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The effect of muscle fatigue on the isometric contractile characteristics of skeletal muscle in the cat
Life Sciences, Vol . 24, pp . 2285-2292 Printed in the U .S .A .
Pergamon Press
THE EFFECT OF MUSCLE FATIGUE ON THE ISOMETRIC CONTRACTILE CHARACTERISTICS OF SKELETAL MUSCLE IN THE CAT Jerrold S . Petrofsky, Adrienne Guard, and Chandler A . Phillips* Department of Physiology, St . Louis University School of Medicine, St . Louis, MO and *Department of Engineering, Wright State University, Dayton, Ohio (Received in final form April 20, 1979) Summary The rise time of an isometric twitch, the tetanic tension, the twitch tetanus ratio, the frequency-tension relationship, and the height of the MUAP (motor unit action potential) were measured in fast twitch (medial gastrocnemius) and slow twitch (coleus) muscles of the cat immediately before, in the middle, and immediately after fatiguing isometric cnntractions at tensions of 30, 50 and 80915 of each muscled initial strength (tetanic tension recorded from the unfatigued muscle) . Although the twitch-tetanus ratio was always less for the soleus than for the medial gastrocnemius muscles, the twitch-tetanus ratio for any one muscle was constant throughout the duration of fatiguing isometric contractions at any of the tensions examined. In contrast, the twitch tension and tetanic tension of the muscles were both less after the contractions, the largest reduction occurring for both muscles during contractions sustained at the lowest isometric tensions . The time to peak tension of an isometric twitch was prolonged for both muscles following the contractions. This was associated with a corresponding shift in the frequency tension relationship such that at the point of muscular fatigue, the muscles tetanized at lower frequencies of stimulation than did the unfatigued muscle . In contrast, the amplitude of the MUAP showed only a modest reduction throughout the duration of the fatiguing contractions . The contractile characteristics and the frequency tension relationship of fast and slow twitch skeletal muscle has been well established in the literature . While slow twitch motor units have a lower actomyosin ATPase activity, lower tetanic strength, and a longer twitch duration (both contraction time and half relaxation time) than fast twitch muscle, these motor units tetanize at lower motor unit firing frequencies than their fast twitch counterparts (1) . Further, because of their high aerobic capacity, slow twitch motor units have substantially longer endurance for both isometric and isotonic contractions (1,2,3,4) . However, although the contractile characteristics of these two types of muscle have been thoroughly examined in unfatigued muscle, little has been done to determine the effect of muscle fatigue on these same characteristics . Brown and Burns found that rabbit muscle tetanized at lower stimulation frequencies after the muscle was fatigued isometrically : here however, only fast twitch muscle was examined with high X100 Hz.) frequencies of stimulation . Fitts and Holloszy (6), found that the V of the coleus (a predominantly slow twitcfi muscle in the rat) was reduced followiri~a ~atiguing isotonic exercise. Finally, we have reported a general shift toward tetanization at lower frequencies of stimulation in isometrically fatigued slow and fast twitch skeletal muscle in the cat (2) . It was the purpose of the present investigation to expand upon our own and the findings of others by examining the properties of the isometric twitch and the frequency 0024-3205/79/242285-07$02 .00/0 Copyright (c) 1979 Pergamon Press Ltd
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tension relationship throughout the duration of a fatiguing isometric contraction in a mixed but largely fast twitch muscle (the medial gastrocnemius) and a slow twitch muscle (the coleus) of the cat (7). Since transmission fatigue has also been implicated in the fatigue process (8), the electrical activity above these two muscles was also assessed throughout the duration of the contractions by recording the MUAP (motor unit action potential) . Methods Female cats weighing an average of 2.8 t 0 .5 kg were used in these experiments . The animals were anesthetized with a-chloralose (75 mg/kg body weight) injected intraperitoneally, and maintained with intravenous booster doses as needed. A heating pad, placed under the cat, maintained the rectal temperature within the range of 37 to 39° C. Blood pressure and heart rate were monitored through an arterial cannula inserted into the left carotid artery . Surgical Preparation Each cat was placed in the prone position with one back leg fixed rigidly to the table by 2 sets of stainless steel pins driven through the knee and lower leg bones. The muscle which was to be tested was exposed and freed from the surrounding muscles. A section of calcareous with its one remaining tendon attached to the test muscle was tied to a low displacement isometric strain gauge dynamometer. Heated liquid paraffin (3839° C), equilibrated initially with a gas mixture of 95% oxygen and 5% CO was circulated over the muscles through a boat made from the overlying skin to m~lntain muscle temperature at 38 t 1 .0°C . A dorsal laminectomy exposed the L6,. L , and S 1 ventral roots which were cut proximally to their emergence from the spina~ cord. The isolated roots were then divided surgically and pooled into 3 groups, each capable of causing the muscle to develop a similar tension following electrical stimulation. Liquid paraffin, maintained at 38°C and oxygenated with a gas mixture consisting of 95% 0 and 5% C0 2 superfused the other. Every 90 minutes, nerve bundles to keep them electrically isolated from one the superfusion of paraffin was changed to Ringer's solution for 5 minutes to ensure that the ventral roots remained viaible. Stimulation Two types of stimulation were used in these experiments. First, to measure the isometric twitch characteristics of the muscle, an indirect (through the motor nerve) or direct (to the muscle) synchronous square wave pulse (pulse width 0.1 ms) was delivered through 2 needle electrodes with the pulse amplitude made sufficiently large to stimulate all motor units in the muscle . Second, to mimic the normal asynchronous pattern of recruitment found during sustained voluntary contractions, (12) indirect stimulation was also delivered sequentially through the 3 bundles of ventral roots dissected apart as described above. One pair of RF isolated stimulating electrodes was placed on each ventral root bundle . The stimulation pulse width was 0.1 ms with a sufficient amplitude to stimulate all the axons in each bundle . The three bundles of axons were stimulated in a rotary sequence at a frequency for each bundle which was varied during the experiments as described below. Distal to the stimulating electrodes, the ventral roots were spread thinly over a second set of electrodes whose purpose was to create an anodal block. The number of active motor units was varied by altering the anodal block voltage (recruitment). The order by which they were recruited, therefore, depended on the resistance of their motor nerves to anodal blockade. In a previous study, it has been demonstrated that a stepwise reduction in anodal block voltage resulted in a continuous reduction in the time to peak
Vol . 24, No . 24, 1979
Isometric Fatigue
228 7
tension of muscle twitdies recorded from the whole medial gastrocnemius muscle, indicating that the recTUitment order proceeded from the slowest to the fastest motor units respectively (9). This pattern of recruitment was used in these experiments since it resembles that which is believed to occur during voluntary isometric contractions (10, 11, 12). The anodal block voltage and stimulation frequency were set and maintained by a digital computer (Intel 8080A microprocessor). During fatiguing isometric contractions, the motor unit stimulation frequency was set initially at 10 Hz while the anodal block voltage was reduced until the target tension was achieved . As the muscle began to fatigue, more motor units were recruited to maintain the target tension. Once all the motor units were recruited, the computer increased the stimulation frequency to maintain the target . A complete description of the computer program is given elsewhere (9). Once an increase in stimulation frequency was ineffective in maintaining the target, the contraction was terminated ; the total length of time the contraction was maintained was called the endurance time . Measurement of the Electric al Activity Above the Muscles The electrical activity above the muscles was measured by a pair of bipolar platinum electrodes . The resultant signal was amplified and recorded on an FM recorder for later analysis . Procedures A series of test contractions was done initially on each muscle to establish the length of the muscle at which it developed its greatest active tension; the muscle was then set at this length throughout the remainder of the experiments . Twenty minutes later, a single synchronous stimulus was applied to both the motor nerve and then to the muscle with sufficient voltage to recruit all of the motor units; 3 min were allowed between these stimuli and the resultant twitches were recorded and analyzed later for the twitch amplitude and the contraction time (time required for the tension during the twitch to rise from S% to 100% of the maximum twitch height). Five minutes later, the muscles were stimulated sequentially as described above at a sufficient voltage to recruit all of the motor units and at a sufficient frequency to tetanus the muscle fully (100 Hz); the stimulation was maintained for a period of 3 sec and the maximum tension recorded in this manner was assumed to be the initial strength of the muscle (S ). The maximum tetanic tension recorded when a similar procedure was used on ~atigued muscles was termed simply the strength of the muscle . After another S min rest period, the frequency tension relationship was recorded by stimulating the muscles sequentially with supramaximal stimulation for 3 sec periods at frequencies of stimulation between 10 and 60 Hz ; 5 min were allowed between each of these contractions . After another' 1S min rest period, a fatiguing isometric contraction was elicited as described under methods at a tension of either 30, SO or 80% of the initial strength . These 3 tensions were chosen since they represent contractions where the tension was maintained initially and throughout most of the contraction by recruitment (3095) and on the other extreme by initially recruiting all of the motor wits and maintaining tension by varying the frequency of stimulation (80%). Thirty minutes were allowed between these contractions and the presentation order of the tensions on any one muscle was selected at random . Each tension was repeated a total of 8 times. Immediately following 2 of these contractions, a single synchronous stimulus was applied to either the sciatic nerve or to the muscle and the resultant twitch was recorded . In the remaining 6 contractions, the muscle was stimulated sequentially in the middle (as assessed from the endurance time of the previous contractions) and immediately after each fatiguing contraction with a sufficient voltage to recruit all of the motor units and at a frequency of either 10, 20, 30, 40, SO or 60 Hz to established the frequency-tension relationship for the isometrically fatigued muscle. These experiments were repeated on each of 8 cats for each type of
2288
Isometric Fatigue
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muscle . Because of the length of the experiments, only muscle In one leg was used on each cat. Results As reported previously, the endurance fa fatlguing isometric contractions was substantially longer for the coleus (the slow twitch muscle) than for the medial gastrocnemius muscle. Further, the endurance far any one muscle was longest when the tension exerted by the muscle was 30% of the initial strength ; the average endurance for all muscles examined here was 1400 175 (S .E.), 210 + 5, and 16 t 1 sec for the coleus and 100 2, 31 3 1, and 6 t 1 sec for the medial gastrocnemius muscles for tensions of 30, 50, and 80% S respectively . The twitch amplitudes and the time to peak tension before and after thes~ fatlguing contractions are shown in Tables 1, 2, and 3 for contractions exerted at 30, 50 and 80% of the initial strength (S ) of the muscles. As expected from the fiber composition of the muscles, the time to p~ak tension of the coleus-muscle was greater and the twitch amplitude was less than in the medial gastrocnemius muscle . TABLE I Contractile Characteristics During A Contraction At 30% S1 Twitch Tension Twitch Tetanus Ratio Time to Peak Tension g/g Soleus
g/g
MUAP
msec
% onset
Onset
785119
0.1710.0
6312 .3
100
End
235111
0.1710.0
8112 .3
9530 .7
Gastrocnemius Onset 1101136
0 .2210.02
3611 .1
100
End
0.2210.02
4511 .9
8730.3
339325
TABLE II Contractile Characteristics During A Contraction At 50% S1 Twits Tension
Twitch Tetanus Ratio
g/g
g/g
msec
% onset
Soleus
Time to Peak Tension
MUAP
Onset
776324
0.1830.05
6232.5
100
End
376318
0.1630.02
84ä.O
9710 .6
Onset
1280129
0.2330.003
3311 .5
100
End
61938
0.25df1 .01
4711 .8
9110 .6
Gastrocnemius
zzss
Isometric Fatigue
Vol . 24, No . 24, 1979
TABLE III Contractile Characteristics During A Contraction At 80% S 1 Twitch Tension g/g
Twitch Tetanus Ratio Time to Peak Tension MUAP msec % onset g/g
Soleus Onset
794f26
0.1510 .01
63 .3
100
End
623118
0 .1610 .01
8812.1
98f0.4
Gastrocnemius Onset 1150131
0.2210.01
3410.49
100
End
0.22~.Oi.007
4611 .0
95fp .5
913113
At the end of the fatiguing contractions, the twitch amplitude had been reduced and the twitch duration had increased for both muscles, the largest changes in both amplitude and duration occurring for contractions at the lowest isometric tensions . However, the twitch amplitude decreased in direct proportion to tetanic tension so twitch tension ratios of a given muscle did not change from the beginning to the end of the fatiguing contractions (P~0.05). In contrast, the MUAP amplitude recorded darin$ the contraction showed a modest but significant reduction for all contractions (P>0.05) for both types of muscle .
FIG . 1 This figure shows the normalized peak tension developed by the coleus muscles of 8 cats 1 the S.E. for brief contractions exerted prior to (o), in the middle (A) and immediately after (p) a fatiguing isometric contraction .
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1979
Along with the increase in the time to peak tension, there was a leftward shift in the frequency-tension diagram . Figures 1 and 2 show the frequency tension relationship recorded as described under Methods, before, in the middle, and at the end of the fatiguing isometric contractions . There was no significant difference in the results recorded at the 3 different tensions for any given frequency of stimulation . Therefore, the data from the 3 different tensions have been averaged together such that each point in Figures 1 (soleus) and 2 (medial gastrocnemius muscles) represents the mean response for all 3 tensions tthe standard error for the unfatigued (o), half fatigued (~) and fatigued muscles (q) . Since the tetanic tension of the muscles was reduced to that of the target tension at the end of the isometric contractions, for simplicity of presentation, the tensions have been normalized in terms of the tension developed by the muscles at a stimulation frequencyof 60 Hz . In these terms, the frequency tension relationship showed, for both muscles, a shift toward the left, allowing the muscles to be tetanized at lower frequencies in the fatigued than the unfatigued state .
à GO ~ 0 40
c
20 10
2O
30
40 50 Frequency ( hz .)
GO
FIG . 2 This figure shows the normalized peak tension developed by the medial gastrocnemius muscles of 8 cats t the S.E . for brief contractions exerted prior to (o), in the middle (pl and immediately after (tÛ a fatiguing isometric contraction . Discussion The results of the present series of experiments are in agreement with those published previously by Brown and Burns (S) where it was shown that fatigued muscle will tetanize at lower frequencies of stimulation than unfatigued muscle. Further, results of the present series of experiments show that twitch duraüon increases in both fast and slow twitch skeletal muscle following fatiguing isometric contractions . One possible mechanism for this response is that, due to their higher fatigability (1,2,3), by the end of
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2291
a fatiguing isometric contraction, the fast twitch motor units may have fatigued to a larger extent than the slow twitch motor units . The muscle twitch at the end of the fatiguing contractions would therefore include proportionately more slow twitch muscle than fast twitch and would therefore have a slower speed of contraction . Although this argument may apply to the medial gastrocnemius muscle, it cannot apply to the coleus muscle where the fiber distribution is entirely slow twitch in the cat (7). It is possible that, for this muscle and for medial gastrocnemius the increase in the twitch duration may be related to some other mechanism, such as a reduction in the actomyosin ATPase, a reduction in the number of actomyosin cross bridges or a reduction in the release or reuptake of Ca ++ from the sarrnplasmic reticulum due to the accumulation of a metabolite . Whatever the mechanism, the large reduction in the tetanic tension compared to the small reduction in the height of the MUAP seems to imply that the point of fatigue is in the contractile apparatus itself . This is supported by the fact that here, and in previous work, the tetanic tension developed by the fatigued muscle was the same for both direct and indirect stimulation (3) . In previous work by others, a reduction in the electrical activity of the muscles has been assumed to show failure of transmission (8) . Although a reduction in electrical activity has been shown here, Luttgau (13) has shown that the contractile characteristics of skeletal muscle may be unaffected by a small change in the height or width of the MUAP, and therefore, fatigue may be entirely in the contractile mechanism during such contractions in fast and slow twitch skeletal muscle. This work was supported by NIH Grant No. 1ROlNS15352-O1 . References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 . 11 . 12. 13 .
R . CLOSE, Physiol . Rev. 52 129-187 (1972). J . S. PETROFSKY, J . S . RINEHART and A. R . LIND, Fed . Proc. 35 291 (1976) . J . S. PETROFSKY, A . R . LIND, Am . J . Physiol. (In Press) (1978) . R . E . BURKE, D. N . LEVINE, P. TSONIS, and F . E. ZAJAC, J. Physiol . 234 723-748 (1973) . G . L . BROWN, and B . D . BURNS, Proc. Roy . Soc . Ser . B 136 182-1% (1949) . R . FITTS and J. Q HOLLOSZY, Am . J . Physiol . 233 C86~G91 (1977) . M. ARIANO, R . B. ARMSTRONG, and V. R. EDGERTON, J. Hist . and Cytochem . 21 51-55 (1973) . . J A . STEPHENS and A. TAYLOR, J . Physiol . 220 1-18 (1972). J. S . PETROFSKY, Med . and Biol . Eng. 16 30268 (1978) C . B. OLSON, D . O . CARPENTER andE. HENNEMAN, Arch . Neurol . 19 591-597 (1968). B. BIGLAND and O .C .J . LIPPOLD, J . Physiol . 125 322-335 (1954) H . S. MILNER-BROWN and R . B . STEIN, J . Physiol . 246 549-571 (1975) . R . FINK, and H . D . LUTTGAU, J. Physiol . 263 215-2~$(1976).
Pergamon Press
THE EFFECT OF MUSCLE FATIGUE ON THE ISOMETRIC CONTRACTILE CHARACTERISTICS OF SKELETAL MUSCLE IN THE CAT Jerrold S . Petrofsky, Adrienne Guard, and Chandler A . Phillips* Department of Physiology, St . Louis University School of Medicine, St . Louis, MO and *Department of Engineering, Wright State University, Dayton, Ohio (Received in final form April 20, 1979) Summary The rise time of an isometric twitch, the tetanic tension, the twitch tetanus ratio, the frequency-tension relationship, and the height of the MUAP (motor unit action potential) were measured in fast twitch (medial gastrocnemius) and slow twitch (coleus) muscles of the cat immediately before, in the middle, and immediately after fatiguing isometric cnntractions at tensions of 30, 50 and 80915 of each muscled initial strength (tetanic tension recorded from the unfatigued muscle) . Although the twitch-tetanus ratio was always less for the soleus than for the medial gastrocnemius muscles, the twitch-tetanus ratio for any one muscle was constant throughout the duration of fatiguing isometric contractions at any of the tensions examined. In contrast, the twitch tension and tetanic tension of the muscles were both less after the contractions, the largest reduction occurring for both muscles during contractions sustained at the lowest isometric tensions . The time to peak tension of an isometric twitch was prolonged for both muscles following the contractions. This was associated with a corresponding shift in the frequency tension relationship such that at the point of muscular fatigue, the muscles tetanized at lower frequencies of stimulation than did the unfatigued muscle . In contrast, the amplitude of the MUAP showed only a modest reduction throughout the duration of the fatiguing contractions . The contractile characteristics and the frequency tension relationship of fast and slow twitch skeletal muscle has been well established in the literature . While slow twitch motor units have a lower actomyosin ATPase activity, lower tetanic strength, and a longer twitch duration (both contraction time and half relaxation time) than fast twitch muscle, these motor units tetanize at lower motor unit firing frequencies than their fast twitch counterparts (1) . Further, because of their high aerobic capacity, slow twitch motor units have substantially longer endurance for both isometric and isotonic contractions (1,2,3,4) . However, although the contractile characteristics of these two types of muscle have been thoroughly examined in unfatigued muscle, little has been done to determine the effect of muscle fatigue on these same characteristics . Brown and Burns found that rabbit muscle tetanized at lower stimulation frequencies after the muscle was fatigued isometrically : here however, only fast twitch muscle was examined with high X100 Hz.) frequencies of stimulation . Fitts and Holloszy (6), found that the V of the coleus (a predominantly slow twitcfi muscle in the rat) was reduced followiri~a ~atiguing isotonic exercise. Finally, we have reported a general shift toward tetanization at lower frequencies of stimulation in isometrically fatigued slow and fast twitch skeletal muscle in the cat (2) . It was the purpose of the present investigation to expand upon our own and the findings of others by examining the properties of the isometric twitch and the frequency 0024-3205/79/242285-07$02 .00/0 Copyright (c) 1979 Pergamon Press Ltd
2286
Isometric Fatigue
Vol . 24, No . 24, 1979
tension relationship throughout the duration of a fatiguing isometric contraction in a mixed but largely fast twitch muscle (the medial gastrocnemius) and a slow twitch muscle (the coleus) of the cat (7). Since transmission fatigue has also been implicated in the fatigue process (8), the electrical activity above these two muscles was also assessed throughout the duration of the contractions by recording the MUAP (motor unit action potential) . Methods Female cats weighing an average of 2.8 t 0 .5 kg were used in these experiments . The animals were anesthetized with a-chloralose (75 mg/kg body weight) injected intraperitoneally, and maintained with intravenous booster doses as needed. A heating pad, placed under the cat, maintained the rectal temperature within the range of 37 to 39° C. Blood pressure and heart rate were monitored through an arterial cannula inserted into the left carotid artery . Surgical Preparation Each cat was placed in the prone position with one back leg fixed rigidly to the table by 2 sets of stainless steel pins driven through the knee and lower leg bones. The muscle which was to be tested was exposed and freed from the surrounding muscles. A section of calcareous with its one remaining tendon attached to the test muscle was tied to a low displacement isometric strain gauge dynamometer. Heated liquid paraffin (3839° C), equilibrated initially with a gas mixture of 95% oxygen and 5% CO was circulated over the muscles through a boat made from the overlying skin to m~lntain muscle temperature at 38 t 1 .0°C . A dorsal laminectomy exposed the L6,. L , and S 1 ventral roots which were cut proximally to their emergence from the spina~ cord. The isolated roots were then divided surgically and pooled into 3 groups, each capable of causing the muscle to develop a similar tension following electrical stimulation. Liquid paraffin, maintained at 38°C and oxygenated with a gas mixture consisting of 95% 0 and 5% C0 2 superfused the other. Every 90 minutes, nerve bundles to keep them electrically isolated from one the superfusion of paraffin was changed to Ringer's solution for 5 minutes to ensure that the ventral roots remained viaible. Stimulation Two types of stimulation were used in these experiments. First, to measure the isometric twitch characteristics of the muscle, an indirect (through the motor nerve) or direct (to the muscle) synchronous square wave pulse (pulse width 0.1 ms) was delivered through 2 needle electrodes with the pulse amplitude made sufficiently large to stimulate all motor units in the muscle . Second, to mimic the normal asynchronous pattern of recruitment found during sustained voluntary contractions, (12) indirect stimulation was also delivered sequentially through the 3 bundles of ventral roots dissected apart as described above. One pair of RF isolated stimulating electrodes was placed on each ventral root bundle . The stimulation pulse width was 0.1 ms with a sufficient amplitude to stimulate all the axons in each bundle . The three bundles of axons were stimulated in a rotary sequence at a frequency for each bundle which was varied during the experiments as described below. Distal to the stimulating electrodes, the ventral roots were spread thinly over a second set of electrodes whose purpose was to create an anodal block. The number of active motor units was varied by altering the anodal block voltage (recruitment). The order by which they were recruited, therefore, depended on the resistance of their motor nerves to anodal blockade. In a previous study, it has been demonstrated that a stepwise reduction in anodal block voltage resulted in a continuous reduction in the time to peak
Vol . 24, No . 24, 1979
Isometric Fatigue
228 7
tension of muscle twitdies recorded from the whole medial gastrocnemius muscle, indicating that the recTUitment order proceeded from the slowest to the fastest motor units respectively (9). This pattern of recruitment was used in these experiments since it resembles that which is believed to occur during voluntary isometric contractions (10, 11, 12). The anodal block voltage and stimulation frequency were set and maintained by a digital computer (Intel 8080A microprocessor). During fatiguing isometric contractions, the motor unit stimulation frequency was set initially at 10 Hz while the anodal block voltage was reduced until the target tension was achieved . As the muscle began to fatigue, more motor units were recruited to maintain the target tension. Once all the motor units were recruited, the computer increased the stimulation frequency to maintain the target . A complete description of the computer program is given elsewhere (9). Once an increase in stimulation frequency was ineffective in maintaining the target, the contraction was terminated ; the total length of time the contraction was maintained was called the endurance time . Measurement of the Electric al Activity Above the Muscles The electrical activity above the muscles was measured by a pair of bipolar platinum electrodes . The resultant signal was amplified and recorded on an FM recorder for later analysis . Procedures A series of test contractions was done initially on each muscle to establish the length of the muscle at which it developed its greatest active tension; the muscle was then set at this length throughout the remainder of the experiments . Twenty minutes later, a single synchronous stimulus was applied to both the motor nerve and then to the muscle with sufficient voltage to recruit all of the motor units; 3 min were allowed between these stimuli and the resultant twitches were recorded and analyzed later for the twitch amplitude and the contraction time (time required for the tension during the twitch to rise from S% to 100% of the maximum twitch height). Five minutes later, the muscles were stimulated sequentially as described above at a sufficient voltage to recruit all of the motor units and at a sufficient frequency to tetanus the muscle fully (100 Hz); the stimulation was maintained for a period of 3 sec and the maximum tension recorded in this manner was assumed to be the initial strength of the muscle (S ). The maximum tetanic tension recorded when a similar procedure was used on ~atigued muscles was termed simply the strength of the muscle . After another S min rest period, the frequency tension relationship was recorded by stimulating the muscles sequentially with supramaximal stimulation for 3 sec periods at frequencies of stimulation between 10 and 60 Hz ; 5 min were allowed between each of these contractions . After another' 1S min rest period, a fatiguing isometric contraction was elicited as described under methods at a tension of either 30, SO or 80% of the initial strength . These 3 tensions were chosen since they represent contractions where the tension was maintained initially and throughout most of the contraction by recruitment (3095) and on the other extreme by initially recruiting all of the motor wits and maintaining tension by varying the frequency of stimulation (80%). Thirty minutes were allowed between these contractions and the presentation order of the tensions on any one muscle was selected at random . Each tension was repeated a total of 8 times. Immediately following 2 of these contractions, a single synchronous stimulus was applied to either the sciatic nerve or to the muscle and the resultant twitch was recorded . In the remaining 6 contractions, the muscle was stimulated sequentially in the middle (as assessed from the endurance time of the previous contractions) and immediately after each fatiguing contraction with a sufficient voltage to recruit all of the motor units and at a frequency of either 10, 20, 30, 40, SO or 60 Hz to established the frequency-tension relationship for the isometrically fatigued muscle. These experiments were repeated on each of 8 cats for each type of
2288
Isometric Fatigue
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muscle . Because of the length of the experiments, only muscle In one leg was used on each cat. Results As reported previously, the endurance fa fatlguing isometric contractions was substantially longer for the coleus (the slow twitch muscle) than for the medial gastrocnemius muscle. Further, the endurance far any one muscle was longest when the tension exerted by the muscle was 30% of the initial strength ; the average endurance for all muscles examined here was 1400 175 (S .E.), 210 + 5, and 16 t 1 sec for the coleus and 100 2, 31 3 1, and 6 t 1 sec for the medial gastrocnemius muscles for tensions of 30, 50, and 80% S respectively . The twitch amplitudes and the time to peak tension before and after thes~ fatlguing contractions are shown in Tables 1, 2, and 3 for contractions exerted at 30, 50 and 80% of the initial strength (S ) of the muscles. As expected from the fiber composition of the muscles, the time to p~ak tension of the coleus-muscle was greater and the twitch amplitude was less than in the medial gastrocnemius muscle . TABLE I Contractile Characteristics During A Contraction At 30% S1 Twitch Tension Twitch Tetanus Ratio Time to Peak Tension g/g Soleus
g/g
MUAP
msec
% onset
Onset
785119
0.1710.0
6312 .3
100
End
235111
0.1710.0
8112 .3
9530 .7
Gastrocnemius Onset 1101136
0 .2210.02
3611 .1
100
End
0.2210.02
4511 .9
8730.3
339325
TABLE II Contractile Characteristics During A Contraction At 50% S1 Twits Tension
Twitch Tetanus Ratio
g/g
g/g
msec
% onset
Soleus
Time to Peak Tension
MUAP
Onset
776324
0.1830.05
6232.5
100
End
376318
0.1630.02
84ä.O
9710 .6
Onset
1280129
0.2330.003
3311 .5
100
End
61938
0.25df1 .01
4711 .8
9110 .6
Gastrocnemius
zzss
Isometric Fatigue
Vol . 24, No . 24, 1979
TABLE III Contractile Characteristics During A Contraction At 80% S 1 Twitch Tension g/g
Twitch Tetanus Ratio Time to Peak Tension MUAP msec % onset g/g
Soleus Onset
794f26
0.1510 .01
63 .3
100
End
623118
0 .1610 .01
8812.1
98f0.4
Gastrocnemius Onset 1150131
0.2210.01
3410.49
100
End
0.22~.Oi.007
4611 .0
95fp .5
913113
At the end of the fatiguing contractions, the twitch amplitude had been reduced and the twitch duration had increased for both muscles, the largest changes in both amplitude and duration occurring for contractions at the lowest isometric tensions . However, the twitch amplitude decreased in direct proportion to tetanic tension so twitch tension ratios of a given muscle did not change from the beginning to the end of the fatiguing contractions (P~0.05). In contrast, the MUAP amplitude recorded darin$ the contraction showed a modest but significant reduction for all contractions (P>0.05) for both types of muscle .
FIG . 1 This figure shows the normalized peak tension developed by the coleus muscles of 8 cats 1 the S.E. for brief contractions exerted prior to (o), in the middle (A) and immediately after (p) a fatiguing isometric contraction .
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Isometric Natigue
Vol . 24, No, 24,
1979
Along with the increase in the time to peak tension, there was a leftward shift in the frequency-tension diagram . Figures 1 and 2 show the frequency tension relationship recorded as described under Methods, before, in the middle, and at the end of the fatiguing isometric contractions . There was no significant difference in the results recorded at the 3 different tensions for any given frequency of stimulation . Therefore, the data from the 3 different tensions have been averaged together such that each point in Figures 1 (soleus) and 2 (medial gastrocnemius muscles) represents the mean response for all 3 tensions tthe standard error for the unfatigued (o), half fatigued (~) and fatigued muscles (q) . Since the tetanic tension of the muscles was reduced to that of the target tension at the end of the isometric contractions, for simplicity of presentation, the tensions have been normalized in terms of the tension developed by the muscles at a stimulation frequencyof 60 Hz . In these terms, the frequency tension relationship showed, for both muscles, a shift toward the left, allowing the muscles to be tetanized at lower frequencies in the fatigued than the unfatigued state .
à GO ~ 0 40
c
20 10
2O
30
40 50 Frequency ( hz .)
GO
FIG . 2 This figure shows the normalized peak tension developed by the medial gastrocnemius muscles of 8 cats t the S.E . for brief contractions exerted prior to (o), in the middle (pl and immediately after (tÛ a fatiguing isometric contraction . Discussion The results of the present series of experiments are in agreement with those published previously by Brown and Burns (S) where it was shown that fatigued muscle will tetanize at lower frequencies of stimulation than unfatigued muscle. Further, results of the present series of experiments show that twitch duraüon increases in both fast and slow twitch skeletal muscle following fatiguing isometric contractions . One possible mechanism for this response is that, due to their higher fatigability (1,2,3), by the end of
Vol . 24, No . 24, 1979
Isometric Fatigue
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a fatiguing isometric contraction, the fast twitch motor units may have fatigued to a larger extent than the slow twitch motor units . The muscle twitch at the end of the fatiguing contractions would therefore include proportionately more slow twitch muscle than fast twitch and would therefore have a slower speed of contraction . Although this argument may apply to the medial gastrocnemius muscle, it cannot apply to the coleus muscle where the fiber distribution is entirely slow twitch in the cat (7). It is possible that, for this muscle and for medial gastrocnemius the increase in the twitch duration may be related to some other mechanism, such as a reduction in the actomyosin ATPase, a reduction in the number of actomyosin cross bridges or a reduction in the release or reuptake of Ca ++ from the sarrnplasmic reticulum due to the accumulation of a metabolite . Whatever the mechanism, the large reduction in the tetanic tension compared to the small reduction in the height of the MUAP seems to imply that the point of fatigue is in the contractile apparatus itself . This is supported by the fact that here, and in previous work, the tetanic tension developed by the fatigued muscle was the same for both direct and indirect stimulation (3) . In previous work by others, a reduction in the electrical activity of the muscles has been assumed to show failure of transmission (8) . Although a reduction in electrical activity has been shown here, Luttgau (13) has shown that the contractile characteristics of skeletal muscle may be unaffected by a small change in the height or width of the MUAP, and therefore, fatigue may be entirely in the contractile mechanism during such contractions in fast and slow twitch skeletal muscle. This work was supported by NIH Grant No. 1ROlNS15352-O1 . References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 . 11 . 12. 13 .
R . CLOSE, Physiol . Rev. 52 129-187 (1972). J . S. PETROFSKY, J . S . RINEHART and A. R . LIND, Fed . Proc. 35 291 (1976) . J . S. PETROFSKY, A . R . LIND, Am . J . Physiol. (In Press) (1978) . R . E . BURKE, D. N . LEVINE, P. TSONIS, and F . E. ZAJAC, J. Physiol . 234 723-748 (1973) . G . L . BROWN, and B . D . BURNS, Proc. Roy . Soc . Ser . B 136 182-1% (1949) . R . FITTS and J. Q HOLLOSZY, Am . J . Physiol . 233 C86~G91 (1977) . M. ARIANO, R . B. ARMSTRONG, and V. R. EDGERTON, J. Hist . and Cytochem . 21 51-55 (1973) . . J A . STEPHENS and A. TAYLOR, J . Physiol . 220 1-18 (1972). J. S . PETROFSKY, Med . and Biol . Eng. 16 30268 (1978) C . B. OLSON, D . O . CARPENTER andE. HENNEMAN, Arch . Neurol . 19 591-597 (1968). B. BIGLAND and O .C .J . LIPPOLD, J . Physiol . 125 322-335 (1954) H . S. MILNER-BROWN and R . B . STEIN, J . Physiol . 246 549-571 (1975) . R . FINK, and H . D . LUTTGAU, J. Physiol . 263 215-2~$(1976).