Europ. J. appl. Physiol. 34, 69--79 (1975) 9 by Springer-Verlag t975
Effects of Fatiguing Isometric Exercise upon Achilles Tendon Reflex and Plantar Flexion Reaction Time Components in Man Keith C. Hayes Department of Kinesiology, University of Waterloo, Waterloo, Ontario Received August 26, 1974 Abstract. Effects of three different fatiguing local muscular exercises upon plantar flexion reaction time and achilles tendon reflex time have been studied in 24 normal males. The Exercise Conditions, each involving a series of 30 maximal voluntary isometric contractions (MVC) of the plantar flexors, differed by allowing either 5, l0 or 20 sec rest interval between each MVC. Decrements in strength ranged from :15% to 34 % MVC. Trend analysis of the fatigue patterns revealed that a cubic orthogonal polynomial equation was sufficient to describe the profile of MVC decrement for all conditions (Table 3). Following the fatiguing exercise, simple visual reaction time (plantar flexion), and its two components, premotor and motor time, failed to demonstrate any change from Pre Exercise Conditions. Achilles Tendon Reflex Times, however, demonstrated a marked augmentation, as manifest in reduced total reflex times, contraction times and half relaxation times. These results may suggest the differential fatigue of motor units employed in the three motor tasks, vlz. MVC, voluntary reaction and achilles tendon reflex. A plausible explanation for the augmentation of the reflex contraction resides in the known potentiating effect of elevated intramuscular temperature. Alternatively, one might postulate a neurally mediated increase in gain of the stretch servomechanism. The possibility of both mechanisms being operative is not excluded. Key words: Reaction Time -- Tendon Reflex -- Fatigue.
Introduction The causal mechanisms underlying numerous phenomena associated with muscular fatigue remain enigmatic. As an example, one might note the disparity that appears to exist in the literature as to the influence of muscular work upon voluntary and involuntary motor responses. Schwab (1949) observed that "the tendon reflex is maintained" in an athlete at a time when he is fatigued to the point of being no longer capable of running and that when all voluntary power of the quadrieeps muscle is lost, a vigorous patellar reflex can be elicited (Schwab and Prichard, 1951). Conversely, Kroll (t974) has reported that following locally fatiguing quadriceps exercise, the time components of a voluntary knee extension reaction time task are unaffected while patellar reflex time components are noticeably lengthened. As the functional elements involved in the execution of discrete voluntary and involuntary responses also subserve more general conditions of motor control (via alpha, gamma, or alpha-gamma coaetivation pathways, see Stein, 1974), it appeared worthwhile to simultaneously investigate the influence of fatigue upon these two different types of response. The majority of previous investigations have considered either volitional (reaction time) or involuntary (reflex time) responses independently and the possibility exists, of
70
K.C. Hayes
course, that the seeming disparate influences of fatigue are an artifact produced by different types of exercise, differences in subjects, different reflexes being measured, or various other aspects of experimental protocol. In addition to the apparent incongruence between voluntary and involuntary response fatigue characteristics there exists some controversy as to the influence of fatigue upon tendon reflexes per se. Historically, investigators have reported either reflex augmentation (Cheah and Tan, t970; Petajan and Eagan, i968; Schwab, 1949; Schwab and Prichard, 195i; Westerman and Gerbrandy, 1969) or reflex depression (Henane, 1968; Henane and Macarez, ~972; Henane and Flandrois, t968a, b; Tipton and Karpovich, 1966; Turtle, 1930) associated with strenuous exercise and although there have been attempts to isolate the independent variables influencing these results, not all of the factors have yet been satisfactorily identified. One possible influence that has not been fully controlled in many investigations is the extent of local muscular impairment. The focus of some studies (on a total body type of fatigue) obviated the need for assessment of this seemingly important parameter. The same comment has also been made of investigations of fatigue effects upon reaction time (Kroll, 1973). The purpose of the present investigation was to determine the effects of a locally fatiguing series of isometric maximal voluntary contractions (M-VC) of triceps surae upon achilles tendon reflex and plantar flexion reaction time components. Three types of fatiguing exercise were investigated. All three involved serial isometric MVC's of ~0 see duration (30 trials) but each Exercise Condition varied as to the inter-trial rest interval. Serial isometric MVC's with varied recuperation periods are well known to induce different fatigue profiles as well as different amounts of local muscular fatigue (Clarke, 197i ; Kroll, t966). Measurements secured from the reflex and reaction times included the Iateneies to onset of the electromyogram (emg), as well as the time lag to the beginning of the overt movement. This fractionation of reponse times was coupled with the normal clinical reflex measurements of contraction and half relaxation characteristics, thus providing more discriminative information and permitting more meaningful comparison of the involuntary and voluntary muscular contractions than might otherwise have been obtained.
Methods Twenty-four healthy volunteer male students served as subjects (Ss). The physical characteristics of the group are shown in Table I. Each subject attended for testing on 7 days, of which the first 2 days served as stabilization and familiarization sessions. These were followed by four main experimental sessions in which three Exercise Conditions and a control (No Exercise Condition) were administered. The 7 and final day constituted
Table 1. Physical characteristics of the subjects S.D. Age (years) Height (inches) Weight (pounds)
22.25 70.75 ~72.87
2.21 2.26
16.59
S.E.
Range
N
0.45 0.46 3.38
20-- 27 67-- 75 142--205
24 24 24
Reflex and Reaction Times Following Exercise
71
a partial replication of Exercise Conditions permitting an estimate of the reliability of the experimental results. Two week intervals separated the 2 stabilization days and a period of not less than 4 days was allowed between each Exercise Condition. The order of administration of Exercise Conditions was balanced in such a way as to minimize any test sequence effects. The Exercise Conditions involved Ss performing a series of 30 isometric maximal voluntary contractions (MVC) of the triceps surae group (plantar flexion). Each MVC was maintained for 10 sec duration and was followed by a recuperation interval of 5 sec (Ex. Condition A), :I0 see (Ex. Condition B) or 20 sec (Ex. Condition C). These plantar flexions (and all subsequent testing) were all performed with S lying prone and firmly secured on a padded wooden bench. Prior to exercise, baseline measures of voluntary plantar flexion reaction time (20 trials), achilles tendon reflex time (10 trials), and isometric plantar flexion MVC (3 trials) were taken. Throughout each Exercise Condition, records of the decrement in MVC were made and then, on completion of the fatiguing series of contractions, reaction, reflex and MVC's were again measured. Recovery was followed through six periods of 5 rain during which 10 reactions, 3 reflexes and I MVC per period were recorded. The measurements of plantar flexion reaction time included Premotor Time (PMT), which is the latency from stimulus to the onset of muscle action potentials (MAPs), and Motor Time (MT), the time lag between MAPs and overt movement, as well as the conventional Total Reaction Time (TRT). The simple reaction to presentation of a visual stimulus (a light flash of 40 msec duration, NE-51 Neon Lamp) consisted of a rapid plantar flexion, virtually isometric, but sufficient to close a microswitch (travel distance 1 ram) located beneath the foot. Muscle action potentials were led off from the motor point of m. soleus through bipolar surface electrodes and were displayed on a Teca Model B2 oscilloscope. PMT was obtained by means of a digital readout device (Teen Model B/CT) attached to the oscilloscope and was recorded in i msec units. Each TRT trial was preceded by a variable fore period (I to 4 see). The probability of occurrence of each fore period was balanced to minimize expectancy and the duration was controlled by a Hunter model l i l - C decade interval timer. Achilles Tendon Reflexes were elicited by allowing a percussion hammer to fall through a constant arc to impact S's achilles tendon at the level of the lateral mall.eolus. The emg of m. solens and the latency of onset of MAPs was recorded in the same manner as for reaction time trials. The force of the reflexly elicited contraction was recorded by means of a strain gauge assembly (Statham Universal Cell Model UC2 with Model UL4-100 necessary head) embedded in the foot plate and recorded on a Beckman Dynograph with paper speed set at 250 mm]sec. Measurement of the contraction characteristics of the reflex response combined the technique of fractionation described by Hayes (t972) with the conventional clinical indices (Lambert et al., ~195:1). They included Total Reflex Time (TRfT), measured from hammer impact to commencement of overt force change, together with its fractionated components; the latency (LAT) of the emg response and the reflex motor time (RfMT). The RfMT measure represents the lag between arrival of the efferent impulse and the overt response, and is a measure of the electromechanical coupling process together with the time in which the muscle generates sufficient tension to accelerate the foot. Additional characteristics of the muscular response that were measured included Contraction Time (CT) and Half Relaxation Time (HRT).
Results Pre Exercise Conditions Analysis of the baseline m e a s u r e m e n t s , i.e., those secured prior to exercise, d e m o n s t r a t e d means, for all measures, t h a t were in a g r e e m e n t with previously r e p o r t e d values. The P r e Exercise Condition m e a s u r e m e n t s are s u m m a r i z e d i n T a b l e 2. This table also includes estimates of the reliability which were based on intraclass correlation coefficients. I n all cases, the intraclass correlation coefficients d e m o n s t r a t e d acceptable reliability for t h e baseline m e a s u r e m e n t s , viz. R = 0.78 to 0.95. The d a t a o b t a i n e d from baseline c o n d i t i o n o n the first 2 stabilization
6.4
Half l~elaxation Time (HRT)
Force (lbs.)
109.4
231.4
Contraction Time (CT)
2.6 3.6 0.5
t2.5 18.0 2.5
6.9
Reflex Motor Time (RfMT)
0.5
33.0
23.5
Reflex Latency (LAT) 1.0
t.f
5.5
56.6 2.3
2.3
6.6
59.1
Motor Time (MT)
Total Reflex Time (TRfT)
4.9
9.7
3.3
16.2
38.5
7.7
1:1,4
20.8
11.2
9.5
7.9
3.7
18.1
228.5
t69.8
Premotor Time (PMT)
17.5
Total Reaction Time (T]~T)
t 3.3
65.0
CV %
369.7
SE
SD
Maximal Voluntary Contraction (lbs.)
2
0.95
0.92
0.78
0.83
0.90
0.90
0.95
0.94
0.93
0.94
R
16.7
188,4
42.2
t0.9
3,1
19.t
29.0
150.2
t94.0
3898.5
g True
5.5
93.7
70.4
24.2
2.2
13.7
4.8
'14.3
40.9
t472.8
a Days error
5.9
203.2
t15.4
15.3
0.5
t4.0
11.7
89.7
$08.2
747.9
a Trials error
Table 2. Pro exercise condition criterion scores. All mean values are in mscc except where indicated. The Intraclass Correlation Coefficient (R) was calculated from data obtained on 7 days. I n the case of TI~T, t)MT, and MT the trials represent the means of a block of 20 trials, n = 24
c~
b.0
Reflex
and Reaction
Times Following Exercise
400/ 575 b
A EXERCISE A o EXERCISE B
10:5 10:10
3501_~_:,,. r " ~ " + ~ + "+-+'N.~
+ EXERCISE C
,0:20
.......
....
73
+_+
225[2oo L:~'
I = I t ! '~ 2 4 6 8
I ' I 10 12
irl~ [ r l [ i t [ l [ i J j I4 16 18 213 22 24 2"5 28 30 TRIALS
Fig. t. Fatigue profiles for isometric maximal voluntary contractions (MVC) of the Triceps surae under three different Exercise Conditions. Each Exercise Condition consisted of 30 trials, each of l0 see duration and the conditions varied as to the intertrial rest interval e. g., Exercise Condition A (10: 5) consisted of I0 see contractions with 5 see rest interval
days (not shown) revealed adequate stabilization of all measures prior to evaluation of the main experimental treatments, viz. Exercise Conditions on Days 3 to 6. Failure to provide an adequate stabilization period prior to treatments has been a recognized limitation of earlier work in this area (Kroll, t965, t973).
Exercise Conditions Exercise Condition A (10:5), involving a series of thirty i0 sec contractions with 5 sec intervals induced a very rapid decrement in plantar flexion MVC values (see Fig. 1). The average NVC decrement, from a first trial mean of 355.8 lbs. to a final trial mean of 234.4 lbs. was approximately 34% MVC. Less rapid declines in MVC were observed for Exercise Conditions B (i0:J0) and C (10:20) although the overall percentage decrements were of the order of 24 % MVC and 15 % MVC, respectively. The overriding feature of the Exercise Conditions analysis was the very evident decrement in isometric MVC of the plantar flexors suggesting" the presence of a substantial degree of local muscular impairment or fatigue. The fatigue profiles of the MVC data, for each Exercise Condition, were analyzed using analyses of variance of trends. These analyses involved the use of orthogonM polynomials and allowed statistical tests of the best fitting curve for each Exercise Condition. Exercise Conditions A and C were found to be best described b y cubic orthogonal polynomial equations although in both cases the statistically significant cubic component represented only a small percentage (0.9% for Ex. Condition A and 2.5% for Ex. Condition ]3) of the overall trend variance. For Exercise Condition ]3 (t0:10) only the linear and quadratic components were significant ( P < 0.05). To provide additional information on the nature of the decrement in MVC, the subjects were classified, afortiori, into High, Medium and Low strength groups, based on their }Pre Exercise MVC means. The analysis of fatigue profiles, with subjects classified b y strength level, is presented in Table 3. Generally the trend analysis supports the contention t h a t the pattern of decrement of high strength subjects is dissimilar to t h a t of low strength subjects and no single equation can
9 P
Effects of Fatiguing Isometric Exercise upon Achilles Tendon Reflex and Plantar Flexion Reaction Time Components in Man Keith C. Hayes Department of Kinesiology, University of Waterloo, Waterloo, Ontario Received August 26, 1974 Abstract. Effects of three different fatiguing local muscular exercises upon plantar flexion reaction time and achilles tendon reflex time have been studied in 24 normal males. The Exercise Conditions, each involving a series of 30 maximal voluntary isometric contractions (MVC) of the plantar flexors, differed by allowing either 5, l0 or 20 sec rest interval between each MVC. Decrements in strength ranged from :15% to 34 % MVC. Trend analysis of the fatigue patterns revealed that a cubic orthogonal polynomial equation was sufficient to describe the profile of MVC decrement for all conditions (Table 3). Following the fatiguing exercise, simple visual reaction time (plantar flexion), and its two components, premotor and motor time, failed to demonstrate any change from Pre Exercise Conditions. Achilles Tendon Reflex Times, however, demonstrated a marked augmentation, as manifest in reduced total reflex times, contraction times and half relaxation times. These results may suggest the differential fatigue of motor units employed in the three motor tasks, vlz. MVC, voluntary reaction and achilles tendon reflex. A plausible explanation for the augmentation of the reflex contraction resides in the known potentiating effect of elevated intramuscular temperature. Alternatively, one might postulate a neurally mediated increase in gain of the stretch servomechanism. The possibility of both mechanisms being operative is not excluded. Key words: Reaction Time -- Tendon Reflex -- Fatigue.
Introduction The causal mechanisms underlying numerous phenomena associated with muscular fatigue remain enigmatic. As an example, one might note the disparity that appears to exist in the literature as to the influence of muscular work upon voluntary and involuntary motor responses. Schwab (1949) observed that "the tendon reflex is maintained" in an athlete at a time when he is fatigued to the point of being no longer capable of running and that when all voluntary power of the quadrieeps muscle is lost, a vigorous patellar reflex can be elicited (Schwab and Prichard, 1951). Conversely, Kroll (t974) has reported that following locally fatiguing quadriceps exercise, the time components of a voluntary knee extension reaction time task are unaffected while patellar reflex time components are noticeably lengthened. As the functional elements involved in the execution of discrete voluntary and involuntary responses also subserve more general conditions of motor control (via alpha, gamma, or alpha-gamma coaetivation pathways, see Stein, 1974), it appeared worthwhile to simultaneously investigate the influence of fatigue upon these two different types of response. The majority of previous investigations have considered either volitional (reaction time) or involuntary (reflex time) responses independently and the possibility exists, of
70
K.C. Hayes
course, that the seeming disparate influences of fatigue are an artifact produced by different types of exercise, differences in subjects, different reflexes being measured, or various other aspects of experimental protocol. In addition to the apparent incongruence between voluntary and involuntary response fatigue characteristics there exists some controversy as to the influence of fatigue upon tendon reflexes per se. Historically, investigators have reported either reflex augmentation (Cheah and Tan, t970; Petajan and Eagan, i968; Schwab, 1949; Schwab and Prichard, 195i; Westerman and Gerbrandy, 1969) or reflex depression (Henane, 1968; Henane and Macarez, ~972; Henane and Flandrois, t968a, b; Tipton and Karpovich, 1966; Turtle, 1930) associated with strenuous exercise and although there have been attempts to isolate the independent variables influencing these results, not all of the factors have yet been satisfactorily identified. One possible influence that has not been fully controlled in many investigations is the extent of local muscular impairment. The focus of some studies (on a total body type of fatigue) obviated the need for assessment of this seemingly important parameter. The same comment has also been made of investigations of fatigue effects upon reaction time (Kroll, 1973). The purpose of the present investigation was to determine the effects of a locally fatiguing series of isometric maximal voluntary contractions (M-VC) of triceps surae upon achilles tendon reflex and plantar flexion reaction time components. Three types of fatiguing exercise were investigated. All three involved serial isometric MVC's of ~0 see duration (30 trials) but each Exercise Condition varied as to the inter-trial rest interval. Serial isometric MVC's with varied recuperation periods are well known to induce different fatigue profiles as well as different amounts of local muscular fatigue (Clarke, 197i ; Kroll, t966). Measurements secured from the reflex and reaction times included the Iateneies to onset of the electromyogram (emg), as well as the time lag to the beginning of the overt movement. This fractionation of reponse times was coupled with the normal clinical reflex measurements of contraction and half relaxation characteristics, thus providing more discriminative information and permitting more meaningful comparison of the involuntary and voluntary muscular contractions than might otherwise have been obtained.
Methods Twenty-four healthy volunteer male students served as subjects (Ss). The physical characteristics of the group are shown in Table I. Each subject attended for testing on 7 days, of which the first 2 days served as stabilization and familiarization sessions. These were followed by four main experimental sessions in which three Exercise Conditions and a control (No Exercise Condition) were administered. The 7 and final day constituted
Table 1. Physical characteristics of the subjects S.D. Age (years) Height (inches) Weight (pounds)
22.25 70.75 ~72.87
2.21 2.26
16.59
S.E.
Range
N
0.45 0.46 3.38
20-- 27 67-- 75 142--205
24 24 24
Reflex and Reaction Times Following Exercise
71
a partial replication of Exercise Conditions permitting an estimate of the reliability of the experimental results. Two week intervals separated the 2 stabilization days and a period of not less than 4 days was allowed between each Exercise Condition. The order of administration of Exercise Conditions was balanced in such a way as to minimize any test sequence effects. The Exercise Conditions involved Ss performing a series of 30 isometric maximal voluntary contractions (MVC) of the triceps surae group (plantar flexion). Each MVC was maintained for 10 sec duration and was followed by a recuperation interval of 5 sec (Ex. Condition A), :I0 see (Ex. Condition B) or 20 sec (Ex. Condition C). These plantar flexions (and all subsequent testing) were all performed with S lying prone and firmly secured on a padded wooden bench. Prior to exercise, baseline measures of voluntary plantar flexion reaction time (20 trials), achilles tendon reflex time (10 trials), and isometric plantar flexion MVC (3 trials) were taken. Throughout each Exercise Condition, records of the decrement in MVC were made and then, on completion of the fatiguing series of contractions, reaction, reflex and MVC's were again measured. Recovery was followed through six periods of 5 rain during which 10 reactions, 3 reflexes and I MVC per period were recorded. The measurements of plantar flexion reaction time included Premotor Time (PMT), which is the latency from stimulus to the onset of muscle action potentials (MAPs), and Motor Time (MT), the time lag between MAPs and overt movement, as well as the conventional Total Reaction Time (TRT). The simple reaction to presentation of a visual stimulus (a light flash of 40 msec duration, NE-51 Neon Lamp) consisted of a rapid plantar flexion, virtually isometric, but sufficient to close a microswitch (travel distance 1 ram) located beneath the foot. Muscle action potentials were led off from the motor point of m. soleus through bipolar surface electrodes and were displayed on a Teca Model B2 oscilloscope. PMT was obtained by means of a digital readout device (Teen Model B/CT) attached to the oscilloscope and was recorded in i msec units. Each TRT trial was preceded by a variable fore period (I to 4 see). The probability of occurrence of each fore period was balanced to minimize expectancy and the duration was controlled by a Hunter model l i l - C decade interval timer. Achilles Tendon Reflexes were elicited by allowing a percussion hammer to fall through a constant arc to impact S's achilles tendon at the level of the lateral mall.eolus. The emg of m. solens and the latency of onset of MAPs was recorded in the same manner as for reaction time trials. The force of the reflexly elicited contraction was recorded by means of a strain gauge assembly (Statham Universal Cell Model UC2 with Model UL4-100 necessary head) embedded in the foot plate and recorded on a Beckman Dynograph with paper speed set at 250 mm]sec. Measurement of the contraction characteristics of the reflex response combined the technique of fractionation described by Hayes (t972) with the conventional clinical indices (Lambert et al., ~195:1). They included Total Reflex Time (TRfT), measured from hammer impact to commencement of overt force change, together with its fractionated components; the latency (LAT) of the emg response and the reflex motor time (RfMT). The RfMT measure represents the lag between arrival of the efferent impulse and the overt response, and is a measure of the electromechanical coupling process together with the time in which the muscle generates sufficient tension to accelerate the foot. Additional characteristics of the muscular response that were measured included Contraction Time (CT) and Half Relaxation Time (HRT).
Results Pre Exercise Conditions Analysis of the baseline m e a s u r e m e n t s , i.e., those secured prior to exercise, d e m o n s t r a t e d means, for all measures, t h a t were in a g r e e m e n t with previously r e p o r t e d values. The P r e Exercise Condition m e a s u r e m e n t s are s u m m a r i z e d i n T a b l e 2. This table also includes estimates of the reliability which were based on intraclass correlation coefficients. I n all cases, the intraclass correlation coefficients d e m o n s t r a t e d acceptable reliability for t h e baseline m e a s u r e m e n t s , viz. R = 0.78 to 0.95. The d a t a o b t a i n e d from baseline c o n d i t i o n o n the first 2 stabilization
6.4
Half l~elaxation Time (HRT)
Force (lbs.)
109.4
231.4
Contraction Time (CT)
2.6 3.6 0.5
t2.5 18.0 2.5
6.9
Reflex Motor Time (RfMT)
0.5
33.0
23.5
Reflex Latency (LAT) 1.0
t.f
5.5
56.6 2.3
2.3
6.6
59.1
Motor Time (MT)
Total Reflex Time (TRfT)
4.9
9.7
3.3
16.2
38.5
7.7
1:1,4
20.8
11.2
9.5
7.9
3.7
18.1
228.5
t69.8
Premotor Time (PMT)
17.5
Total Reaction Time (T]~T)
t 3.3
65.0
CV %
369.7
SE
SD
Maximal Voluntary Contraction (lbs.)
2
0.95
0.92
0.78
0.83
0.90
0.90
0.95
0.94
0.93
0.94
R
16.7
188,4
42.2
t0.9
3,1
19.t
29.0
150.2
t94.0
3898.5
g True
5.5
93.7
70.4
24.2
2.2
13.7
4.8
'14.3
40.9
t472.8
a Days error
5.9
203.2
t15.4
15.3
0.5
t4.0
11.7
89.7
$08.2
747.9
a Trials error
Table 2. Pro exercise condition criterion scores. All mean values are in mscc except where indicated. The Intraclass Correlation Coefficient (R) was calculated from data obtained on 7 days. I n the case of TI~T, t)MT, and MT the trials represent the means of a block of 20 trials, n = 24
c~
b.0
Reflex
and Reaction
Times Following Exercise
400/ 575 b
A EXERCISE A o EXERCISE B
10:5 10:10
3501_~_:,,. r " ~ " + ~ + "+-+'N.~
+ EXERCISE C
,0:20
.......
....
73
+_+
225[2oo L:~'
I = I t ! '~ 2 4 6 8
I ' I 10 12
irl~ [ r l [ i t [ l [ i J j I4 16 18 213 22 24 2"5 28 30 TRIALS
Fig. t. Fatigue profiles for isometric maximal voluntary contractions (MVC) of the Triceps surae under three different Exercise Conditions. Each Exercise Condition consisted of 30 trials, each of l0 see duration and the conditions varied as to the intertrial rest interval e. g., Exercise Condition A (10: 5) consisted of I0 see contractions with 5 see rest interval
days (not shown) revealed adequate stabilization of all measures prior to evaluation of the main experimental treatments, viz. Exercise Conditions on Days 3 to 6. Failure to provide an adequate stabilization period prior to treatments has been a recognized limitation of earlier work in this area (Kroll, t965, t973).
Exercise Conditions Exercise Condition A (10:5), involving a series of thirty i0 sec contractions with 5 sec intervals induced a very rapid decrement in plantar flexion MVC values (see Fig. 1). The average NVC decrement, from a first trial mean of 355.8 lbs. to a final trial mean of 234.4 lbs. was approximately 34% MVC. Less rapid declines in MVC were observed for Exercise Conditions B (i0:J0) and C (10:20) although the overall percentage decrements were of the order of 24 % MVC and 15 % MVC, respectively. The overriding feature of the Exercise Conditions analysis was the very evident decrement in isometric MVC of the plantar flexors suggesting" the presence of a substantial degree of local muscular impairment or fatigue. The fatigue profiles of the MVC data, for each Exercise Condition, were analyzed using analyses of variance of trends. These analyses involved the use of orthogonM polynomials and allowed statistical tests of the best fitting curve for each Exercise Condition. Exercise Conditions A and C were found to be best described b y cubic orthogonal polynomial equations although in both cases the statistically significant cubic component represented only a small percentage (0.9% for Ex. Condition A and 2.5% for Ex. Condition ]3) of the overall trend variance. For Exercise Condition ]3 (t0:10) only the linear and quadratic components were significant ( P < 0.05). To provide additional information on the nature of the decrement in MVC, the subjects were classified, afortiori, into High, Medium and Low strength groups, based on their }Pre Exercise MVC means. The analysis of fatigue profiles, with subjects classified b y strength level, is presented in Table 3. Generally the trend analysis supports the contention t h a t the pattern of decrement of high strength subjects is dissimilar to t h a t of low strength subjects and no single equation can
9 P
