EXPERIMENTAL
NEUROLOGY
64, 414-427 (1979)
Excitation Frequency and Muscle Fatigue: Electrical Responses During Human Voluntary and Stimulated Contractions B.
BIGLAND-RITCHIE,
D. A.
JONES,
AND
J. J.
WOODS’
Quinnipiac College and the John B. Pierce Foundation, New Haven, Connecticut06519, and the Department of Human Metabolism, University College Hospital Medical School, London WCIE 6JJ Received
August
16, 1978;
revision
received
January
8, 1979
Changes in the electrical activity of the human adductor pollicis muscle during fatiguing maximal voluntary contractions (MVCs) were compared to those resulting from equal periods of maximal ulnar nerve stimulation at different frequencies. In each case the force and smoothed, rectified EMG (SRE) were monitored continuously, and the area of the evoked surface action potential (SAP) was measured at intervals. During high-frequency stimulation (50 and 80 Hz), both the SRE and SAP area increased in the first 10 to 20 s, thereafter declining to very low values. With low-frequency stimulation (20 HZ), both increased gradually throughout the contraction. The increases in SAP area were related to a slowing of conduction velocity. In all experiments in which the frequency of stimulation was constant, changes in the SRE and SAP area mirrored one another. In sustained MVCs, the rate offorce loss was less than during high-frequency stimulation. SAPS evoked by periodic single maximal shocks to the nerve increased initially in area but then remained relatively constant. The SRE no longer paralleled the SAP; it generally increased initially, but then declined roughly in proportion to the force. When the nerve was maximally stimulated at a progressively reduced frequency (80 to 20 Hz), force loss and SAP area were similar to those recorded during an MVC. The SRE was also similar in form. It is concluded that (a) during continuous high-frequency stimulation, much of the fatigue is due to failure of electrical propagation, probably largely at the muscle fiber membrane; and (b) in voluntary contractions where no similar failure was observed, muscle fatigue is minimized by a progressive reduction in motor unit activation.
Abbreviations: EMG-electromyogram; SRE-smoothed, rectified electromyogram; SAP-surface action potential; MVC-maximal voluntary contraction. ’ We are grateful to Prof. R. H. T. Edwards for his help and encouragement. The work was supported by grant NS-14756 from the U.S. Public Health Service; by the Muscular Dystrophy Associations of the U.S.A. and Great Britain; and by the Wellcome Trust. 414 0014-4886/79/050414-14$02.00/O Copyright All rights
0 1979 by Academic Press, Inc. of reproduction in any form reserved.
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INTRODUCTION In the preceding study, Jones et al. (12) proposed that during a sustained maximum voluntary contraction (MVC) of the adductor pollicis muscle, force generation is optimized by a successive reduction in the average motor neuron firing frequency, and that this frequency reduction appears to minimize conduction failure at peripheral sites. Those conclusions were based on force measurements during both voluntary and stimulated contractions in which it was found that (a) high-frequency nerve stimulation (50 to 80 Hz) was required to tetanize fully an unfatigued muscle and to match the peak force generated in a maximum voluntary contraction (3); (b) continuous stimulation at high frequency led to a more rapid loss of force than was seen in a sustained MVC; (c) in fatigued muscle more force was generated by low- than by high-frequency stimulation; and (d) the force generated during a sustained MVC could be matched by nerve stimulation only if the initial high frequency was progressively reduced. Those conclusions were based on measurement of only the mechanical responses of the muscle to different types of stimulation. If, as predicted, the observed differences in rate of force loss were due to changes in electrical propagation, this should be detected by electrical recording from the muscle. The present study was designed to repeat many of the earlier human experiments while recording both the smooth rectified EMG (SRE) continuously and the area of the evoked action potential periodically, during both maximal voluntary and stimulated contractions. Although the experimental conditions were similar to those of the earlier study, many of the details of procedure, subjects, equipment, and so forth, differed slightly. The results, however, support the hypothesis that (a) fatigue from high-frequency stimulation results from failure of electrical activation of the muscle, and (b) the natural firing frequency decreases during the course of an MVC with the result that the type of failure seen with continuous high-frequency stimulation is minimized. Some of these findings were reported briefly in preliminary communications (2, 4). METHODS The subjects used in these experiments were volunteers, mainly the investigators themselves, but also physical therapy students who routinely undergo nerve and muscle stimulation as part of their clinical training. All gave their informed consent. Experiments were done on the adductor pollicis muscle. The methods for force recording and stimulation were similar to those described previously (9, 12). Only a brief description is given here. In the present
416
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JONES,
AND
WOODS
experiments, however, no occlusion of circulation was used because the sustained contractions were not interrupted, and preliminary experiments showed the results were not affected by this procedure. Force recording during maximal voluntary and stimulated contractions was made using a dynamometer modified from Merton (16). The strain gauge output was displayed on one channel of a dual channel pen recorder. Stimulation. The ulnar nerve was stimulated at the wrist with pulses of 50-100 ps. Maximal response was judged by action potential size; when this was achieved the voltage was increased by about 20% to ensure continued maximal response. This was checked periodically throughout the experiment. Electromyogram recordings were made from two adhesive silver/silver chloride cup electrodes (IO-mm diameter), one over the surface of the muscle, the other on the tip of the forefinger. Care was taken in placing the electrodes such that only single action potentials were recorded in response to single shocks to the nerve. These electrodes were used to record both the continuous SRE and the individual evoked surface action potentials (SAP) during voluntary contractions and in response to nerve stimulation. After suitable amplification the signals for SRE recording were full-waverectified and smoothed (tc, 0.2 to 0.4 s) before being displayed on the second channel of the pen recorder. Single evoked compound action potentials were photographed from an oscilloscope, and their total area above and below the isoelectric line was measured by planimetry. A typical action potential with the area measured is shown in the inset of Fig. 2. Conduction time from stimulus artifact to the onset of the action potential was measured as an indicator of changes in conduction velocity. The results were generally expressed as percentage change from an initial value at the start of the contraction. Because the shape and size of the compound action potential changes when the muscle contracts and alters its physical configuration with respect to the surface electrodes, the initial values were generally taken 2 s after the onset of contraction. Wherever possible mean values +SE are given. RESULTS Stimulated Contractions at Constant Frequency. The muscle was maximally stimulated via the ulnar nerve for 60 s at a constant frequency (either20,50, or 80 Hz), while force and SRE were recorded (Fig. 1A). At 2 s, 10 s, and at subsequent 10 s intervals, individual evoked surface action potentials were recorded (Fig. 1B). At 20 Hz the force remained relatively constant while the SRE increased gradually throughout the contraction. At the higher frequencies (such as were required to match the force of an MVC in unfatigued muscle; Fig. 1A, left), the force decreased rapidly so that after
2
ih,
20/set
20/set
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I\
J
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I
30
\, j
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:
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,
I
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80/set
30
60
FIG. 1. A-force (lower record) and smoothed, rectified EMG (SRE) recorded when the adductor pollicis muscle was maximally stimulated 60 s at a frequency of 20, 50, and 80 Hz, respectively. Force produced during a brief maximal voluntary contraction (MVC) is also shown for comparison. B-action potentials recorded at 10-s intervals during the contractions shown above. The numbers on the left indicate the time in seconds after the start of the contraction.
50 60
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BIGLAND-RITCHIE,
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30 s less force was generated than after a similar period of stimulation at 20 Hz (12). The SRE, however, showed an initial increase, nearly doubling in the first 10 to 20 s, before declining rapidly. During contractions at 20 Hz there was little change in the amplitude of the individual action potentials, but there was an overall slowing and prolongation of the waveform (Fig. 1B). At 50 Hz the slowing and prolongation were more marked, reaching a maximum at about 20 s, after which the amplitude declined rapidly. At 80 Hz the same prolongation was seen, but the action potentials began to fail even before 10 s of stimulation. At this time the action potential was so prolonged that its total area could no longer be measured. Measurements were made of the action potential area during the stimulated contractions (Fig. 2, inset). For any given frequency, changes in the SRE were directly proportional to changes in the action potential area (Fig. 2). Thus the striking increase in SRE at the onset of the contractions stimulated at higher frequencies was clearly due to the prolongation and increased total area of the individual action potentials. Throughout these contractions there was also a continuous increase in the conduction time from stimulus artifact to the onset of the action potential. If this was mainly due to slowing of conduction velocity along the muscle fiber membrane rather than in the presynaptic terminals, it would account for the prolongation and increase in action potential area. In the latter part of the high frequency-stimulated contractions, the loss of amplitude and subsequent failure of the action potentials reduced their area, and this also resulted in a parallel decrease in the SRE.
--. M
l
SAP SRE
TIME
Isec)
FIG. 2. Changes in surface action potential area (SAP, 0) and smoothed, rectified EMG (SRE, 0) when the muscle was stimulated maximally for 60 s at 20, 50, and 80 Hz. Values 2 SE. Inset: area and conduction time measurements.
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20
01
’ IO
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’
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01 0
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FIG. 3. A-changes in surface action potential area (SAP, l ), smoothed, rectified EMG (SRE, 0), and force (0) when the muscle was stimulated maximally at 80 Hz for 45 s, followed by stimulation at 20 Hz. B-changes in SRE in relation to SAP area times frequency. Data taken from three experiments like that shown in A. Stimulus frequency: 80 (0) and 20 Hz (0).
Effect of Sudden Reduction of Stimulus Frequency. When a muscle was fatigued by high-frequency stimulation, a sudden reduction in the frequency resulted in more force being generated (12). Figure 3A shows the effect of stimulating for 45 s at 80 Hz followed by continued stimulation at 20 Hz. When the frequency was reduced the action potential area returned to near the original value, and the SRE also increased. The force increased to a value close to that which would have occurred had 20 Hz stimulation been used throughout. The recovery of force when the action potential was restored indicates that the rapid loss of force during high-frequency stimulation was probably due to failure of electrical propagation. Throughout contraction the SRE was directly proportional to the product of action potential area and frequency (Fig. 3B). The results illustrated in Figs. 2 and 3B thus confirm that the SRE of maximally stimulated muscles is determined by two factors, the area of the action potential and the frequency of stimulation (21). Consequently, if the SRE and action potential areas are known, changes in the stimulation frequency during a contraction can be estimated. Voluntary Contractions. Subjects were asked to sustain a maximum voluntary contraction for 60 s. Once every 10 s the ulnar nerve was
420
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stimulated with a single maximal shock, and the evoked action potential was recorded. In unfatigued muscle the initial MVC force was similar to that obtained when stimulating at 50 to 80 Hz, but the voluntary force was better maintained than the high-frequency tetanic contraction. The SRE during an MVC generally increased during the first 10 s of the contraction before gradually decreasing. Evoked action potentials recorded during the contraction showed little change in amplitude, but did show a slight prolongation of the peaks during the first 10 to 15 s with the result that the total action potential area generally increased (Figs. 4 and 5B). This was accompanied by a corresponding change in conduction time. Figure 4 gives the mean data for eight such contractions showing that the action potential area increased by about 40% in the first 10 s and then remained constant, and the SRE, after the initial increase, subsequently declined roughly in parallel with the force as described by Stephens and Taylor (20). Stimulated Contractions with a Continuous Decrease in Frequency. Jones et al. (12) showed that the time course of MVC force decay can be
5 - II0 :z E f F - 105 c I? s P s
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FIG. 4. Mean values + SE for force, smoothed, rectified EMG (SRE), surface action potential (SAP) area, and conduction time (CT) from eight maximal voluntary contractions of one subject sustained for 60 s. Each is expressed as a percentage of the initial value measured 2 s after the start of a contraction.
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60
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I
I
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Timr (SECI I 60
I 30
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(1.00)
(1.00)
(1.41) (I.421
(1.33)
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(I.521 (1.36)
(I.271 (I.521 t
5msec
(1.22) (I.061
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5. A-comparison of the smoothed, rectified EMG record during a maximal voluntary contraction (MVC, irregular line) with that of a stimulated contraction (smoother line) in which the frequency of stimulation was decreased as shown on the abscissa. (The amplification was four times greater for the MVC). B, C-action potentials recorded at 10-s intervals during similar experiments. B, an MVC, and C, a stimulated contraction in which the frequency was steadily reduced. Numbers on left show time in seconds after onset of contraction. Numbers on right of each record show relative action potential area. Stimulation frequency as shown in A. FIG.
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imitated by a stimulated contraction in which the frequency is continuously decreased. This procedure was repeated, and in addition to force, SRE and individual action potentials were recorded (Fig. 5). Not only was the time course of the MVC force decay matched, but also the electrical activity of the muscle. Individual action potentials were well maintained and similar in area to those obtained during an MVC. The SREs for the stimulated contractions were similar in form but had to be scaled down by a factor of approximately four to match that of the MVC. This reflects mainly the difference between the synchronized electrical stimulation of the muscle as opposed to the asynchronous voluntary input. DISCUSSION Jones et al. (12) compared the time courses of force decline during repetitive stimulation at different frequencies and during an MVC. They found that the force declined more slowly during a voluntary contraction than during prolonged nerve stimulation at a frequency high enough to match the initial force of a voluntary contraction. Thus, in fatigued muscle, high-frequency stimulation suppressed force generation. They concluded that this type of suppression must be reduced during prolonged voluntary contractions by a progressive decrease in motor neuron firing frequency. In the present work we examined the electrical activity of the muscle during similar contractions to see whether or not this also indicates changes in firing frequency, and also to investigate the causes of high-frequency force fatigue. Electrical Propagation Tetanic Stimulation. The excessive loss of force that occurs during continuous high frequency stimulation appears to be mainly due to failure of electrical propagation for it is accompanied by a rapid decline in the size of the evoked action potential, and largely recovers when the latter is restored. During the initial period of stimulation there is a marked broadening of the action potential with little loss of amplitude. This results in a substantial increase in total action potential area and is associated with a corresponding increase in conduction time. If the changes in conduction time are mainly due to slowing of conduction velocity along the muscle fiber membrane this would account for both the broadening and the increase in area of the action potential; it would also indicate a reduced muscle membrane excitability. If, on the other hand, the slowing of conduction and subsequent electrical failure are mainly in the presynaptic nerve terminals or result from increased synaptic delay, the compound action potential might broaden because of increased dispersion between units, but only with a loss of amplitude; its total area would not increase.
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Similar changes in both action potential area and conduction velocity can be seen in the records of Naess and Storm-Mathisen (19) when rabbit and human muscle were stimulated via the nerve at different frequencies. Slowing of conduction velocity during fatiguing human voluntary contractions were also observed by DeLuca and Forrest (7) Lindstrom et al. (14), and also by Mortimer et al. (18) in cat muscle stimulated both via the nerve and directly. Using direct stimulation of curarized rat diaphragm muscle, KrjneviE and Miledi (13) found that the fiber membrane threshold may increase several-fold during only a few minutes of high-frequency stimulation and that few fibers could conduct for more than a minute when stimulated continuously at 50 Hz. These findings were attributed mainly to changes in transmembranal cation concentrations. Mortimer ef al. (18) found that changes in conduction velocity induced by stimulating small areas of muscle while occluding the blood supply were almost abolished when the muscle was perfused with oxygen-free dextran solution. They concluded that the slowing was due to the accumulation of some metabolite, probably lactic acid, rather than to ischemia as such. A more likely explanation, however, is that there was a reduction in the [Na+] and an increase of [K+] in the extracellular fluid during activity, which would certainly slow conduction (6, 10). This could occur because (a) during contraction the intramuscular pressure occludes the circulation so that the muscle becomes a closed system and behaves like an isolated preparation (8); and (b) there may not be time between action potentials for the sodium pump to restore the Na+ and K+ exchange that results from the passage of each impulse. This would account for the finding in the previous study (12) that reducing the extracellular Na+ accelerated the rate of force fatigue in the isolated curarized preparation in the same way as did an increase in stimulus frequency. Changes in cation concentrations can be roughly calculated from estimates of total membrane area, cation fluxes, etc. KrnjeviE and Miledi (13) calculated that after 10,000 impulses at 50 Hz the intracellular Na+ in rat diaphragm muscle may rise by 70 mM; and there would be a similar decrease in intracellular K+. Because the interfiber inulin spaces in rat limb muscles are as low as one-fourth that of the intracellular volume (1 l), the changes in extracellular cation concentrations will be correspondingly greater. Mouse soleus muscle is inexcitable in 20 mM [K+]. Changes in extracellular cation concentrations could, thus, reduce the excitability of the muscle surface membrane sufficiently to account for all aspects of high-frequency force fatigue observed in these experiments, even in the total absence of neuromuscular block, whereas neuromuscular block alone could not. It seems more likely, however, that some
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AND
WOODS
presynaptic failure and block resulting from reduced end-plate potentials are also occurring simultaneously, but the relative contributions of each cannot be determined from the present experiments. Voluntary Contractions. The technique of recording evoked synchronous action potentials superimposed on a maximum voluntary contraction was used primarily to follow changes in electrical propagation at the neuromuscularjunction (16). Stephens and Taylor (20) used double shocks, making their measurements from the second impulse, to minimize interference by the voluntary activity. In preliminary experiments we found that double shocks did not change the measurements of area or improve the stability of the baseline which was not generally a problem. During an MVC the most notable feature of the recorded action potentials was that even at the end of the voluntary contractions they were well maintained and showed much less change than that associated with continuous high-frequency stimulation (Figs. 1, 5): There was neither the extreme prolongation nor the decline in amplitude. Instead there was an increase in area which was maintained throughout the contraction. Thus we may conclude that in adductor pollicis failure of electrical propagation, whatever the cause, cannot be the main reason for the loss of force during the first 60 s of a maximum voluntary contraction. These results differ from those of Stephens and Taylor (20) who found a 35% decrease in the size of the action potential after a 60-s sustained MVC of the first dorsal interosseous muscle. This finding, together with the simultaneous decline in the SRE, led them to conclude that fatigue resulted mainly from failure of neuromuscular transmission. It may be that the first dorsal interosseous muscle is more susceptible to neuromuscular block than is the adductor pollicis. Alternatively the discrepancy may result from the method of measuring action potential areas. We measured the whole area on either side of the isoelectric line, and these results correlate closely with changes in the surface EMG recorded simultaneously (Fig. 2). For example, the striking increase in SRE at the onset of high-frequency stimulation is paralleled by an equal increase in action potential area. Stephens and Taylor (20) measured only a portion of the total action potential area, and their method may not have adequately taken into account the slight changes in form of the action potential resulting from the slowing of conduction velocity (Figs. 4, 5). When measured their way we found the parallel between changes in the action potential size and surface EMG during high frequency stimulation was lost. Frequency Changes during a Prolonged Maximal Voluntary Contraction. Rectifying and smoothing the surface EMG activity is a commonly used averaging technique, and Woods et al. (21) showed that the signal produced during synchronous stimulation was proportional to the number
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of activated muscle fibers, the area of the individual action potentials, and to their frequency of activation. This is also shown in Figs. 1 and 3. Consequently if all fibers are active and the SRE and the area of the action potential are known, an estimate of changes in frequency can be made. During constant-frequency stimulation changes in action potential area were always accompanied by a parallel change in the SRE. During sustained MVC this was not so. Simultaneous recording of SRE and evoked action potentials show that for the subject shown in Fig. 4 the former decreased to approximately 40% of the initial value whereas the action potential area increased to 140% and remained near this value for the 60 s of voluntary contraction. If all motor neurons remained active, and if the SRE, evoked action potential area, and frequency during voluntary contractions have the same relationship as that observed during stimulated contractions, it follows that during the course of the 60-s MVC the natural firing frequency declined by a factor of 140/40 or 3.5 times, a value similar to that required during nerve stimulation to imitate a sustained MVC (Fig. 5). This conclusion makes two assumptions. First, the single supramaximal shock to the ulnar nerve activates the same number of fibers as are functioning during voluntary activity. If a proportion of motor neurons had stopped firing, even though the muscle fibers supplied by them were still capable of being stimulated, then the synchronous action potential would be disproportionately greater than the SRE recorded during the voluntary contraction. This is unlikely as the force never increased when a sustained MVC was interrupted by maximal nerve stimulation (12), although this can sometimes occur during fatigue of other muscles (3). Second, the changes in SRE observed during an MVC were assumed not to be due to changes in synchronization between individual motor unit action potentials. In Fig. 4 the SRE resulting from synchronous stimulation was four times greater than that from the asynchronous MVC for equal rates of force loss. Mimer-Brown and Stein (17) found that in contractions of unfatigued muscle there was little synchronization. If synchronization increases as fatigue progresses, the SRE in the later part of the contraction would increase rather than decline with time. It is not clear whether the changes in firing frequency occur in all motor neurons or only in those with the highest natural frequency. The matter can be resolved only by direct recording from different single motor unit types. But this is difficult during truly maximal contractions due to interference from surrounding units (1, 5). Direct evidence for a change in firing frequency was obtained by Marsden et al. (15) who recorded from aberrant motor units of the adductor pollicis innervated from the medial nerve, after total blockage of the ulnar nerve. Maximal contractions resulted in an initial discharge frequency of 180 Hz, decreasing rapidly to 50 Hz in the first
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second or so and thereafter to 20 Hz in the next 30 s, after which it remained relatively constant. These results are in the same range as the frequency changes we found most effective in matching both the force and electrical activity of an MVC. If such a reduction in motor neuron firing frequency does occur it appears to optimize force generation during fatiguing contractions by avoiding, or reducing, the failure of electrical propagation which would otherwise occur. REFERENCES 1. BIGLAND, B., AND 0. C. .I. LIPPOLD. 1954. Motor unit activity in the voluntary contraction of human muscle. J. Physiol. (London) 125: 322-335. 2. BIGLAND-RITCHIE, B., R. H. T. EDWARDS,,AND D. A. JONES. 1977. Fatigue in sustained isometric voluntary contractions. Physiology (Canada) 8: 28. 3. BIGLAND-RITCHIE, B., D. A. JONES, G. P. HOSKING, AND R. H. T. EDWARDS. 1978. Central and peripheral fatigue in sustained maximum voluntary contractions of human quadriceps muscle. Clin. Sci. Mol. Med. 54: 609-614. 4. BIGLAND-RITCHIE, B., J. J. WOODS, AND D. A. JONES. 1978. Electrical activity during fatigue of sustained maximal voluntary and stimulated contractions. In 4th International Congress on Neuromuscular Diseases, Abstr. 287. Montreal, Canada. 5. CLAMANN, H. P. 1970. Activity of single motor units during isometric tension. Neurology (Minneapolis) 20: 254-260. 6. COLQUHOUN, D., AND J. M. RITCHIE. 1972. The interaction at equilibrium between tetrodotoxin and mammalian non-myelinated nerve fibers. J. Physiol. (London) 221: 533-553. 7. DELUCA, C. J., AND W. J. FORREST. 1973. Some properties of motor unit action potential trains recorded during constant force isometric contractions in man. Kybernetik 12: 160-168. 8. EDWARDS, R. H. T. 1975. Muscle fatigue. Postgrad. Med. J. 51: 137-143. 9. EDWARDS, R. H. T., D. K. HILL, D. A. JONES, AND P. A. MERTON. 1977. Fatigue of long duration in human skeletal muscle after exercise. J. Physiol. (London) 272: 769-778. 10. HODGKIN, A. L., AND B. K. KATZ. 1949. The effect of sodium ions on the electrical activity of the giant axon of the squid. J. Physiol. (London) 108: 37-77. 11. HOH, J. F. Y ., AND B. SALAFSKY. 1971. Effects of nerve cross-union on rat intracellular potassium in fast-twitch and slow-twitch rat muscles. J. Physiol. (London) 216: 171-179. 12. JONES, D. A., B. BIGLAND-RITCHIE, AND R. H. T. EDWARDS. 1979. Excitation frequency and muscle fatigue: Mechanical responses during voluntary and stimulated contractions. Exp. Neurol. 64: 401-413. 13. KRNJEVI~, K., AND R. MILEDI. 1958. Failure of neuromuscular propagation in rats. J. Physiol. (London) 140: 440-461. 14. LINDSTROM, L., R. MAGNUSSON, AND I. PETERSEN. 1970. Muscular fatigue and action potential conduction velocity changes studied with frequency analysis of EMG signals. Electromyography 10: 341-356. 15. MARSDEN, C. D., J. C. MEADOWS, AND P. A. MERTON. 1971. Isolated single motor units in human muscle and their rates of discharge during maximal voluntary effort. J. Physiol. (London) 217: 12-13P. 16. MERTON, P. A. 1954. Voluntary strengthandfatigue. J. Physiol. (London) 128: 553-564.
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17. MILNER-BROWN, H. S., AND R. B. STEIN. 1975. The relation between the surface electromyogram and muscle force. J. Physiol. (London) 246: 549-569. 18. MORTIMER, J. T., R. MAGNUSSON, AND I. PETERSEN. 1970. Conduction velocity in ischaemic muscle: effect on EMG frequency spectrum. Am. J. Physiol. 219: 1324-
1329.
NAESS, K., AND A. STROM-MATHISON. 19.55. Fatigue of sustained tetanic contractions. Acta Physiol. Stand. 34: 351-366. 20. STEPHENS, J. A., AND A. TAYLOR. 1972. Fatigue of maintained voluntary muscle contraction in man. J. Physiol. (London) 220: l- 18. 21. WOODS, J. J., D. A. JONES, AND B. BIGLAND-RITCHIE. 1978. Components ofthe surface EMG during stimulated and voluntary contractions. Med. Sri. Sports 10(l): 67. 19.
NEUROLOGY
64, 414-427 (1979)
Excitation Frequency and Muscle Fatigue: Electrical Responses During Human Voluntary and Stimulated Contractions B.
BIGLAND-RITCHIE,
D. A.
JONES,
AND
J. J.
WOODS’
Quinnipiac College and the John B. Pierce Foundation, New Haven, Connecticut06519, and the Department of Human Metabolism, University College Hospital Medical School, London WCIE 6JJ Received
August
16, 1978;
revision
received
January
8, 1979
Changes in the electrical activity of the human adductor pollicis muscle during fatiguing maximal voluntary contractions (MVCs) were compared to those resulting from equal periods of maximal ulnar nerve stimulation at different frequencies. In each case the force and smoothed, rectified EMG (SRE) were monitored continuously, and the area of the evoked surface action potential (SAP) was measured at intervals. During high-frequency stimulation (50 and 80 Hz), both the SRE and SAP area increased in the first 10 to 20 s, thereafter declining to very low values. With low-frequency stimulation (20 HZ), both increased gradually throughout the contraction. The increases in SAP area were related to a slowing of conduction velocity. In all experiments in which the frequency of stimulation was constant, changes in the SRE and SAP area mirrored one another. In sustained MVCs, the rate offorce loss was less than during high-frequency stimulation. SAPS evoked by periodic single maximal shocks to the nerve increased initially in area but then remained relatively constant. The SRE no longer paralleled the SAP; it generally increased initially, but then declined roughly in proportion to the force. When the nerve was maximally stimulated at a progressively reduced frequency (80 to 20 Hz), force loss and SAP area were similar to those recorded during an MVC. The SRE was also similar in form. It is concluded that (a) during continuous high-frequency stimulation, much of the fatigue is due to failure of electrical propagation, probably largely at the muscle fiber membrane; and (b) in voluntary contractions where no similar failure was observed, muscle fatigue is minimized by a progressive reduction in motor unit activation.
Abbreviations: EMG-electromyogram; SRE-smoothed, rectified electromyogram; SAP-surface action potential; MVC-maximal voluntary contraction. ’ We are grateful to Prof. R. H. T. Edwards for his help and encouragement. The work was supported by grant NS-14756 from the U.S. Public Health Service; by the Muscular Dystrophy Associations of the U.S.A. and Great Britain; and by the Wellcome Trust. 414 0014-4886/79/050414-14$02.00/O Copyright All rights
0 1979 by Academic Press, Inc. of reproduction in any form reserved.
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INTRODUCTION In the preceding study, Jones et al. (12) proposed that during a sustained maximum voluntary contraction (MVC) of the adductor pollicis muscle, force generation is optimized by a successive reduction in the average motor neuron firing frequency, and that this frequency reduction appears to minimize conduction failure at peripheral sites. Those conclusions were based on force measurements during both voluntary and stimulated contractions in which it was found that (a) high-frequency nerve stimulation (50 to 80 Hz) was required to tetanize fully an unfatigued muscle and to match the peak force generated in a maximum voluntary contraction (3); (b) continuous stimulation at high frequency led to a more rapid loss of force than was seen in a sustained MVC; (c) in fatigued muscle more force was generated by low- than by high-frequency stimulation; and (d) the force generated during a sustained MVC could be matched by nerve stimulation only if the initial high frequency was progressively reduced. Those conclusions were based on measurement of only the mechanical responses of the muscle to different types of stimulation. If, as predicted, the observed differences in rate of force loss were due to changes in electrical propagation, this should be detected by electrical recording from the muscle. The present study was designed to repeat many of the earlier human experiments while recording both the smooth rectified EMG (SRE) continuously and the area of the evoked action potential periodically, during both maximal voluntary and stimulated contractions. Although the experimental conditions were similar to those of the earlier study, many of the details of procedure, subjects, equipment, and so forth, differed slightly. The results, however, support the hypothesis that (a) fatigue from high-frequency stimulation results from failure of electrical activation of the muscle, and (b) the natural firing frequency decreases during the course of an MVC with the result that the type of failure seen with continuous high-frequency stimulation is minimized. Some of these findings were reported briefly in preliminary communications (2, 4). METHODS The subjects used in these experiments were volunteers, mainly the investigators themselves, but also physical therapy students who routinely undergo nerve and muscle stimulation as part of their clinical training. All gave their informed consent. Experiments were done on the adductor pollicis muscle. The methods for force recording and stimulation were similar to those described previously (9, 12). Only a brief description is given here. In the present
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BIGLAND-RITCHIE,
JONES,
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experiments, however, no occlusion of circulation was used because the sustained contractions were not interrupted, and preliminary experiments showed the results were not affected by this procedure. Force recording during maximal voluntary and stimulated contractions was made using a dynamometer modified from Merton (16). The strain gauge output was displayed on one channel of a dual channel pen recorder. Stimulation. The ulnar nerve was stimulated at the wrist with pulses of 50-100 ps. Maximal response was judged by action potential size; when this was achieved the voltage was increased by about 20% to ensure continued maximal response. This was checked periodically throughout the experiment. Electromyogram recordings were made from two adhesive silver/silver chloride cup electrodes (IO-mm diameter), one over the surface of the muscle, the other on the tip of the forefinger. Care was taken in placing the electrodes such that only single action potentials were recorded in response to single shocks to the nerve. These electrodes were used to record both the continuous SRE and the individual evoked surface action potentials (SAP) during voluntary contractions and in response to nerve stimulation. After suitable amplification the signals for SRE recording were full-waverectified and smoothed (tc, 0.2 to 0.4 s) before being displayed on the second channel of the pen recorder. Single evoked compound action potentials were photographed from an oscilloscope, and their total area above and below the isoelectric line was measured by planimetry. A typical action potential with the area measured is shown in the inset of Fig. 2. Conduction time from stimulus artifact to the onset of the action potential was measured as an indicator of changes in conduction velocity. The results were generally expressed as percentage change from an initial value at the start of the contraction. Because the shape and size of the compound action potential changes when the muscle contracts and alters its physical configuration with respect to the surface electrodes, the initial values were generally taken 2 s after the onset of contraction. Wherever possible mean values +SE are given. RESULTS Stimulated Contractions at Constant Frequency. The muscle was maximally stimulated via the ulnar nerve for 60 s at a constant frequency (either20,50, or 80 Hz), while force and SRE were recorded (Fig. 1A). At 2 s, 10 s, and at subsequent 10 s intervals, individual evoked surface action potentials were recorded (Fig. 1B). At 20 Hz the force remained relatively constant while the SRE increased gradually throughout the contraction. At the higher frequencies (such as were required to match the force of an MVC in unfatigued muscle; Fig. 1A, left), the force decreased rapidly so that after
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FIG. 1. A-force (lower record) and smoothed, rectified EMG (SRE) recorded when the adductor pollicis muscle was maximally stimulated 60 s at a frequency of 20, 50, and 80 Hz, respectively. Force produced during a brief maximal voluntary contraction (MVC) is also shown for comparison. B-action potentials recorded at 10-s intervals during the contractions shown above. The numbers on the left indicate the time in seconds after the start of the contraction.
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30 s less force was generated than after a similar period of stimulation at 20 Hz (12). The SRE, however, showed an initial increase, nearly doubling in the first 10 to 20 s, before declining rapidly. During contractions at 20 Hz there was little change in the amplitude of the individual action potentials, but there was an overall slowing and prolongation of the waveform (Fig. 1B). At 50 Hz the slowing and prolongation were more marked, reaching a maximum at about 20 s, after which the amplitude declined rapidly. At 80 Hz the same prolongation was seen, but the action potentials began to fail even before 10 s of stimulation. At this time the action potential was so prolonged that its total area could no longer be measured. Measurements were made of the action potential area during the stimulated contractions (Fig. 2, inset). For any given frequency, changes in the SRE were directly proportional to changes in the action potential area (Fig. 2). Thus the striking increase in SRE at the onset of the contractions stimulated at higher frequencies was clearly due to the prolongation and increased total area of the individual action potentials. Throughout these contractions there was also a continuous increase in the conduction time from stimulus artifact to the onset of the action potential. If this was mainly due to slowing of conduction velocity along the muscle fiber membrane rather than in the presynaptic terminals, it would account for the prolongation and increase in action potential area. In the latter part of the high frequency-stimulated contractions, the loss of amplitude and subsequent failure of the action potentials reduced their area, and this also resulted in a parallel decrease in the SRE.
--. M
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FIG. 2. Changes in surface action potential area (SAP, 0) and smoothed, rectified EMG (SRE, 0) when the muscle was stimulated maximally for 60 s at 20, 50, and 80 Hz. Values 2 SE. Inset: area and conduction time measurements.
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FIG. 3. A-changes in surface action potential area (SAP, l ), smoothed, rectified EMG (SRE, 0), and force (0) when the muscle was stimulated maximally at 80 Hz for 45 s, followed by stimulation at 20 Hz. B-changes in SRE in relation to SAP area times frequency. Data taken from three experiments like that shown in A. Stimulus frequency: 80 (0) and 20 Hz (0).
Effect of Sudden Reduction of Stimulus Frequency. When a muscle was fatigued by high-frequency stimulation, a sudden reduction in the frequency resulted in more force being generated (12). Figure 3A shows the effect of stimulating for 45 s at 80 Hz followed by continued stimulation at 20 Hz. When the frequency was reduced the action potential area returned to near the original value, and the SRE also increased. The force increased to a value close to that which would have occurred had 20 Hz stimulation been used throughout. The recovery of force when the action potential was restored indicates that the rapid loss of force during high-frequency stimulation was probably due to failure of electrical propagation. Throughout contraction the SRE was directly proportional to the product of action potential area and frequency (Fig. 3B). The results illustrated in Figs. 2 and 3B thus confirm that the SRE of maximally stimulated muscles is determined by two factors, the area of the action potential and the frequency of stimulation (21). Consequently, if the SRE and action potential areas are known, changes in the stimulation frequency during a contraction can be estimated. Voluntary Contractions. Subjects were asked to sustain a maximum voluntary contraction for 60 s. Once every 10 s the ulnar nerve was
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stimulated with a single maximal shock, and the evoked action potential was recorded. In unfatigued muscle the initial MVC force was similar to that obtained when stimulating at 50 to 80 Hz, but the voluntary force was better maintained than the high-frequency tetanic contraction. The SRE during an MVC generally increased during the first 10 s of the contraction before gradually decreasing. Evoked action potentials recorded during the contraction showed little change in amplitude, but did show a slight prolongation of the peaks during the first 10 to 15 s with the result that the total action potential area generally increased (Figs. 4 and 5B). This was accompanied by a corresponding change in conduction time. Figure 4 gives the mean data for eight such contractions showing that the action potential area increased by about 40% in the first 10 s and then remained constant, and the SRE, after the initial increase, subsequently declined roughly in parallel with the force as described by Stephens and Taylor (20). Stimulated Contractions with a Continuous Decrease in Frequency. Jones et al. (12) showed that the time course of MVC force decay can be
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FIG. 4. Mean values + SE for force, smoothed, rectified EMG (SRE), surface action potential (SAP) area, and conduction time (CT) from eight maximal voluntary contractions of one subject sustained for 60 s. Each is expressed as a percentage of the initial value measured 2 s after the start of a contraction.
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Timr (SECI I 60
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5. A-comparison of the smoothed, rectified EMG record during a maximal voluntary contraction (MVC, irregular line) with that of a stimulated contraction (smoother line) in which the frequency of stimulation was decreased as shown on the abscissa. (The amplification was four times greater for the MVC). B, C-action potentials recorded at 10-s intervals during similar experiments. B, an MVC, and C, a stimulated contraction in which the frequency was steadily reduced. Numbers on left show time in seconds after onset of contraction. Numbers on right of each record show relative action potential area. Stimulation frequency as shown in A. FIG.
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imitated by a stimulated contraction in which the frequency is continuously decreased. This procedure was repeated, and in addition to force, SRE and individual action potentials were recorded (Fig. 5). Not only was the time course of the MVC force decay matched, but also the electrical activity of the muscle. Individual action potentials were well maintained and similar in area to those obtained during an MVC. The SREs for the stimulated contractions were similar in form but had to be scaled down by a factor of approximately four to match that of the MVC. This reflects mainly the difference between the synchronized electrical stimulation of the muscle as opposed to the asynchronous voluntary input. DISCUSSION Jones et al. (12) compared the time courses of force decline during repetitive stimulation at different frequencies and during an MVC. They found that the force declined more slowly during a voluntary contraction than during prolonged nerve stimulation at a frequency high enough to match the initial force of a voluntary contraction. Thus, in fatigued muscle, high-frequency stimulation suppressed force generation. They concluded that this type of suppression must be reduced during prolonged voluntary contractions by a progressive decrease in motor neuron firing frequency. In the present work we examined the electrical activity of the muscle during similar contractions to see whether or not this also indicates changes in firing frequency, and also to investigate the causes of high-frequency force fatigue. Electrical Propagation Tetanic Stimulation. The excessive loss of force that occurs during continuous high frequency stimulation appears to be mainly due to failure of electrical propagation for it is accompanied by a rapid decline in the size of the evoked action potential, and largely recovers when the latter is restored. During the initial period of stimulation there is a marked broadening of the action potential with little loss of amplitude. This results in a substantial increase in total action potential area and is associated with a corresponding increase in conduction time. If the changes in conduction time are mainly due to slowing of conduction velocity along the muscle fiber membrane this would account for both the broadening and the increase in area of the action potential; it would also indicate a reduced muscle membrane excitability. If, on the other hand, the slowing of conduction and subsequent electrical failure are mainly in the presynaptic nerve terminals or result from increased synaptic delay, the compound action potential might broaden because of increased dispersion between units, but only with a loss of amplitude; its total area would not increase.
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Similar changes in both action potential area and conduction velocity can be seen in the records of Naess and Storm-Mathisen (19) when rabbit and human muscle were stimulated via the nerve at different frequencies. Slowing of conduction velocity during fatiguing human voluntary contractions were also observed by DeLuca and Forrest (7) Lindstrom et al. (14), and also by Mortimer et al. (18) in cat muscle stimulated both via the nerve and directly. Using direct stimulation of curarized rat diaphragm muscle, KrjneviE and Miledi (13) found that the fiber membrane threshold may increase several-fold during only a few minutes of high-frequency stimulation and that few fibers could conduct for more than a minute when stimulated continuously at 50 Hz. These findings were attributed mainly to changes in transmembranal cation concentrations. Mortimer ef al. (18) found that changes in conduction velocity induced by stimulating small areas of muscle while occluding the blood supply were almost abolished when the muscle was perfused with oxygen-free dextran solution. They concluded that the slowing was due to the accumulation of some metabolite, probably lactic acid, rather than to ischemia as such. A more likely explanation, however, is that there was a reduction in the [Na+] and an increase of [K+] in the extracellular fluid during activity, which would certainly slow conduction (6, 10). This could occur because (a) during contraction the intramuscular pressure occludes the circulation so that the muscle becomes a closed system and behaves like an isolated preparation (8); and (b) there may not be time between action potentials for the sodium pump to restore the Na+ and K+ exchange that results from the passage of each impulse. This would account for the finding in the previous study (12) that reducing the extracellular Na+ accelerated the rate of force fatigue in the isolated curarized preparation in the same way as did an increase in stimulus frequency. Changes in cation concentrations can be roughly calculated from estimates of total membrane area, cation fluxes, etc. KrnjeviE and Miledi (13) calculated that after 10,000 impulses at 50 Hz the intracellular Na+ in rat diaphragm muscle may rise by 70 mM; and there would be a similar decrease in intracellular K+. Because the interfiber inulin spaces in rat limb muscles are as low as one-fourth that of the intracellular volume (1 l), the changes in extracellular cation concentrations will be correspondingly greater. Mouse soleus muscle is inexcitable in 20 mM [K+]. Changes in extracellular cation concentrations could, thus, reduce the excitability of the muscle surface membrane sufficiently to account for all aspects of high-frequency force fatigue observed in these experiments, even in the total absence of neuromuscular block, whereas neuromuscular block alone could not. It seems more likely, however, that some
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presynaptic failure and block resulting from reduced end-plate potentials are also occurring simultaneously, but the relative contributions of each cannot be determined from the present experiments. Voluntary Contractions. The technique of recording evoked synchronous action potentials superimposed on a maximum voluntary contraction was used primarily to follow changes in electrical propagation at the neuromuscularjunction (16). Stephens and Taylor (20) used double shocks, making their measurements from the second impulse, to minimize interference by the voluntary activity. In preliminary experiments we found that double shocks did not change the measurements of area or improve the stability of the baseline which was not generally a problem. During an MVC the most notable feature of the recorded action potentials was that even at the end of the voluntary contractions they were well maintained and showed much less change than that associated with continuous high-frequency stimulation (Figs. 1, 5): There was neither the extreme prolongation nor the decline in amplitude. Instead there was an increase in area which was maintained throughout the contraction. Thus we may conclude that in adductor pollicis failure of electrical propagation, whatever the cause, cannot be the main reason for the loss of force during the first 60 s of a maximum voluntary contraction. These results differ from those of Stephens and Taylor (20) who found a 35% decrease in the size of the action potential after a 60-s sustained MVC of the first dorsal interosseous muscle. This finding, together with the simultaneous decline in the SRE, led them to conclude that fatigue resulted mainly from failure of neuromuscular transmission. It may be that the first dorsal interosseous muscle is more susceptible to neuromuscular block than is the adductor pollicis. Alternatively the discrepancy may result from the method of measuring action potential areas. We measured the whole area on either side of the isoelectric line, and these results correlate closely with changes in the surface EMG recorded simultaneously (Fig. 2). For example, the striking increase in SRE at the onset of high-frequency stimulation is paralleled by an equal increase in action potential area. Stephens and Taylor (20) measured only a portion of the total action potential area, and their method may not have adequately taken into account the slight changes in form of the action potential resulting from the slowing of conduction velocity (Figs. 4, 5). When measured their way we found the parallel between changes in the action potential size and surface EMG during high frequency stimulation was lost. Frequency Changes during a Prolonged Maximal Voluntary Contraction. Rectifying and smoothing the surface EMG activity is a commonly used averaging technique, and Woods et al. (21) showed that the signal produced during synchronous stimulation was proportional to the number
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of activated muscle fibers, the area of the individual action potentials, and to their frequency of activation. This is also shown in Figs. 1 and 3. Consequently if all fibers are active and the SRE and the area of the action potential are known, an estimate of changes in frequency can be made. During constant-frequency stimulation changes in action potential area were always accompanied by a parallel change in the SRE. During sustained MVC this was not so. Simultaneous recording of SRE and evoked action potentials show that for the subject shown in Fig. 4 the former decreased to approximately 40% of the initial value whereas the action potential area increased to 140% and remained near this value for the 60 s of voluntary contraction. If all motor neurons remained active, and if the SRE, evoked action potential area, and frequency during voluntary contractions have the same relationship as that observed during stimulated contractions, it follows that during the course of the 60-s MVC the natural firing frequency declined by a factor of 140/40 or 3.5 times, a value similar to that required during nerve stimulation to imitate a sustained MVC (Fig. 5). This conclusion makes two assumptions. First, the single supramaximal shock to the ulnar nerve activates the same number of fibers as are functioning during voluntary activity. If a proportion of motor neurons had stopped firing, even though the muscle fibers supplied by them were still capable of being stimulated, then the synchronous action potential would be disproportionately greater than the SRE recorded during the voluntary contraction. This is unlikely as the force never increased when a sustained MVC was interrupted by maximal nerve stimulation (12), although this can sometimes occur during fatigue of other muscles (3). Second, the changes in SRE observed during an MVC were assumed not to be due to changes in synchronization between individual motor unit action potentials. In Fig. 4 the SRE resulting from synchronous stimulation was four times greater than that from the asynchronous MVC for equal rates of force loss. Mimer-Brown and Stein (17) found that in contractions of unfatigued muscle there was little synchronization. If synchronization increases as fatigue progresses, the SRE in the later part of the contraction would increase rather than decline with time. It is not clear whether the changes in firing frequency occur in all motor neurons or only in those with the highest natural frequency. The matter can be resolved only by direct recording from different single motor unit types. But this is difficult during truly maximal contractions due to interference from surrounding units (1, 5). Direct evidence for a change in firing frequency was obtained by Marsden et al. (15) who recorded from aberrant motor units of the adductor pollicis innervated from the medial nerve, after total blockage of the ulnar nerve. Maximal contractions resulted in an initial discharge frequency of 180 Hz, decreasing rapidly to 50 Hz in the first
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second or so and thereafter to 20 Hz in the next 30 s, after which it remained relatively constant. These results are in the same range as the frequency changes we found most effective in matching both the force and electrical activity of an MVC. If such a reduction in motor neuron firing frequency does occur it appears to optimize force generation during fatiguing contractions by avoiding, or reducing, the failure of electrical propagation which would otherwise occur. REFERENCES 1. BIGLAND, B., AND 0. C. .I. LIPPOLD. 1954. Motor unit activity in the voluntary contraction of human muscle. J. Physiol. (London) 125: 322-335. 2. BIGLAND-RITCHIE, B., R. H. T. EDWARDS,,AND D. A. JONES. 1977. Fatigue in sustained isometric voluntary contractions. Physiology (Canada) 8: 28. 3. BIGLAND-RITCHIE, B., D. A. JONES, G. P. HOSKING, AND R. H. T. EDWARDS. 1978. Central and peripheral fatigue in sustained maximum voluntary contractions of human quadriceps muscle. Clin. Sci. Mol. Med. 54: 609-614. 4. BIGLAND-RITCHIE, B., J. J. WOODS, AND D. A. JONES. 1978. Electrical activity during fatigue of sustained maximal voluntary and stimulated contractions. In 4th International Congress on Neuromuscular Diseases, Abstr. 287. Montreal, Canada. 5. CLAMANN, H. P. 1970. Activity of single motor units during isometric tension. Neurology (Minneapolis) 20: 254-260. 6. COLQUHOUN, D., AND J. M. RITCHIE. 1972. The interaction at equilibrium between tetrodotoxin and mammalian non-myelinated nerve fibers. J. Physiol. (London) 221: 533-553. 7. DELUCA, C. J., AND W. J. FORREST. 1973. Some properties of motor unit action potential trains recorded during constant force isometric contractions in man. Kybernetik 12: 160-168. 8. EDWARDS, R. H. T. 1975. Muscle fatigue. Postgrad. Med. J. 51: 137-143. 9. EDWARDS, R. H. T., D. K. HILL, D. A. JONES, AND P. A. MERTON. 1977. Fatigue of long duration in human skeletal muscle after exercise. J. Physiol. (London) 272: 769-778. 10. HODGKIN, A. L., AND B. K. KATZ. 1949. The effect of sodium ions on the electrical activity of the giant axon of the squid. J. Physiol. (London) 108: 37-77. 11. HOH, J. F. Y ., AND B. SALAFSKY. 1971. Effects of nerve cross-union on rat intracellular potassium in fast-twitch and slow-twitch rat muscles. J. Physiol. (London) 216: 171-179. 12. JONES, D. A., B. BIGLAND-RITCHIE, AND R. H. T. EDWARDS. 1979. Excitation frequency and muscle fatigue: Mechanical responses during voluntary and stimulated contractions. Exp. Neurol. 64: 401-413. 13. KRNJEVI~, K., AND R. MILEDI. 1958. Failure of neuromuscular propagation in rats. J. Physiol. (London) 140: 440-461. 14. LINDSTROM, L., R. MAGNUSSON, AND I. PETERSEN. 1970. Muscular fatigue and action potential conduction velocity changes studied with frequency analysis of EMG signals. Electromyography 10: 341-356. 15. MARSDEN, C. D., J. C. MEADOWS, AND P. A. MERTON. 1971. Isolated single motor units in human muscle and their rates of discharge during maximal voluntary effort. J. Physiol. (London) 217: 12-13P. 16. MERTON, P. A. 1954. Voluntary strengthandfatigue. J. Physiol. (London) 128: 553-564.
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17. MILNER-BROWN, H. S., AND R. B. STEIN. 1975. The relation between the surface electromyogram and muscle force. J. Physiol. (London) 246: 549-569. 18. MORTIMER, J. T., R. MAGNUSSON, AND I. PETERSEN. 1970. Conduction velocity in ischaemic muscle: effect on EMG frequency spectrum. Am. J. Physiol. 219: 1324-
1329.
NAESS, K., AND A. STROM-MATHISON. 19.55. Fatigue of sustained tetanic contractions. Acta Physiol. Stand. 34: 351-366. 20. STEPHENS, J. A., AND A. TAYLOR. 1972. Fatigue of maintained voluntary muscle contraction in man. J. Physiol. (London) 220: l- 18. 21. WOODS, J. J., D. A. JONES, AND B. BIGLAND-RITCHIE. 1978. Components ofthe surface EMG during stimulated and voluntary contractions. Med. Sri. Sports 10(l): 67. 19.
