido Lift is a valuable new piece of equipment that is capable of evaluating parameters not currently evaluated by other machines. It can show deviations in movement patterns in different planes and is able to measure acceleration and deceleration, velocity, and both peak and average force. It is able to provide reliable and valid evaluations of lifting capacity in both isokinetic and gravity-inertia modes.
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ACKNOWLEDGMENTS Data for this study were collected at the Irvine Medical Center. The authors gratefully acknowledge their appreciation for this support. Recognition is also due to Janet Grant and Jean Kuhns of ERIC for their assistance with preparation of the manuscript.
REFERENCES 1. Bigos S, Spengler D, Martin N, Zeh J, Fisher L, Nachemson A, Wang M. Back injuries in industry: a restrospective study II. Injury factors. Spine 1986; 11(3): 246-251. 2. Chaffin DB. Manual materials handling: The cause of over-exertion injury and illness in industry. J Environ Pathol Toxicot 1979; 2(5): 31-66. 3. Troup J, Martin J, Lloyd D. Back pain in industry: A prospective study. Spine 1981; 6(1): 61-69. 4. Chaffin D, Herrin G, Keyserling W. Preemployment strength testing. J Occup Med 1978; 20(6): 403-408. 5. Kroemer K. An isoinertial technique to assess individual lifting capability. Hum Factors 1983; 25(2): 493-506. 6. Menon K, Freivalds A. Repeatability of dynamic strength tests. Proc Hum Factors Soc 1985; 29(1): 517-520. 7. Kroemer IC Human strength terminology, measurement, and interpretation of data. Hum Factors 1970; 12: 297-313. 8. Karwowski W, Yates J. Reliability of the psychophysical approach to manual lifting of liquids by females. Ergonomics 1986; 29: 237-248. 9. Mital A. The psychophysical approach in manual l i f t i n g - a verification study. Hum Factors 1983; 25(5): 485-491. 10. Mayer T, Barnes D, Kishino N, Nichols G, Gatchel R, Mayer H, Mooney V. Progressive isoinertial lifting evaluation: I. A standardized protocol and normative database. Spine 1988; 13(9): 993-997. 11. Matheson L, Mooney V, Jarvis G, Caiozzo V, Lichter R, DeBerry C, Pottinger J, Levin K, Backlund K. Progressive lifting capacity with masked weights: Reliability study. Presented at the International Society for the Study of the Lumbar Spine, Boston, Massachusetts, June 1990. 12. Thistle H, Hislop H, Moffroid M, Lowman E. Isokinetic contraction. A new concept of resistive exercise. Arch Phys Med 1967; 48: 279-282. 13. Aitkens S, Lord J, Bernauer E, McCrory M. Analysis of the validity of the Lido Digital Isokinetic System (Abstract). Phys Ther 1987; 67(5): 757. 14. Burdett R, Van Swearingen J. Reliability of isokinetic muscle endurance tests. J Orthop Sports Phys Ther 1987; 8(10): 484-488. 15. Langrana N, Lee C, Alexander H, Mayott C. Quantitative assessment of back strength using isokinetic testing. Spine 1984; 9(3): 287-290. 16. McCrory M, Aitkens S, Avery C, Bernauer E. Reliability and validity of the Lido Active Isokinetic Rehabilitation System. Med Sci Sports Exerc 1989; 21(2): $52. 17. Rose S, Delitto A, Crandell C. Reliability of isokinetic trunk muscle performance (Abstract). Phys Ther 1988; 68(5): 824. 18. Smith S, Mayer T, Gatchel R, Becker T. Quantification of lumbar function: Part 1. Isometric and multispeed isokinetie trunk strength measures in sagittal and axial planes in normal subjects. Spine 1985; 10(8): 757-764. 19. Frykman P, Harmon E, Vogel J. Using a new dynamometer to compare three lift styles. Med Sci Sports Exerc 1988; 20(2): 87. 20. Porterfield J, Mostardi R, King S, Ariki P, Moats E, No D. Simulated lift testing using computerized isokinetics. Spine 1987; 12(7): 683-687. 21. Rothstein J, Lamb R, Mayhew T. Clinical uses of isokinetic measurements: Critical issues. Phys Ther 1987; 67: 1840-1844. 22. Timm K. Isokinetic lifting simulation: A normative data study. J Orthop Sports Phys Ther 1988; 10(5): 156-166. 23. Kishino N, Mayer T, Gatchel R, Parrish M, Anderson C, Gustin L, Mooney V. Quantification of lumbar function: Part 4. Isometric and isokinetic lifting simulation in normal subjects and low back dysfunction patients. Spine 1985; 10(10): 921-927.
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24. Feiring D, Ellenbecker T, Derscheid G. Test-retest reliability of the Biodex Isokinetic Dynamometer. J Orthop Sports Phys Ther 1990; 11(7): 298-300. 25. Lord J, Aitkens S, McCrory M, Bernauer E. Reliability of the Lido Digital Isokinetic System for the measurement of muscular strength (Abstract). Phys Ther 1987; 67(5): 757. 26. Tredinnick T, Duncan P. Reliability of measurements of concentric and eccentric isokinetic loading. Phys Ther 1988; 68(5): 656-659. 27. Pytel J, Kamon E. Dynamic strength tests as a predictor for maximal acceptable lifting. Ergonomics 1981; 24: 663-672. 28. Kim J, Marras W. Quantitative trunk muscle electromyography during lifting at different speeds. lnt J lnd Ergonomics 1987; 1: 219-229. 29. Garg A, Mital A, Asfour S. A comparison of isometric strength and dynamic lifting capacity. Ergonomics 1980; 23: 13-27.
The Reliability and Validity of Two New Tests of Maximum Lifting Capacity Joanette Alpert, 1 Leonard Matheson, 2,5 William Beam, 3 and Vert Mooney 4
This study investigated the test-retest reliability and validity of the isokinetic and gravity~inertia modes of the Lido Lift TM and the Progressive Lifting Capacity H test (PLC II). M a x i m u m lifting capacity tests were performed in the isokinetic and gravity~inertia modes on the Lido Lift and with the P L C II with 29 healthy male subjects. Pearson product-moment correlations were calculated to determine the test-retest reliabilities and to compare P L C H test values with isokinetic and gravity~inertia test values. The correlations for the isokinetic, gravity inertia, and the P L C II were r = .90, .82, and .91, respectively. The correlations between the isokinetic and gravity~inertia tests and the P L C H were r = .64 and .74 during the test and r = .72 and .81 during the retest. The results of this study support the reliability of both Lido Lift TM tests and of the P L C II, and the validity of the isokinetic and gravity~inertia tests when compared to the P L C II. KEY WORDS: lift capacity; isokinetic; gravity inertia.
INTRODUCTION Several studies have documented the prevalence of low back pain in industry (1-3). Bigos et al. (1) found that back injuries were more commonly associated with material handling and lifting than with slips and falls. Chaffin (2) reported that, in industry and medicine, it is important that objective evaluative tools be developed to accurately assess a worker's functional capacity. In particular, there is a need for a standardized test or group of tests that can accurately assess lifting capacity. Although various tests have been shown to be reliable, there is no lifting test which has been shown to be more valid than another. Controversy exists in the literature with respect to which type of testing is the most valid and practical. IOccucare Industrial Health Clinics, Pomona, California. Employment and Rehabdltatlon Institute of Cahforma, Santa Ana, California. 3California State University, Fullerton, California. 4Irvine Medical Center, Irvine, California. 5Correspondence should be directed to Leonard Matheson, Ph.D., Employment and Rehabilitation Institute of California, Suite 101, 600 South Grand Avenue, Santa Ana, California 92705. .
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Historically, the evaluation of lifting capacity has centered around isometric, free weight, and more recently, isokinetic testing. Isometric (static) tests involve contracting a muscle or group of muscles with no shortening of the muscle and allowing no motion to occur. These tests have frequently been the tests of choice because they have been shown to be reliable, easy to administer, and the equipment needed is not too costly. Chaffin et al. (4) proposed a set of isometric lifting tests. Acceptable test-retest reliability has been demonstrated with these tests. However, since lifting is a dynamic activity, it has also been said that the testing of lifting should be dynamic (5). Because most activities are dynamic, isometric testing may not provide the most accurate assessment of an individual's capabilities. Menon and Freivalds (6) concluded from their study, which compared isometric tests with isokinetic tests, that since the correlation between dynamic and static strengths was significant, dynamic tests should be the tests of choice in that dynamic tests were better indicators of work capacity in "real world" settings. The validity of using dynamic tests in which a free weight is used, i.e., using a crate or box with a weight in it, lies in the fact that this is indeed what the individual actually does in a work setting. This simulates more closely the actual activity and may give a better evaluation of the individual's lifting capacity, instead of "predicting" it from the results of an isometric test. Kroemer (7) reported that it was at best very difficult, if not impossible, for dynamic strength to be predicted from a test of static strength, mainly because the mechanical and physiological processes that occur during each test are so different. A free weight lifting test involves a dynamic movement of a fixed load. Different protocols have been utilized with this fixed load concept. A "psychophysical" approach has been utilized by Karwowski and Yates (8) and Mital (9). As described by Mital (9), the psychophysical approach requires a subject to randomly adjust a load during a period of 20-25 minutes until an acceptable load is found that the subject estimates can be lifted repetitively during a regular 8-hour work shift. The weight that the subject chooses is considered the maximum weight that the subject can comfortably lift for the 8-hour period. Karwowski and Yates (8) found this approach reliable only for low and moderate lifting frequencies (not greater that six lifts per minute). In a study by Mital (9), the psychophysical method tended to overestimate the maximum acceptable weight of lifting. He compared the results of the psychophysical test to an actual 12-hour session of lifting, and found that none of the ten subjects were able to work with the weight they had chosen during the psychophysical test. Given these findings, the validity of the psychophysical approach is questionable. Another free weight lifting approach is the "isoinertial" technique. Kroemer (5) described this technique as one in which the individual lifts a constant mass which is progressively increased to the maximum that the individual can lift. The study reported a coefficient of variation of 3.2-3.9% in an overhead lift and 5.27.8% in a knuckle-height lift. The same subjects showed much higher variability with static tests. The coefficient of variation with the static tests ranged from 11.6 to 15.4%.
Reliability and Validity of Lifting Tests
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Test-retest reliability of a test which is based on an adaptation of this technique, the progressive isoinertial lifting evaluation, "PILE" (10), showed a correlation of r = .87 for the lift of a progressively loaded milk crate with a frequency of four lifts every 20 seconds from floor to 76 cm and r = .93 for the lift from 76 cm to 137 cm. Similar to the PILE is the progressive lifting capacity test "PLC I" (11). The PLC I utilizes "masked weights" with a similar weighting schema and a frequency of four lifts every 30 seconds. An evaluation of the test-retest reliability of this lifting capacity test demonstrated a correlation coefficient of r = .77 for the tift from floor to 76 cm and r = .81 for the lift from 76 cm to 122 cm with a frequency of four lifts every 30 seconds. Isokinetic testing has been shown to be a reliable means of evaluating strength in the peripheral joints, in the spine, and more recently in lifting. The concept of isokinetic exercise was introduced in 1967 (12). Isokinetic performance allows motion to occur against accommodating resistance at a fixed speed. Isoldnetic testing has been found to be reliable in several studies (13-18). Isokinetic lifting simulation was shown to be reliable in studies done by Frykman et al. (19) and Porterfield et al. (20). Some critical issues have been raised regarding the use of isokinetic testing of functional activities such as lifting (21, 22). Although isokinetic testing is dynamic, it does not mimic functional activities since work is not usually performed at a fixed speed. Kishino et al. (23) felt that although isokinetic equipment can be a useful tool in industry and rehabilitation, it had important equipment limitations that limited its ability to predict performance in an actual work setting. Timm (22) also felt that since lifting is a three-dimensional activity and is composed of various combinations of isometric, isotonic, and isokinetic components, isokinetic technology may not be totally effective as an evaluative or rehabilitative tool. Mayer et al. (10) compared isokinetic lifting (Cybex Liftask) with the PILE and found the correlations between the two tests to be low. It was concluded that the tests do not evaluate the same parameters and cannot be substituted for each other. Although both tests are dynamic, they are quite different. The PILE is an unconstrained lifting test allowing the individual to move at any speed and utilize various postures. The isokinetic test limits acceleration and velocity. The Lido Lift (Loredan Biomedical, Davis, California) has been designed to evaluate dynamic lifting capacity in isokinetic and gravity-inertia modes. The object to be lifted is attached to an articulating arm that measures movement in three dimensions. The gravity-inertia mode involves moving a fixed load designed to reproduce a real lifting situation. In this mode, the force of gravity and the inertia of the weight being lifted is simulated by the machine. This feature currently is unique to this piece of equipment. While this is a potentially useful piece of equipment, no data are available on its reliability or validity. The purposes of the current study were: (1) to investigate the reliability in a test-retest design of the isokinetic and gravity-inertia modes of the Lido Lift, (2) to investigate the validity of the Lido Lift in comparison to the PLC II, and (3) to investigate the reliability of the progressive lifting capacity test (PLC II).
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METHODS Subjects Twenty-nine male volunteers participated in this study. Ages ranged from 20 to 59 years with a mean of 32 years. Height ranged from 173 to 196 cm with a mean of 180 cm, and weight ranged from 68 to 109 kg with a mean of 83 kg (Table I). In response to questioning, subjects reported no limitation of activity due to back pain, no history of back surgery, and no reported back injury within the previous 6 months.
Procedures All subjects underwent an orientation and signed a consent document. Subjects completed a general health questionnaire and the experimenter performed a brief musculoskeletal evaluation. A submaximal cardiovascular test, to screen for abnormal heart rate and/or blood pressure response to exercise, was performed if the subject was "at higher risk" for cardiovascular disease (as described by The American College of Sports Medicine, 1986). Only one subject required this test. A 9 minute graded submaximal exercise test was performed. This subject had normal heart rate and blood pressure responses and was admitted to the subject pool. The health questionnaire, musculoskeletal evaluation, and the cardiovascular screening were reviewed for any abnormalities which would have placed the subject at risk during the lifting tests. No subjects were excluded from the study based on these screenings. Upon completion of the screening procedures, subjects were guided through 5 minutes of stretching exercises and 5 minutes on a stationary bicycle at low resistance to serve as a warm-up prior to the initiation of testing. A Fitron bicycle ergometer was used for the graded submaximal exercise test and for the warm-up prior to testing. Subjects participated in the three types of maximal lifting tests on two separate days. The test-retest interval ranged from 4 to 12 days, for all but one subject who had a 22-day test-retest interval. All subjects were given the same instructions and performed the same tests in the same order during the test and the retest. The order of testing and inter-test intervals were standardized for all subjects: (1) isokinetic practice and test, (2) 5-minute rest, (3) progressive gravity-inertia test
Table I. Physical Characteristics of the Subjectsa Variable (units) Age (yr)
Mean
(SD)
Minimum
Maximum
32.0
(9.4)
20.0
59.0
Height (cm)
180.0
(5.4)
173.0
196.0
Weight (kg)
83.0
(10.4)
68.0
109.0
an = 29.
Reliability and Validity of Lifting Tests
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beginning at 50% of the average isokinetic maximum and progressing in 10-pound increments, (4) 20-minute rest period, and (5) progressive PLC II test beginning at 50% of the average isokinetic maximum and progressing in 10-pound increments. The following instructions (italicized words were verbally emphasized) were read to each subject during the test procedures: Isokinetic: In this test, you will use the "T" handle (demonstrate) placed at 6 inches above the floor. I want you to bend at your hips and knees, and keep your head up (demonstrate). In this test, the speed is fixed at 18 inches per second, and I want you to pull up as hard as you can until you hit the "stop" (there is a cushioned stop at 42 inches), and then let go of the handles (demonstrate). You will do six repetitions for practice. I want you to take these fairly easy, giving about 50% of your maximum effort. The purpose of the practice trials is to get you comfortable with the machine. Then, you will do four test trials. I will tell you when to start each time. I will say "get ready, 5, 4, 3, Z 1 go.t" In the test trials, I'm looking for a 100% maximum effort. It's extremely important than you give me your full effort for each test trial. Remember, I'm looking for a 100% maximum effort. Do you have any questions? OK, let's begin now! Gravity-inertia: In this test you will be using the small box attachment, and you will use these grips (demonstrate). You will lift each weight once, and you will do one lift every 30 seconds. I will time you. You will lift the box from the floor, set it on the shelf, and then step back. I will tell you when to initiate each lift. I will say, "get ready, 5, 4, 3, 2, 1 go.t" You will be lifting weights that will progressively get heavier. After each lift, I will ask you how you feel, and I will ask you if you want to add weight. If you say yes, I will add weight, and we will proceed.
I want a maximum safe lift, that you can dependably reproduce 10-12 times per day. Do you have any questions? Now remember, I want a maximum safe lift, that you can dependably reproduce ]0-12 times per day. OK, let's begin.
PLC II: In this test, I'm going to ask you to pick up this milk crate from the floor and put it on this bench right here and then put it back on the floor. I want you to do this same thing one time each 30 seconds, and then rest, and then if you're doing OK, FII ask you to do it again with more weight. When you finish each lift, I'll ask you if you can lift the crate with more weight. If you can, we'll keep going, but if you can't that's OK, just let me know. Remember, it's O K to stop the test. I don't mind, I don't want you to hurt yourself!
I want a maximum safe lift, that you can dependably reproduce 10-12 times per day. Do you have any questions? OK? Watch me. This is what l'd like you to do. See?
Remember, I want a maximum safe lift, that you can dependably reproduce 10-12 times per day. OK, let's start the test now!
All subjects wore leather gloves and comfortable non-binding work or athletic clothes. Subjects wore a Tunturi heart rate monitor which was attached to the earlobe for the duration of the testing session. Blood pressure was monitored periodically throughout the testing with a Baumanometer Calibrate V-lok blood pressure cuff. The Lido Lift was used to evaluate isokinetic and gravity-inertia lifting capacity (Fig. 1). Operated by an IBM PS 30/286 computer, the device has a fixed arm mechanism on which various "T" handles or boxes can be attached. All force
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Fig. 1. Lido Lift (Loredan Biomedical,Davis, California).
and velocity variables were controlled through the computer. The computer printed out the result of any test on command. Before each test, the machine zeroed itself out for height of the fixed arm and for weight. The strain gauge was calibrated at the factory. The isokinetic test was performed on the Lido Lift through the use of a "T" handle with rotating padded hand grips and a 12-inch span between the hand grips. Vertical velocity was fixed at .46 m/sec (18 inches per second). The subject stood facing the machine in a squatting position and grasped the handgrips, one in each hand, placed at 15 cm (6 inches) above the floor. The subject lifted the handle to a height of 107 cm (42 inches). Six submaximal practice trials (subjects were instructed to give 50% effort) were performed to allow the subject to become comfortable with the machine, followed
Reliability and Validity of Lifting Tests
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by four test trials. For the test trials, subjects were asked to give a maximal effort. Peak force for each trial was measured, and the average peak force using the best three out of four repetitions was calculated. The gravity-inertia test was performed on the Lido Lift. In this mode, the load was fixed in pounds, and was programmed into the computer. Subjects lifted from the floor a box which was attached to the machine, and set it on a shelf placed at 76 cm (30 inches) above the floor. The dimensions of the box were 29 cm across by 24 cm deep and 23 cm high. The padded hand holds were 18 cm above the bottom of the box, centered 12 cm from the front and rear edge of the box. After placing the box on the shelf, the subject stepped back and the experimenter pushed the box off the shelf to allow it to fall slowly to the floor. The initial load for each subject was determined by averaging the peak force values obtained during the best time out of four test trials in the isokinetic test, and taking 50% of the number. There were no practice trials preceding this test. One lift was performed with each load at 30-second intervals. The load was increased in 4.5-kg (10-pound) increments until the subject reached his maximum lift. The third test performed was the progressive lifting capacity test (PLC II). This involved lifting and lowering a load in a plastic milk crate, using gloves and hand-holds. Each subject's starting load was equal to the starting load used during the gravity-inertia test on the Lido Lift. As with gravity-inertia, the subject was not informed of the starting weight. Subjects lifted the crate from the floor and placed it on a workbench that was 76 cm (30 inches) high. The crate was then lowered back to the floor. Similar to the gravity-inertia test, one lift was performed every 30 seconds, and one lift was performed with each load. However, this was a liftlower task. The load was increased in 4.5-kg increments until the subject reached maximum lift. A standard steel reinforced milk crate which weighed 2.5 kilograms was used for the PLC tests. The crate measured 48 cm wide by 32 cm deep, and was 28 cm tall. The hand holds were 25 cm from the bottom of the crate, centered 16 cm from the front and rear edge of the box. Color coded weight canisters were used as the free weights. The canisters weighted either 2.3 kg, 4.5 kg, or 9.1 kg, and were all the same size, 28 cm tall and 10 cm in diameter. Subjects were not informed of their performance, starting loads, or increments of additional loads during and between tests and testing days. In the isokinetic and gravity-inertia modes, subjects were not able to view the computer screen. When testing was completed, subjects were contacted by telephone to perform a debriefing. The purpose of the debriefing was to determine whether or not there were any problems resulting from the test experience, and to provide the subject with feedback about his performance.
Variables
In the isokinetic mode, the average peak force was calculated from the best three out of four test trials. In this study for both the gravity-inertia and the PCL
Alpert, Matheson, Beam, and Mooney
20
II tests "maximum safe lift" was defined as a safe and reproducible maximum (GIMAX and PLCMAX, respectively). Subjects were instructed to continue until a self-selected maximum safe lift was achieved, one that could be dependably reproduced 10-12 times per day. This was similar to an "infrequent occasional" lifting requirement and was meant to be different from a one-time maximum lift that might not be able to be reproduced. If postural changes were observed which would put the subject at risk for injury, the experimenter would discontinue the test and call the previous lift the subject's maximum. These postures included using the whole body to stabilize or move the box, coming up onto the toes with either heel off the floor, or shrugging the shoulders in an effort to reach the shelf. A maximum weight of 113 kilograms was possible on the Lido Lift in the isokinetic mode. Four out of 29 subjects were unable to be evaluated in this mode because their maximum was beyond the limits of the machine. Because the starting load for the gravity-inertia and PLC II tests used 50% of the average peak force values from the isokinetic test, starting load for the four subjects unable to be tested to the maximum in the isokinetic mode was determined by starting at 50% of body weight. The gravity-inertia mode had a limit of 90.7 kg, which was reached by seven of the 29 subjects. Two of these stopped because it was their maximum, but the other five were stopped by the tester because the machine was unable to evaluate them at a higher weight. These subjects were unable to elicit their maximum in the gravity-inertia test, so the sample size in this aspect of the study was decreased to 24 (from 29).
RESULTS Physical characteristics of the subjects (age, height, and weight) were not related to test performance except with regard to a weak but statistically significant inverse relationship between age and isokinetic test performance in both the test and the retest (r = -.499 and r -- -.403, respectively).
Reliability Minimum, maximum, mean, and standard deviation for test and retest force values are presented in Table II. Coefficients of variation for each subject's set of isokinetic test trials ranged from 2.0 to 15.5% (mean = 7.8%) for the test and from 2.2 to 13.9% (mean = 6.8%) for the retest. Test-retest correlation coefficients are presented in Table III. All correlations are statistically significant at p ___.05.
Validity Pearson product-moment correlation coefficients were computed to determine the relationship between the isokinetic test and the PLC II and the gravity-inertia test and the PLC II. Results are presented in Table IV. Values were also normalized
Reliability and Validity of Lifting Tests
21
Table II. Mean Force Values for Isokinetic and Gravity-Inertia Modes of Lido Lift and for PLC II at Test and Retest Mean
(SD)
Minimum
Maximum
Isoldnetic mode (kg) Test trial Retest trial
81.2 82.7
(20.4) (22.3)
47.4 42.6
125.2 130.5
Gravity-inertia mode (kg) Test trial Retest trial
74.8 74.9
(11.9) (11.3)
46.4 44.1
90.9 90.5
PLC II (kg) Test trial Retest trial
71.9 73.6
(12.5) (14.1)
42.3 40.0
99.1 109.3
Type of test (units)
Table III. Correlation Coefficients Describing Reliability of Lifting Capacity Tests Type of test
Test-retest reliability (r)
Lido Lift (isokinetic)
0.90
Lido Lift (gravity-inertia)
0.81
PLC II
0.90
by dividing each variable by the subject's body weight. The correlations between these normalized values are presented in Table V. All correlations are statistically significant at p _< .05. Absolute lifting capacity values are classified by percentile for the test and retest in the isokinetic gravity-inertia, and PLC II tests in Table VI. The average of the best three of the four peak force values that were obtained in the isokinetic test ranged from 47.3 to 125.0 kg, and at the retest ranged from 42.5 to 130.2 kg. The normalized (body weight) values are presented in Table VII. The predictive validity of the isokinetic and gravity-inertia tests can be evaluated through the use of regression equations which, along with scatter graphs of the retest data, are shown in Figs. 2 and 3. Although the isokinetic test does not account for as much of the variance of lifting capacity as does the gravity-inertia test, Fisher's-z comparison demonstrated no statistically significant difference between the two.
DISCUSSION
Reliability of Isokinetic Test Performance. A correlation coefficient of r = .90 was reported for isokinetic test performance. This is comparable to other studies,
22
Alpert, Matheson, Beam, and Mooney
Table IV. Correlation Coefficients Describing Validity of Lifting Capacity Tests Validity compared with PLC II (r) Type of test
Test
Retest
Lido Lift (isokinetic)
0.64
0.72
Lido Lift (gravity-inertia)
0.72
0.81
Table V. Correlation Coefficients Describing Validity of Lifting Capacity Tests Normalized by Body Weight Validity compared with PLC II (r) Type of test
Test
Retest
Lido Lift (isokinetic)
0.68
0.73
Lido Lift (gravity-inertia)
0.80
0.87
y = .44x |
.3
+ .445,
|
R-squared: !
.532 !
.2
|
0
.*
.1
0
1 .g
0
.8
.....
.4
.6
~
.i
0
2
1' IKMAX/BW
~176176176176
o
1:2
1:4
1.6
Fig. 2. Regression of isokinetic test on PLC II; 95% confidence intervals for the true mean of PLC II. PLCMAX/BW = maximum weight lifted in PLC test divided by body weight; IKMAX/BW = maximum weight lifted in Lido isokinetic test divided by body weight.
Reliability and Validity of Lifting Tests
y
=
23
.738x
+
.169,
R-squared:
.755
.1 1,
0
.g,
o0"
"'"
L.
.8,
.7 .6
.5 .4 .4 GIMAX/BW
Fig. 3. Regression of gravity-inertia on PLC II; 95% confidence intervals for the true mean of PLC II. PLCMAX/BW = maximum weight lifted in PLC test divided by body weight; GIMAX/BW = maximum weight lifted in Lido gravity-inertia test divided by body weight.
including Burdent and Van Swearingen (14) and Rose et at (17). Although this was the first study to investigate the test-retest reliability of the isokinetic mode on the Lido Lift, studies investigating other isokinetic devices have reported correlation coefficients greater than r = .80. Frykman et aL (19) found test-retest reliability of peak force with the Cybex Liftask to be r = .82. Studies evaluating isokinetic knee performance have reported correlation coefficients ranging from r = .47 to .98 (24-26). Rose et al. (17) reported acceptable test-retest reliability (greater than r = .80) for average peak torque on the Lido back isokinetic dynamometer. Reliability of Gravity-Inertia and P L C H Test Performance. The present study was the first to examine the reliability of the gravity-inertia and PLC II tests. Both tests were found to be reliable. The test-retest correlation coefficients for the PLC II were comparable to that reported by Matheson and his colleagues (11) for the original PLC and by Mayer et al. (10) for the PILE. The main difference in testing protocols for those tests and the tests used in the present study were the end-point criteria. There were three defined end-point criteria for the PLC I and for the PILE. These were: (1) a psychophysical end-point, (2) a heart rate end-point, and (3) a proportion of ideal body weight end-point. Subjects in the present study were instructed to continue until a "maximum safe lift" was reached which could be dependably reproduced 10-12 times per day. Each subject used his own perception of the load as a psychophysical end-point. There were no heart rate or ideal body weight end-points. The test-retest interval for the PILE was only 1 day, while the interval in the present study averaged 8 days. Also, as with the PLC I, in the PLC II the weights in the crate were masked and subjects were unable to visually evaluate the amount of each lift. It would seem unlikely that
24
Alpert, Matheson, Beam, and Mooney Table VI. Normative Values for Lifting Capacity Tests in Healthy Adult Males
Percentile rank
Lido Lift
Lido Lift
(Isokinetic mode)
(Gravity-inertia)
PLC II
Test
Retest
Test
Retest
Test
Retest
90th
112.20
111.90
90.50
88.50
92.30
88.90
75th
95.00
98.50
86.10
83.80
78.90
80.10
50th
77.60
80.30
72.70
76.40
70.70
74.10
25th
66.50
66.60
66.60
68.30
61.60
65.30
lOth
56.30
52.50
62.00
63.10
61.30
62.70
Table VII. Normative Values for Relative Force (kg/kg) of Lifting Capacity Tests in Healthy Adult Males
Percentile rank
Lido Lift
Lido Lift
(Isokinetic mode)
(Gravity-inertia)
PLC II
Test
Retest
Test
Retest
Test
Retest
90th
1.36
1.31
1.13
1.08
1.10
1.05
75th
1.14
1.20
1.06
1.04
0.96
0.98
50th
1.03
1.06
0.87
0.96
0.88
0.91
25th
0.80
0.86
0.83
0.84
0.73
0.75
lOth
0.62
0.61
0.71
0.68
0.67
0.71
s u b j e c t s w e r e a b l e to r e m e m b e r t h e a m o u n t o f w e i g h t t h a t was lifted d u r i n g t h e initial test. I n this s t u d y , t h e e n d - p o i n t c r i t e r i a f o r t h e g r a v i t y - i n e r t i a t e s t a n d t h e P L C II w e r e t h e s a m e . S u b j e c t s w e r e i n s t r u c t e d to c o n t i n u e t h e t e s t w i t h a d d i t i o n a l w e i g h t u n t i l a " m a x i m u m s a f e lift" w a s a c h i e v e d , o n e t h a t c o u l d b e " d e p e n d a b l y reproduced 10-12 times per day." In the present study, a "reproducible" maxim u m w a s s o u g h t . T h i s was t h o u g h t to b e m o r e a p p l i c a b l e to an i n d u s t r i a l s e t t i n g t h a n w o u l d b e a " o n e r e p e t i t i o n m a x i m u m . " K r o e m e r (7) d i s c u s s e d t h e d i f f e r e n c e b e t w e e n m a x i m u m a n d o p t i m u m a n d t h a t m a x i m u m r e f e r s to a n " a m o u n t " a n d o p t i m u m r e f e r s to a " c o n d i t i o n . " C o n s i d e r i n g t h e s e d e f i n i t i o n s , t h e p r e s e n t s t u d y w a s s e e k i n g an " o p t i m u m " lift in t h e g r a v i t y - i n e r t i a a n d P L C II tests, o n e w h i c h w o u l d b e f u n c t i o n a l for u s e in i n d u s t r y . K r o e m e r also felt t h a t i n c o n s i s t e n c i e s in t e r m i n o l o g y , l a c k o f c a l i b r a t i o n o f d e v i c e s , t h e i n a b i l i t y to c o n t r o l f o r m o t i v a t i o n , a n d t h e l a c k o f s t a n d a r d i z a t i o n o f i n s t r u c t i o n s all h a v e c o n t r i b u t e d to i n c o n s i s t e n t i n t e r p r e t a t i o n s o f t h e r e s u l t s o f m a n y s t r e n g t h s t u d i e s . A n e f f o r t
Reliability and Validity of Lifting Tests
25
was made in the present study to define "maximum" and to use standardized instructions. In addition to the instructions given to the subjects regarding when to tell the tester to stop increasing the load, the tester observed the subjects' body mechanics and postural changes. Only if the subject was using body mechanics or assumed a position which would put him at risk for injury, was the test halted. Although subjects were encouraged to use the legs for power, and not the back, each used the posture of choice. It was interesting to note that postural changes were observed as subjects approached what was defined in this study as maximum. These changes included: (1) rising onto toes and/or shrugging the shoulders to clear the 76 cm shelf, and (2) going from a "leg lift" (using the legs for power and keeping the lumbar curve in a neutral position) to a "back lift" (knee extension preceding back extension). Mayer et aL (10) have discussed the "weak link" phenomenon. A whole body motion, such as that required by a lift of this nature, will be limited by the weakest part of the whole, whether it is upper body, lower body, or trunk strength. The weak link may be revealed through compensatory postural changes observed as the individual nears his maximum capacity. It appeared in this study as if the weak link provides consistent limitations on performance in that whole body lifting in all three modes demonstrated high degrees of reliability. In summary, all three of the tests evaluated in this study appear to be reliable tools for evaluating lifting capacity. Validity of Isokinetic and Gravity-Inertia Test Performance. Lifting results on the gravity-inertia mode of the Lido Lift accounted for a higher percentage of the variance on the PLC II than the lifting data from the isokinetic test mode. This may have been due to the similarity in lifting tasks between the gravity-inertia mode and the PLC II. The isokinetic test in this study was done at .46 m/sec (18 inches per second). Pytel and Kamon's study (27) showed that a simulated lift using an isokinetic motion at .73 m/sec (29 inches per second) was highly correlated with the maximal dynamic lift (fixed load) in that study. Correlation coefficients were r = .87 for men and r = .92 for women. Mayer et al. (10) compared a simulated isokinetic lift on the Cybex Liftask at .46 m/sec (18 inches per second) and .76 m/sec (30 inches per second) with the PILE, a functional dynamic lift described previously. Subjects in that study were patients who had chronic low back pain and underwent several weeks of rehabilitation between initial evaluation and discharge. Correlation coefficients for the male subjects ranged from r = .61 to .63 at the initial test, and from r = .38 to .45 at discharge. Pytel and Kamon (27) also evaluated an isokinetic lift at .97 m/sec (38 inches per second) that did not correlate as well with the maximal dynamic lift. It may be that there is a "velocity window" within which isokinetic test performance is able to be adequately predictive of lifting capacity. That such would be the case is suggested by Kim and Marras (28) in which extension velocities of 15-30 degrees per second were found to be typical of men lifting with weights that were substantial but did not overload. This may be an age-related phenomenon, although the effect does not appear to be robust, at least in the range of 20-59 years. The correlation coefficients found in the present study were close to that reported by Mayer et al. (10) for the male subjects at .46 m/sec (18 inches per second) at the initial test. In summary, the
26
Alpert, Matheson, Beam, and Mooney
ability of an isokinetic lift to predict performance in a functional dynamic lift was demonstrated in the present study but there appears to be a relationship between age and isokinetic test performance. By design, the gravity-inertia and PLC II tests were very similar. Both tests involved lifting a box up from the floor to a shelf placed at 76 cm (30 inches) above the floor. Starting loads were the same and were determined individually by each subject's performance in the isokinetic test. One lift was done at each weight at 30-second intervals, and loads were increased in 4.5-kg (10-pound) increments. The gravity-inertia mode differed from the PLC II in that the box was smaller. The grip span across the milk crate was nearly twice that of the box used for the gravity-inertia test. The milk crate needed to be large in order to safely accommodate the mass of the masked weight canisters. Several of the subjects reported that lifting the crate was difficult because of the wide stance that was necessary. Additionally, the gravity-inertia box was lifted but not lowered. Lifting in these tests involved unrestricted motion. Subjects lifted the box, took a step, and set it on a shelf using the posture and movement pattern of choice. In the gravity-inertia test, however, since the box was attached to the machine, less motion occurred during the lift. It was not literally a "free weight" lift, as was the PLC II. Garg et al. (29) studied box size in an investigation which compared isometric strength with dynamic lifting capacity. One of the conclusions of that study was that the length of the box did not impact lifting capacity, but the increase in distance away from the body in the sagittal plane did decrease lifting capacity significantly. In the current study, this difference (12 cm for the gravity-inertia box and 16 cm for the PLC II crate) may have been functionally significant. The box in the gravity-inertia test was lifted but not lowered. This was done because it was difficult to tell if the subject was lowering the box independently, or if the machine was assisting. Subjects lifted and lowered in the PLC II because the investigator was unable to lower the loads for the subjects due to the large amount of weight being lifted. Although the weights lifted in the PLC II were on average 4% lower than those lifted in the gravity inertia test, this may have happened because more work was done in the PLC II. More energy was required when both lifting and lowering occurred. The correlation coefficients of r = .72 in test and r = .81 in retest shows that maximal lifting capacity as demonstrated by the PLC II can be predicted by testing in the gravity-inertia mode on the Lido Lift. This can be improved by normalizing values in terms of the subject's body weight (r = .80 in test and r = .87 in retest). Either of these approaches is reasonable and defensible. Fatigue may have played a role in differences that occurred between the two tests although subjects reported that the 20-minute rest period between the gravity-inertia and PLC II tests was adequate and there were no subjective reports of fatigue. Of all three tests (isokinetic, gravity-inertia, and PLC II), the PLC II was reported by all subjects to be the most demanding and was completed last in order. In addition, learning may have had an effect on test performance. The correlations at the retest, for both isokinetic/PLC II and gravity-inertia/PLC II were higher,
Reliability and Validity of Lifting Tests
27
although not to a statistically significant degree. This may be due to a learning effect. The correlation at the initial test is most important however, because in an actual testing situation the individual may be tested only once. In summary, the correlation between the gravity-inertia and PLC II was acceptable and accounts for 52% of the variance (66% in retest). If these tests are normalized for body weight, substantially more of the variance (64% in test and 76% in retest) in PLC II performance can be accounted for by gravity-inertia performance. Mean values in the gravity-inertia tests were higher than those in the PLC II, but they were highly correlated. This was also the case with the isokinetic tests. The gravity-inertia mode and the isokinetic mode on the Lido Lift appear to be valid ways to evaluate lifting capacity. If a more functional test is desired, the gravity-inertia mode would be the test of choice, although it utilizes isokinetic test results to set the starting weight. This approach appears to be quite effective in producing a reliable and valid result. The following conclusions were drawn as a result of this investigation: (1) The three maximal lifting capacity tests that were evaluated in a test-retest design were found to be reliable. Test-retest correlation coefficients for the isokinetic, gravity-inertia, and P L C II tests were r = .90, r = .81, and r = .90, respectively. (2) Although the isokinetic and gravity-inertia tests demonstrated higher means than those in the PLC II, both tests can be useful tools in evaluating maximal lifting capacity. The correlation between the gravity-inertia test and the PLC II was higher although not significantly different from that between the isokinetic test and the PLC II (r = .72 vs. r = .64 in test and r = .72 vs. r = .81 in retest). The ability of isokinetic testing at different speeds to predict lifting capacity needs further exploration so that an optimal range of testing speeds may be ascertained. There appears to be a weak but statistically significant relationship between age and isokinetic test performance that may dictate a slower velocity for older evaluees. The velocity of isokinetic lift in the current study (.46 m/sec or 18 inches per second) may be too fast for some evaluees, especially those with spinal injuries. Additional research is needed to investigate the behavioral attributes of lifting. The observation and description of postural changes as an individual nears maximal lifting capacity may help clinicians determine what is a safe and reproducible maximum. End-point criteria also need to be clearly defined, and terminology must be standardized. Lastly, the evaluation of new tests and computerized equipment needs to be an ongoing process. The Lido Lift is a valuable new piece of equipment that is capable of evaluating parameters not currently evaluated by other machines. It can show deviations in movement patterns in different planes and is able to measure acceleration and deceleration, velocity, and both peak and average force. It is able to provide reliable and valid evaluations of lifting capacity in both isokinetic and gravity-inertia modes.
28
Alpert, Matheson, Beam, and Mooney
ACKNOWLEDGMENTS Data for this study were collected at the Irvine Medical Center. The authors gratefully acknowledge their appreciation for this support. Recognition is also due to Janet Grant and Jean Kuhns of ERIC for their assistance with preparation of the manuscript.
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