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Isometric and Isokinetic Measurement of Hamstring and Quadriceps Strength Janet P. Lord, MD, Susan G. Aitkens,

BA, Megan A. McCrory,

BS, Edmund

M. Bernauer, PhD

Lord JP, Aitkens SG, McCrory MA, Bernauer EM. Isometric and isokinetic measurement of hamstring ABSTRACT. and quadriceps strength. Arch Phys Med Rehabil 1992;73:324-30. l This study evaluates quadriceps strength at varying movement velocities in a group of healthy subjects. Specific objectives included (1) investigating the effect of angular velocity on peak torque (PT) (2) investigating the effect of angular velocity on joint angle at peak torque (JAPT) and (3) evaluating whether the use of a relatively new isokinetic testing device yields new insights to previously accepted relationships between angular velocity and PT or JAPT. Twenty healthy subjects were tested for isometric and isokinetic knee extension and flexion strength on a LID0 Active Isokinetic Rehabilitation System at velocities of O”, 60”, 120”, and lSO”/sec. Isometric m (O”/sec) was found to be highly correlated with isokinetic PT for both extensors and flexors at all velocities (r = .88-.93, pi.01). PT declined significantly as angular velocity increased for both extension (pc.01) and flexion (pc.05). A relationship between isometric and isokinetic JAPT was significant for extension only at 60” and 180”/sec (r = .48-.52, pc.05). These results indicate that knee isometric and isokinetic PT as measured on the LID0 Active system are highly related for both extension and flexion. PT declined as isokinetic angular velocity increased. Isometric and isokinetic JAPT are significantly related only for extension. 0 1992 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and

Rehabilitation

Strength may be defined as the ability of muscle to exert force and can be measured isometrically, isotonically, or isokinetically. Isometric measurement has been popular because of its technical simplicity and low cost. However, because most functional activities involve movement, values obtained with isometric testing may not reflect the same muscular capability generally used in daily activities or sports.’ Isotonic testing has the advantage of more closely approximating functional movements, but is difficult to standardize because movement velocity, muscle substitutions, and fatigue from multiple trials all confound reliable results. Thus, isokinetic testing, which provides a highly reproducible technique for dynamic strength testingZe4has gained popularity as an approach to the measurement of dynamic human strength.5 Isokinetic devices control angular velocity by providing an accommodating resistance to muscular torque developed during the course of a contraction.’ In an isokinetic system. increasing the force of muscular contraction does From the Department of Physical Medicine and Rehabilitation, University ofCalifornia. Davis (Dr. Lord): Department ofpediatric Rehabilitation. Children’s Hospital Oakland (Dr. Lord): and the Department ofPhysical Education. UniversityofCalifornia, Davis (Ms. Aitkens. Ms. McCrory. and Dr. Bernauer). This work was supported by a grant from the National Institute on Disability Rehabilitation Research ‘X08300078. Submitted for publication February 5, 1990. Accepted in revised form March 7. 1991. A commercial party with a direct or indirect interest m the subject matter of this article has conferred or will confer a financial benefit upon one or more of the authors. Reprint requests to Janet Lord. MD. Department of Pediatric Rehabilitation, Children’s Hospital Oakland. 747 52nd Street. Oakland. CA 94609. 1~ 1992 by the American Congress of Rehabilitation Medicine and the American Academy

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not change the angular velocity of contraction, but is translated into increased torque. Joint angles can also be measured, and other variables, such as work and power, can be calculated. When angular velocity is constrained to O”/sec, the measured torque is the same as isometric torque. The relationship of isokinetically measured strength to isometric and isotonic measured strength has not yet been fully evaluated. Several researchers have examined the relationship between isometric and isokinetic peak torque (PT) and the joint angle at which peak torque is reached (JAPT) using the Cybex” system.6-” Some consistent relationships have emerged. In general, angle-specific torque is consistently higher when measured isometrically than isokinetically, and as angular velocity increases, PT values decrease.6~8-‘2 Technology for the isokinetic measurement of strength has been evolving since Hislop and Perrine’s original article in 1967.13 The first commercially available and therefore most widely described piece of equipment for isokinetic measurement is the Cybex isokinetic dynamometer. Recently, a number of other companies have become involved in the manufacture of isokinetic equipment. Resulting technical changes have attempted to improve measurement precision for both PT and JAPT.14.15 New measurement techniques bring up the question of the relationship between angular velocity and isokinetic performance, this time in the context of whether new technology yields different answers from previous measurement techniques. This study considers the following null hypotheses (1) angular velocity (O”, 60”, 120”. 180”/sec) does not significantly affect the value of PT generated around a joint, (2) angular velocity (0”, 60”, 120”. and 180”/sec) does not sig-

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nificantly affect JAPT. In addition, results from this study are compared with previous studies that used Cybex equipment to evaluate whether a relatively new isokinetic testing device yields new insights to previously accepted relationships between angular velocity and PT or JAPT.

METHODS Subjects Twenty subjects. I 1 men and nine women, aged 20 to 39 years (30.6 i 6.3) volunteered for this study. All subjects were healthy and had no previous musculoskeletal injuries to the lower extremities. Weight ranged from 48.15kg to 1 12.5kg (7 1.95 2 16.1) and height ranged from I52Scm to 183.25cm (169.5 -t 9.0). All subjects gave informed consent.

Instrumentation A LID0 Active Isokinetic Rehabilitation Systemb was used to measure both isokinetic and isometric PT as well as JAPT. Test-retest reliability of isokinetically measured PT using the Cybex 11dynamometer has been evaluated, revealing that PT remains stable both within testing sessions and across a two week interval provided subjects have at least one maximal warm-up trial. I6When similar evaluation was done using LID0 equipment, PT’s measured two days apart showed no significant changes between test days. Intraclass correlation coefficients ranged from .93 to .95.” Since published data regarding test-retest reliability for JAPT was not available, I9 subjects were tested in this laboratory on two separate test dates. Test sessions were scheduled two days apart and measured PT and JAPT for both knee flexion and extension. Reliability of both PT” and JAPT was verified by student f-test.

Test protocol All subjects were tested for both isokinetic and isometric knee extension and flexion strength on the side of their dominant hand. Since the study compared isometric and isokinetically measured PT and JAPT on the same side, it was not believed that specific dominance was of particular importance. Before testing, subjects were familiarized with both the equipment and the test protocol. The protocol was explained, and three to four submaximal repetitions of knee extension/flexion were performed. The protocol was developed through repeated trials with healthy subjects and involved a process of shortening the total testing time until the evaluators believed that fatigue was not a significant factor in any subject’s performance. The entire test protocol required approximately I5 minutes to complete. Subjects were stabilized in the exercise chair at the pelvis and ipsilateral thigh to prevent excessive movement and were instructed to grasp stabilization handles during the test. A technician positioned the extension/flexion axis of knee (identified as a horizontal line through the femoral condylesi8) coincident with the machine axis of rotation.

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Subjects were instructed to work as hard and fast as possible when maximal effort was required, ,411testing was performed by one technician during a single test session for each subject. The isokinetic test protocol included an initial warmup of eight reciprocal extension/flexion repetitions at 60”/sec. with the effort level gradually increasing from approximately 35%’effort to maximal effort. After a IO-second rest, the subjects performed three maximal test repetitions at 60”/sec. A two-minute rest was then given, followed by three warm-up repetitions of increasing intensity at 120”/ sec. a ten-second rest, then three maximal test repetitions at that angular velocity. After another two-minute rest, subjects repeated three warm-up repetitions of increasing intensity, this time at 180”/sec. They then underwent testing at 180”/sec in the same manner as for the previous angular velocities. After isokinetic testing at I80’/sec. another twominute rest was given. Isometric testing immediately followed the isokinetic test. Subjects were tested in 10” increments throughout a 70” range of motion. Testing started at 90” of knee flexion (as measured by a goniometer) and progressed through extension to 20” from straight. At each joint angle, the subjects performed one maximal extension effort followed immediately by a reciprocal maximal flexion effort. Each repetition lasted approximately three seconds. A 20-second rest was given before advancing to the next joint angle. A post-hoc repeatability study was performed to verify that neither warm-up effect nor fatigue affected the measured torques during isometric testing. Using a load cell and digital force indicator, eight healthy subjects were tested for torque generation over nine maximal isometric knee extensor contractions each at 120” of extension with a 20-second rest between each effort. The load cell was linked to a strap that was placed around the subject’s ankle. The strap position was not allowed to vary during testing. Subjects were instructed to give a maximal effort each time. A one-way repeated measures ANOVA was performed on the data. No significant trend was found over the nine contractions (p = .9883), indicating that there was no fatigue or warm-up effect over nine isometric contractions with a 20-second rest between them.

Data Analysis Our goal was to evaluate the relationship between isometric and isokinetic PT and JAPT. which required that we first determine mean PT and mean J.4PT for each angular velocity, including O”/sec (isometric). Relationships among PT and JAPT measured at different angular velocities were then evaluated for statistical significance. The first portion of the analysis included determination of PT and JAPT for each angular velocity. Values for PT and JAPT for each subject at each angular velocity (0”. 60”, 120”. and 180”/sec) were measured, with isometric angular velocity considered to be O”/sec. For isokinetic trials, PT was defined as the highest torque developed for each movement over all trials at a given angular velocity as determined from the computerized torque curve for each subject. Data were collected at a sampling Arch Phys Med Rehabil Vd 73, April 1992

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rate of IOOHz. and computer software accompanying the LID0 system included a facility that isolated PT for any given torque curve. Maximum peak values were typically reached on the second repetition at each angular velocity. Isometric PT was defined as the highest measured torque value at O”/sec. Determination of JAPT during isokinetic testing was calculated based on the point in the range of motion at which PT was achieved. Torque was a continuously measured variable throughout the range of motion. Therefore, JAPT was considered to be the joint angle corresponding to measured PT at each given angular velocity. In contrast, isometric torque was measured discreetly at given joint angles at 10” increments. Torque was then plotted against joint angle, and a curve of torque versus joint angle was developed using a second-order polynomial regression analysis. JAPT was then determined as that value corresponding to PT on the fitted curve of torque versus joint angle. Evaluation of the relationships between measurements done at different angular velocities followed once PT and JAPT values were determined. For this portion of the analysis, the mean PT or JAPT across all subjects was used. Data analysis proceeded in three steps. First, to investigate the relationship between isokinetic PT and isometric PT. linear regression analyses were performed using isometric PT as the independent variable and isokinetic PT at each of the three isokinetic test velocities as dependent variables. To investigate the relationship between isokinetic JAPT and isometric JAPT, linear regression analyses were performed using isometric JAPT as the independent variable and isokinetic JAPT at each of the three isokinetic angular velocities as dependent variables. Finally, to investigate the effect of angular velocity on maximum PT and JAPT, we plotted the mean PT and JAPT for all subjects against the angular velocities at which those peak values were achieved. Statistical verification of differences between PT and JAPT at different angular velocities was done with a two-way ANOVA (using subject as the within-group factor and angular velocity as the between-groups factor) and Tukey’s LSD post-hoc test. All analyses were performed separately for extension and flexion.

A-.-A

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A

163 136 109 81 54 27 0 0

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Fig 2-Linear regression analyses of isometric lT versus isokinetic FT at each isokinetic test velocity (flexion).

RESULTS Angular Velocity and FT The relationship of angular velocity to PT was evaluated in several ways. First, when subjected to linear regression analysis, isokinetic PT demonstrated a significant relationship to isometric PT (R = .88 to R = .93) with higher isometric PTs predicting higher isokinetic PTs in a generally linear fashion (figs 1 and 2) for all three angular velocities tested in this study. In addition. as angular velocity increased, PT decreased, with the decrement more obvious in extension than in flexion (fig 3). The ANOVA (table I) evaluating the effect of increasing angular velocity on PT for both knee flexion and extension were significant (knee extension: F = 83.55, ~~01: knee flexion; F = 39.33, p angular velocity. with PT decreasing as angular \,eloci t! i nc~reases (decreasing slope of the regression lines \vith inc,reasing angular vclocityLh.X-’ ’ Two other studies present conflicting tindings. Osternig’s group, measuring both isometric and isokinetic torques of the triceps brachii. reported low correlations between isometric and isokinetic measurements and concluded that static and d! namic muscle strength Mt’rt’ two distinct phenomena. Details of their instrumcniati~.,n and limb stabili-

Table 1: ANOVA Results: Effect of Increasing Test Velocity on Peak Torque for Knee Extension and Flexion

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zation are not provided. The discrepancy between this study and the Osternig group’s work may be due to the different joints studied (knee extension/flexion versus elbow extension) or to the different isokinetic velocities cvaluated. In addition. depending on the details upper extremity stabilization, subjects could have been using muscle substitutions (eg. shoulder extension) in inconsistent fashion during testing. The other conflicting study. done by used a Cybex II isokinetic loading Perrine and Edgerton.” dynamometer to test knee maximum knee extension torque at velocities ranging from 0” to 288”/sec and found a biphasic force-velocity curve. Low angular velocity isokinetic measurements demonstrated torques comparable to isometric values. As velocities exceeded 144”/sec: however. there was a rapid fall off in isokinetic torque. They poslulated neural mechanisms to explain their findings.

of

Effect of Angular Velocity on JAP’I’ Relationships between isometric and isokinetic JAPT were less clearly defined than relationships involving PT. making it impossible to definitively reject the hypothesis that angular velocity was unrelated to the joint angle at which peak torque was reached. A signilicant relationship

Table 2: ANOVA Results: Effect of Increasing Test Velocity on Joint Angle at Peak Torque for Knee Extension and Flexion

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Fig 6-Mean JAPT at different isokinetic test velocities. bars represent standard deviations from the mean.

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Fig 7-Polynomial regression curves used to ralculate isometric JAPI‘ for extension and flexion. *Extension: 0 deg/sec significantly different (pbc\ isokinetic d~~namometer. The Cybex allows an earl! period of fret accleration before reaching the preset angular Lclocit! at which resistance is oflered. Subjects using the equipment will. therefore. pass through a fraction of the joint range of motion before reaching the preset angular velocit! and beginning to generate measured torque. Rccause subjects generate considerable momentum before reaching the desired angular velocity. they will tend to over\hijot that angular \.elocity before the machine can offer the appropriate amount of resistance to prevent this.“‘-” Con\cqucntl). the rcsistancr offered will reach a high transient le\cl in an attempt to return the angular velocity to the preset \alut*. This will then slow the limb as subjects accommodate to this lncreascd resistance and begin to generate maximum torque at the desired angular velocit). All ofthi goes on while subjects mo\e through the joint range of motion. creating oscillations df overshoot/undershoot of the preset angular \cIocity and generating potentially invalid torque data fi)r (he cart! portion ofjoint range.“-‘3 Thus PT may be recorded as inaccurately high.” while JAPI’. which may have ocsurred before subjects reached the preset angular \,elocity. ma! also be inaccurate11 recorded.” .&cording to the manufacturer’s claims. the LID0 Actibc provides gradual]! incrra