Afterhyperpolarization time-course and minimal discharge rate in low threshold motor units in humans

The alpha-motoneurone afterhyperpolarization (AHP) duration correlates with a number of its muscle unit properties in animal preparations. In humans, the interval death rate (IDR) analysis has been used to estimate the time course of human motoneurone AHP based on the pattern of motor unit firing. The purpose of this experiment was first, to examine the relationship between estimated AHP time course and the minimal firing rate of the motor unit and second, to examine the relationship between the AHP and motor unit contractile properties in the tibialis anterior (TA) muscle. Motor unit data were obtained from the TA muscle during low force isometric contractions lasting 600 s. Muscle unit twitch characteristics were determined using spike-triggered averaging (STA) and the motoneurone AHP time course was estimated using the IDR analysis. Minimal discharge rate and derecruitment threshold torque were determined for 2 s preceding motor unit derecruitment. The AHP time constant and minimal discharge rate were negatively correlated, whereas the derecruitment threshold torque was not associated with the AHP time constant. The estimated AHP duration, however, is considerably shorter than the mean ISI of the minimal discharge rate suggesting that synaptic noise and AHP duration are important factors in dictating the minimal discharge rate in low force voluntary contractions in humans. The AHP time constant did not vary significantly with motor unit twitch amplitude; however, significant positive relationships were found between the AHP time constant and the temporal properties of the motor unit twitch. The calculated AHP time course using the IDR analysis, therefore, is a reasonable estimate and coupled with motor unit properties attained with STA, it provides a powerful method to describe low-threshold motor units.

Sway-dependent modulation of the triceps surae H-reflex during standing

Previous research has shown that changes in spinal excitability occur during the postural sway of quiet standing. In the present study, it was of interest to examine the independent effects of sway position and sway direction on the efficacy of the triceps surae Ia pathway, as reflected by the Hoffman (H)-reflex amplitude, during standing. Eighteen participants, tested under two different experimental protocols, stood quietly on a force platform. Percutaneous electrical stimulation was applied to the posterior tibial nerve when the position and direction of anteroposterior (A-P) center of pressure (COP) signal satisfied the criteria for the various experimental conditions. It was found that, regardless of sway position, a larger amplitude of the triceps surae H-reflex (difference of 9–14%; P = 0.005) occurred when subjects were swaying in the forward compared with the backward direction. The effects of sway position, independent of the sway direction, on spinal excitability exhibited a trend ( P = 0.075), with an 8.9 ± 3.7% increase in the H-reflex amplitude occurring when subjects were in a more forward position. The observed changes to the efficacy of the Ia pathway cannot be attributed to changes in stimulus intensity, as indicated by a constant M-wave amplitude, or to the small changes in the level of background electromyographic activity. One explanation for the changes in reflex excitability with respect to the postural sway of standing is that the neural modulation may be related to the small lengthening and shortening contractions occurring in the muscles of the triceps surae.

Cortical and spinal modulation of antagonist coactivation during a submaximal fatiguing contraction in humans

This study investigates the control mechanisms at the cortical and spinal levels of antagonist coactivation during a submaximal fatiguing contraction of the elbow flexors at 50% of maximal voluntary contraction (MVC). We recorded motor-evoked potentials in the biceps brachii and triceps brachii muscles in response to magnetic stimulation of the motor cortex (MEP) and corticospinal tract (cervicomedullary motor-evoked potentials--CMEPs), as well as the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus, before, during, and after the fatigue task. The results showed that although the coactivation ratio did not change at task failure, the MVC torque produced by the elbow flexors declined by 48% (P < 0.01) with no change in MVC torque for the elbow extensors. While the MEP and CMEP areas (normalized to Mmax) of the biceps brachii increased ( approximately 50%) over the first 40% of the time to task failure and then plateaued, both responses in the triceps brachii increased ( approximately 150-180%) gradually throughout the fatigue task. In contrast to the monotonic increase in the MEP and CMEP of the antagonist muscles, the H-reflex of the triceps brachii exhibited a biphasic modulation, increasing during the first part of the contraction before declining subsequently to 65% of its initial value. Collectively, these results suggest that the level of coactivation during a fatiguing contraction is mediated by supraspinal rather than spinal mechanisms and involves differential control of agonist and antagonist muscles.

Control of the triceps surae during the postural sway of quiet standing

AIM

The present study investigated how the triceps surae are controlled at the spinal level during the naturally occurring postural sway of quiet standing.

METHODS

Subjects stood on a force platform as electrical stimuli were applied to the posterior tibial nerve when the center of pressure (COP) was either 1.6 standard deviations anterior (COP(ant)) or posterior (COP(post)) to the mean baseline COP signal. Peak-to-peak amplitudes of the H-reflex and M-wave from the soleus (SOL) and medial gastrocnemius (MG) muscles were recorded to assess the efficacy of the Ia pathway.

RESULTS

A significant increase in the H(max) : M(max) ratio for both the SOL (12 +/- 6%) and MG (23 +/- 6%) was observed during the COP(ant) as compared to the COP(post) condition. The source of the modulation between COP conditions cannot be determined from this study. However, the observed changes in the synaptic efficacy of the Ia pathway are unlikely to be simply a result of an altered level of background electromyographic activity in the triceps surae. This was indicated by the lack of differences observed in the H(max) : M(max) ratio when subjects stood without postural sway (via the use of a tilt table) at two levels of background activity.

CONCLUSIONS

It is suggested that the phase-dependent modulation of the triceps surae H-reflexes during the postural sway of quiet standing functions to maintain upright stance and may explain the results from previous studies, which, until now, had not taken the influence of postural sway on the H-reflex into consideration.

Factors Affecting the Common Modulation of Bilateral Motor Unit Discharge in Human Soleus Muscles

The purpose of this study was to determine the factors that influence the co-modulation of motor unit discharge rate in soleus muscles of both legs during upright standing. Single motor units were recorded from the left and right soleus muscles under three experimental conditions: standing quietly with the eyes open and closed, standing with the eyes closed while vibration was applied to one Achilles tendon, and swaying voluntarily or producing variable low-force isometric contractions at a frequency of 0.05 Hz. Correlations in motor unit discharge rate between left and right soleus motor units were assessed using common drive analysis. The results showed that common drive to motoneurons of the two muscles did not differ between standing with the eyes open or closed, but there was an order effect with the second task having significantly lower common drive than the first. Common drive was also significantly lower when vibration was applied to one leg compared with when no vibration was applied. Common drive was higher as subjects swayed anteriorly as compared with when they swayed posteriorly. There were no significant differences in common drive across phases of the variable isometric force contraction. Common drive was higher during voluntary sway than during variable force production; both of these values were significantly lower than those derived from the quiet standing task. These results suggest that proprioceptive and sub-cortical inputs contribute to the co-modulation of the firing rate of soleus motor unit pairs of the left and right leg during standing posture.

Reliability of the interval death rate analysis for estimating the time course of the motoneurone afterhyperpolarization in humans

The reliability of the afterhyperpolarization (AHP) time course, as estimated by the interval death rate (IDR) analysis was evaluated both within and between investigators. The IDR analysis uses the firing history of a single motor unit train at low tonic firing rates to calculate an estimate of the AHP time course [Matthews PB. Relationship of firing intervals of human motor units to the trajectory of post-spike after-hyperpolarization and synaptic noise. J Physiol 1996;492:597-628]. Single motor unit trains were collected from the tibialis anterior (TA) to determine intra-rater reliability (within investigator). Data from the first dorsal interosseus (FDI), collected in a previous investigation [Gossen ER, Ivanova TD, Garland SJ. The time course of the motoneurone afterhyperpolarization is related to motor unit twitch speed in human skeletal muscle. J Physiol 2003;552:657-64], were used to examine the inter-rater reliability (between investigators). The lead author was blinded to the original time constants and file identities for the re-analysis. The intra-rater reliability of the AHP time constant in the TA data was high (r(2)=0.88; p<0.001; ICC=0.91). The inter-rater reliability for the FDI data was also strong (r(2)=0.92; p<0.001; ICC=0.95). The standard error of measurement was 0.61 ms for the TA and 0.55 ms for FDI. It is concluded that the interval death rate analysis is a reliable tool for estimating the AHP time course with experienced investigators.

Recovery of standing balance and health-related quality of life after mild or moderately severe stroke

OBJECTIVE

To examine the physiologic and functional recovery of standing balance and health-related quality of life (HRQOL) in people after mild and moderate stroke.

DESIGN

Inception cohort study with evaluations at 1 month and 3 months poststroke.

SETTING

Laboratory.

PARTICIPANTS

Twenty-nine volunteers who had sustained a stroke. Subjects were categorized into mild and moderate groups.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Functional balance was assessed (Clinical Outcome Variables Scale [COVS]) and physiologic measures (electromyography, postural sway) were taken when subjects stood quietly on a force platform and when they performed a rapid unilateral arm-raise perturbation. The Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36) was administered to evaluate HRQOL.

RESULTS

Subjects in the mild group were approaching maximal scores on the COVS (87.7+/-4.1/91) at 3 months poststroke, yet had significant impairment in paretic muscle activation patterns when compared with healthy subjects. Subjects in the moderate group had increased paretic muscle activation over the 2 months, accompanied by significant increases of 10.7+/-5.9 points on the COVS. For both groups, there was significantly less postural sway on the paretic than the nonparetic leg and significant improvements in the SF-36 (physical component) over time.

CONCLUSIONS

Subjects recovering from a stroke showed a significant improvement in physical HRQOL and functional and physiologic balance, yet the physiologic balance recovery was not complete even in the mild group.

Low-frequency common modulation of soleus motor unit discharge is enhanced during postural control in humans

The maintenance of quiet stance requires the activation of muscles bilaterally. The soleus muscles in each leg share a common function in standing; that is, each muscle acts to control antero-posterior (AP) sway on its own side. We sought to determine the extent to which oscillations in motor unit discharge were related in motor unit pairs of the soleus muscles during postural and voluntary isometric tasks, both within and between legs. Subjects stood quietly for 5 min or performed a voluntary isometric plantarflexion contraction in a seated position. During the postural tasks, the excursions of AP sway between legs were highly correlated (rho = 0.86 +/- 0.06). The strength of common modulation of motor unit discharge rates was assessed using time- and frequency-domain analyses. The time-domain common drive analysis revealed that the strongest correlation in motor unit discharge modulation occurred in the postural task with unilateral pairs (rho = 0.71 +/- 0.13) being more strongly correlated than bilateral pairs (rho = 0.50 +/- 0.16). Common modulation of motor unit discharge was lowest for the voluntary tasks, with rho = 0.38 +/- 0.11 and 0.16 +/- 0.08 for unilateral and bilateral pairs, respectively. Similarly, the frequency-domain coherence analysis demonstrated an identical ordering effect, with the largest maximum pooled coherence occurring during standing posture in unilateral (0.070 at 1.6 Hz) and bilateral (0.055 at 1.6 Hz) recordings, whereas minimal coherence was observed in the voluntary task in both unilateral and bilateral recordings within the 0-5 Hz range. These results indicate that in the soleus muscle, common modulation of motor unit discharge is greater during postural tasks than during voluntary isometric tasks and can be observed in both bilateral and unilateral motor unit pairs. Differences in the extent of co-modulation of motor unit discharge between tasks may be attributed to either differences in the descending control or differences in the proprioceptive input between postural and isometric tasks.

Influence of Age and Gender of Healthy Adults on Scoring Patterns on the Community Balance and Mobility Scale

Purpose: The purpose of the study was to describe and compare scoring patterns on the Community Balance and Mobility Scale (CB&M) in healthy men and women between 30 and 59 years of age.

Methods: Following criterion testing on 5 individuals, 90 healthy volunteers who met the inclusion criteria and consented to participate in the study were tested on the CB&M. The subjects were recruited through quota sampling in three age categories: 30 to 39 years, 40 to 49 years, and 50 to 59 years, with 15 men and 15 women in each age category. The groups were not matched for any anthropometric variables.

Results: Women within the age category of 50 to 59 years had significantly lower scores on the CB&M compared with all other age and gender categories.

Conclusions: The results from this study indicate that although the items on the CB&M were appropriate for the 30- to 59-year age categories, some items still posed a challenge to healthy participants because most were unable to score full points on the CB&M. The normative data from the present study could help clinicians put the CB&M scores of middle-aged patients into context. These data may also be useful in making recommendations regarding the safe integration of patients back into the community following mild stroke.

Synchronization of motor units in human soleus muscle during standing postural tasks

During standing posture, the soleus muscles acts to control sway in the anteroposterior (AP) direction. The soleus muscles bilaterally share a common function during standing tasks. We sought to determine whether common descending inputs, as evidenced by the synchronization of bilateral motor unit pairs, were employed as a strategy to control this common function. Single motor units were recorded from the soleus muscles in subjects who stood on adjacent force platforms for 5 min with their eyes open or closed. While standing with the eyes open, only 4/39 bilateral motor unit pairs showed significant synchronization. Similarly, only 3/36 motor unit pairs were significantly synchronized during the eyes closed task. The low incidence of synchronization was observed despite a high correlation in the amount of sway in the AP direction between legs in both the eyes open and eyes closed tasks (rho = 0.80 and rho = 0.83, respectively). When the extent of synchronization was assessed between pairs of motor units within the same leg with the eyes open, 10/12 pairs were synchronized. Furthermore, when pairs of soleus motor units were recorded both bilaterally and unilaterally during voluntary isometric ankle plantarflexion, only 4/30 bilateral pairs showed significant synchronization, whereas 19/24 unilateral pairs had significant synchronization. In this study, there was little evidence of the existence of synchronization between bilateral soleus motor unit pairs in either postural tasks or voluntary isometric contractions. In cases in which bilateral synchronization was observed, it was considerably weaker than the synchronization of motor units within a single soleus muscle. The results of this study reveal that it is rather uncommon for bilateral soleus motoneurons to receive common descending synaptic inputs, whereas two motoneurons within a single soleus muscle do.

The time course of the motoneurone afterhyperpolarization is related to motor unit twitch speed in human skeletal muscle

The relationship between the electrophysiological properties of motoneurones and their muscle units has been established in animal models. A functionally significant relationship exists whereby motoneurones with long post-spike afterhyperpolarizations (AHPs) innervate slow contracting muscle units. The purpose of this study was to determine whether the time course of the AHP as measured by its time constant is associated with the contractile properties of its muscle unit in humans. Using an intramuscular fine wire electrode, 46 motor units were recorded in eight subjects as they held a low force contraction of the first dorsal interosseus muscle for approximately 10 min. By applying a recently validated transform to the interspike interval histogram, the mean voltage versus time trajectory of the motoneurone AHP was determined. Spike-triggered averaging was used to extract the muscle unit twitch from the whole muscle force with strict control over force variability and motor unit discharge rate (interspike intervals between 120 and 200 ms). The AHP time constant was positively correlated to the time to half-force decay (rho = 0.36, P < 0.05) and twitch duration (rho = 0.57, P < 0.001); however, time to peak force failed to reach significance (rho = 0.27, P < 0.07). These results suggest that a similar functional relationship exists in humans between the motoneurone AHP and the muscle unit contractile properties.

Modulation of motor unit discharge rate and H-reflex amplitude during submaximal fatigue of the human soleus muscle

Declining motor unit discharge rates and H-reflex amplitude have been observed in separate experiments during fatiguing submaximal contractions in humans. The purpose of this experiment was to investigate motor unit discharge rate, H-reflex amplitude, and twitch contractile properties concurrently during a fatiguing submaximal isometric contraction of the ankle plantarflexors. Eleven healthy subjects performed fatiguing contractions of low force (25% maximal voluntary contraction (MVC)) or high force (42-66% MVC). Hoffmann (H)-reflexes, muscle compound action potentials (M-waves), twitch contractile properties, and motor unit discharges were recorded from the soleus muscle. In the low-force fatigue task, motor unit firing rate increased gradually over time, whereas the resting H-reflex was significantly depressed at 15% of endurance time and remained quasiconstant for the rest of the task. This suggests that the processes mediating the resting H-reflex depression are relatively independent of those modulating the motor unit firing rate during a low-force fatigue task. In the high-force fatigue task, a decline in the average motor unit discharge rate was accompanied by a decrease in the resting H-reflex amplitude and a prolongation of the twitch half-relaxation time (HRT) at the completion of the fatigue task. Overall, motor unit firing rate was modulated in parallel with changes in the twitch HRT, consistent with the muscle wisdom hypothesis.

Postural muscle activity during bilateral and unilateral arm movements at different speeds

The muscle activation patterns in anterior and posterior leg muscles were investigated with two types of perturbations to standing balance. Subjects stood with each foot on adjacent force platforms and performed arm flexion movements to shoulder height. Nine subjects performed ten repetitions unilaterally and bilaterally at 100, 75, 50, 25, and 12.5% of maximal acceleration as measured by an accelerometer placed on the dominant hand. Four subjects also performed the fastest movements while leaning forwards and backwards. The area and latency of the EMG activity from the quadriceps (QUAD), hamstrings (BF), soleus (SOL), and tibialis anterior (TA) were measured bilaterally, along with the excursions of the center of pressure (COP) during each movement. In both unilateral and bilateral tasks, subjects showed a scaling of EMG area and COP excursion with the acceleration of the arm movement. Prior to movement onset, significant scaling of EMG area with movement speed occurred in both unilateral and bilateral tasks in most muscles. Following movement onset, EMG areas scaled significantly to movement speed in only the anterior musculature, with the exception of the left BF. The latency of BF was consistent for the four fastest movements. Only the slowest movements resulted in a significant rightward shift of the BF EMG latency. During the unilateral task, the ipsilateral hamstrings were activated significantly earlier than in the bilateral task and the contralateral hamstrings were activated significantly later. It was also observed that subjects utilized one of two different strategies to maintain balance. Five individuals displayed simultaneous anterior/posterior muscle activation while the other four displayed a reciprocal pattern of activation. Regardless of the initial standing position (leaning forwards or backwards), subjects used the same simultaneous or reciprocal activation strategy. The results indicate that muscle activation patterns change with different tasks, but remain the same during variations of the same task.

Ischemia sensitivity and motoneuron afterhyperpolarization in human motor units

The purpose of this study was to examine whether motor units, grouped by speed of contraction in the human first dorsal interosseous muscle, differed in their sensitivity to ischemia and motoneuron afterhyperpolarization (AHP) time-course. Motor units were recorded while subjects held an abduction force for approximately 10 min. Subsequently, subjects abducted for 4-5 min under ischemic conditions. Motor unit twitches derived using spike-triggered averaging were allocated into "fast" or "slow" contracting groups based on twitch time to peak (TTP) force. Motor units in the "slow" group had a greater sensitivity to ischemia than the "fast" group. When upper and lower quartiles of TTP were compared, motor units with slow TTP had long AHP time-constants (as estimated by an interspike interval histogram transform). Thus, motor units grouped by speed of contraction differed in both their sensitivity to ischemia and motoneuron AHP time-course. This provides preliminary evidence that the estimated AHP time-constant may be used to deduce motor-unit type in humans.

Recovery of standing balance and functional mobility after stroke11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated

OBJECTIVE

To examine the extent to which recovery of functional balance and mobility is accompanied by change in a few specific physiologic measures of postural control.

DESIGN

Longitudinal prospective study.

SETTING

Laboratory setting in Ontario.

PARTICIPANTS

Twenty-seven volunteers (age, 64.2+/-13.7y) undergoing 4 weeks of rehabilitation after stroke participated. At initial testing, patients were 32.7+/-18.4 days poststroke and exhibited a moderate level of motor recovery (lower-extremity and postural control, stages 3-4 on the Chedoke-McMaster Stroke Assessment Impairment Inventory).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Three functional measures (Berg Balance Scale, Clinical Outcome Variables Scale, gait speed) were assessed. Three physiologic measures (electromyographic data of hamstrings and soleus muscles bilaterally, postural sway, arm acceleration) were taken while subjects stood quietly on a force platform and while they performed a rapid shoulder flexion movement of the nonparetic upper extremity.

RESULTS

After 1 month of rehabilitation, there was an overall significant improvement in all outcome measures (functional, physiologic). However, 10 patients failed to show any improvement in the electromyographic activation of hamstrings muscle on the paretic side in response to the rapid arm movement. These patients compensated by increasing the anticipatory activation of the nonparetic hamstrings.

CONCLUSION

After stroke, patients showed improvement in both physiologic and functional measures of balance and mobility over a 1-month period. We have identified some patients who may be using compensatory strategies to increase function. The factors that may predict those patients who are likely to use compensatory strategies awaits further study.

Factors affecting the stability of the spike-triggered averaged force in the human first dorsal interosseus muscle

The reproducibility of motor unit twitches obtained using spike-triggered averaging (STA) was examined in the human first dorsal interosseus. For each motor unit (30 total) a series of STA twitches was derived using a 30 s averaging window. Within each averaging window, eight independent measures characterizing motor unit discharge and whole muscle force properties were recorded. These included the mean and standard deviation (S.D.) of the interspike interval (ISI), the mean and S.D. of pre and post-trigger ISIs used in averaging, and mean and S.D. of whole muscle force. To determine the relative importance of the independent variables on twitch reproducibility, the variables were used in a multiple regression analysis performed on STA twitch peak force (PF), time to peak force (TTP) and time of half-force decay (HFD). It was found that PF was significantly correlated to the mean and S.D. of whole muscle force, and mean post-trigger ISI. TTP was significantly correlated to the S.D. of the post-trigger ISI and mean whole muscle force while HFD was related to the mean and S.D. of the pre-trigger ISI and the mean post-trigger ISI. It was concluded that by minimizing whole muscle force variability and the mean and S.D. of acceptable ISIs used in the STA process, the reproducibility of the STA twitch is improved.

Effect of force level and training status on contractile properties following fatigue

This study examined the effects of fatigue on the contractile properties of the twitch contraction evoked by the H-reflex and the maximal M-wave. Untrained subjects demonstrated more pronounced slowing of half relaxation time (HRT) in the H-reflex twitch than in the M-wave twitch and also more slowing of HRT in high force vs. low force contractions. Endurance-trained subjects, however, demonstrated a shortening of the H-reflex twitch. The twitch contractile properties following fatigue are dependent on force level and training status.

Changes in motor unit discharge rate are not associated with the amount of twitch potentiation in old men

This study examined, in nine old men (82 +/- 4 yr), whether there is an association between the magnitude of change in motor unit discharge rate and the amount of twitch potentiation after a conditioning contraction (CC). The evoked twitch force and motor unit discharge rate during isometric ramp-and-hold contractions (10-18 s) of the triceps brachii muscle at 10, 20, and 30% of the maximal voluntary contraction were determined before and 10 s, 2 min, 6 min, and 11 min after a 5-s CC at 75% maximal voluntary contraction. After the CC, there was a potentiation of twitch force (approximately twofold), and the discharge rate of the 47 sampled motor units declined (P < 0.05) an average of 1 Hz 10 s after the CC, compared with the control condition. The CC had no effect on the variability (coefficient of variation) of both force and discharge rate, as well as the electromyographic activity recorded over the triceps brachii and biceps brachii muscles. In contrast to our earlier study of young men (Klein CS, Ivanova TD, Rice CL, and Garland SJ, Neurosci Lett 316: 153-156, 2001), the magnitude of the reduction in discharge rate after the CC was not associated (r = 0.06) with the amount of twitch potentiation. These findings suggest an age-related alteration in the neural strategy for adjusting motor output to a muscle after a CC.

Reflex inhibition during muscle fatigue in endurance-trained and sedentary individuals

Reflex inhibition of the motoneuron pool following fatiguing contractions may be mediated by the build-up of byproducts of fatigue. Endurance training is accompanied by neuromuscular adaptations that would alter the production and/or clearance of metabolic substrates. The purpose of the study was to determine the extent of reflex inhibition during and after fatigue in endurance-trained individuals compared to sedentary controls. Subjects produced isometric ankle plantarflexion contractions at 30% of maximal voluntary contraction (MVC) until their MVC torque declined by 30%. H-reflexes were measured during a brief rest period every 3 min as well as superimposed upon the contraction every minute. Both groups of subjects experienced a similar amount of reflex inhibition by the end of the fatiguing protocol, although the endurance time was twice as long for the endurance-trained subjects. The endurance-trained subjects showed a greater reduction in H-reflex amplitude early in the fatiguing protocol compared to the sedentary subjects. These experiments have demonstrated that the neuromuscular processes associated with fatigue-related reflex inhibition must be multi-faceted and cannot be explained solely by small-diameter afferents responding to the byproducts of muscle contraction.

The muscular wisdom hypothesis in human muscle fatigue

The muscular wisdom hypothesis proposed that the slowing of the motor unit discharge rate during sustained maximal isometric contractions serves to minimize fatigue. The purpose of this review is to examine the applicability of the muscular wisdom hypothesis during other forms of contraction, i.e., prolonged submaximal isometric or dynamic contractions.

Motor unit discharge rate following twitch potentiation in human triceps brachii muscle

It has been proposed that during brief voluntary contractions, twitch potentiation may sustain force output despite a decline in motor unit discharge rate. This study examined the evoked twitch force and motor unit discharge rates during submaximal voluntary contractions of the triceps brachii muscle before and after a 5 s conditioning contraction (CC) at 75% of maximal voluntary force. After the CC, twitch force potentiated ( approximately 1.3-2-fold), and the discharge rate in 33 of 35 motor units declined significantly by 1-6 Hz. The increase in twitch force was significantly correlated with the decline in discharge rate (r=-0.74). These findings suggest that the extent of the decrease in motor unit discharge rate following a CC is associated with the magnitude of twitch potentiation.

Muscle vibration sustains motor unit firing rate during submaximal isometric fatigue in humans

1. In keeping with the 'muscular wisdom hypothesis', many studies have documented that the firing rate of the majority of motor units decreased during fatiguing isometric contractions. The present study investigated whether the application of periodic muscle vibration, which strongly activates muscle spindles, would alter the modulation of motor unit firing rate during submaximal fatiguing isometric contractions. 2. Thirty-three motor units from the lateral head of the triceps brachii muscle were recorded from 10 subjects during a sustained isometric 20 % maximal voluntary contraction (MVC) of the elbow extensors. Vibration was interposed on the contraction for 2 s every 10 s. Twenty-two motor units were recorded from the beginning of the fatigue task. The discharge rate of the majority of motor units remained constant (12/22) or increased (4/22) with fatigue. Six motor units demonstrated a reduction in discharge rate that later returned toward initial values; these motor units had higher initial discharge rates than the other 16 motor units. 3. In a second series of experiments, four subjects held a sustained isometric 20 % MVC for 2 min and then vibration was applied as above for the remainder of the contraction. In this case, motor units initially demonstrated a decrease in firing rate that increased after the vibration was applied. Thus muscle spindle disfacilitation of the motoneurone pool may be associated with the decline of motor unit discharge rate observed during the first 2 min of the contraction. 4. In a third set of experiments, seven subjects performed the main experiment on one occasion and repeated the fatigue task without vibration on a second occasion. Neither the endurance time of the fatiguing contraction nor the MVC torque following fatigue was affected by the application of vibration. This finding calls into question the applicability of the muscular wisdom hypothesis to submaximal contractions.

Blood flow in the triceps brachii muscle in humans during sustained submaximal isometric contractions

The main purpose of this study was to determine the extent to which blood flow through the profunda artery within the triceps brachii muscle may be compromised during maintained low-force isometric fatiguing contractions. Doppler ultrasound techniques were used to record mean blood velocity and arterial diameter of the profunda brachii artery during sustained isometric contractions of 20% maximal voluntary contraction. The arterial diameter did not change throughout the contraction. Thus, blood velocity was considered to be an indicator of blood flow. The mean blood velocity increased initially and then remained constant during the contraction period. When compared to rest [0.06 (SD 0.03) m s-1] mean blood velocity was significantly larger at the start of the contraction [0.13 (SD 0.07) m s-1] and larger yet during recovery following the contraction [0.30 (SD 0.14) m s-1]. Although blood flow through the conduit artery did not drop during the contraction, the post-contraction hyperaemia suggested that circulatory compromise might have occurred at the level of the capillary beds.

Role of limb movement in the modulation of motor unit discharge rate during fatiguing contractions

Motor unit firing rates of the triceps brachii muscle have been shown to decline during sustained isometric contractions, but not if the fatiguing contraction incorporates arm movements. The purpose of this study was to determine the impact of the actual physical displacement of the limb on the maintenance of motor unit discharge rate during dynamic muscle fatigue. An isometric force pulse paradigm was used to recreate the motor unit activity patterns that occur during a dynamic contraction. With this paradigm, the variable force output that would occur during a dynamic contraction remained intact, but the movement of the limb was eliminated. Motor unit firing rates declined in the isometric force pulse protocol. Thus, factors related to the actual movement of the limb appear to enable the maintenance of motor unit discharge rates during fatigue.

Motor unit double discharges: statistical anomaly or functional entity?

Motor unit double discharges, or doublets, have been described as two consecutive motor unit discharges that occur with a short interspike interval of 2.5 - 20 ms (Simpson, 1969). Double discharges have been reported in the literature for over 70 years. For instance, Eccles and Hoff (1932) found that double discharges were elicited occasionally at the onset of a crossed extension reflex in the soleus muscle of the anaesthetized cat. With the use of electrical stimulation protocols, short interspike intervals inserted at the beginning of a stimulation train have been shown to increase both the peak force and rate of rise of force production, and also decrease the range of fatigue. The extent to which double discharges occur in naturally-occurring voluntary behaviours remains relatively unexplored. This review examines the issue of whether double discharges occur solely because of an intrinsic property of motoneurones, thereby representing a "statistical anomaly," or whether they may result from a neural control strategy to augment force production, i.e., a "functional entity."

Discharge patterns in human motor units during fatiguing arm movements

The purpose of this study was to determine whether short interspike intervals (ISIs of <20 ms) would occur naturally during voluntary movement and would increase in number with fatigue. Thirty-four triceps brachii motor units from nine subjects were assessed during a fatigue task consisting of fifty extension and fifty flexion elbow movements against a constant-load opposing extension. Nineteen motor units were recorded from the beginning of the fatigue task; the number of short ISIs was 7.1 +/- 4.1% of the total number of ISIs in the first one-third of the task (unfatigued state). This value increased to 11.8 +/- 5.9% for the last one-third of the task (fatigued state). Fifteen motor units were recruited during the fatigue task and discharged, with 16.4 +/- 6.0% of short ISIs in the fatigued state. For all motor units, the number of short ISIs was positively correlated (r2 = 0.85) with the recruitment threshold torque. Short ISIs occurred most frequently at movement initiation but also occurred throughout the movement. These results document the presence of short ISIs during voluntary movement and their increase in number during fatigue.

Motor unit discharge rate is not associated with muscle relaxation time in sustained submaximal contractions in humans

The muscle wisdom hypothesis suggests that motor unit discharge rates decrease in parallel with the slowing of muscle contractile properties during fatigue. In this study, the discharge rates of single motor units and the muscle contractile properties were measured during a sustained submaximal contraction. The majority of motor units that were active from the beginning of the task demonstrated a decline in discharge rate in the absence of any slowing of muscle relaxation time. These findings suggest that the muscle wisdom hypothesis may not apply to sustained submaximal contractions.

Postural responses to unilateral arm perturbation in young, elderly, and hemiplegic subjects

OBJECTIVE

To compare postural responses during standing associated with self-paced unilateral arm flexion exhibited by young and elderly healthy subjects and subjects with hemiplegia from stroke.

DESIGN AND SETTING

Descriptive cross-sectional study in a laboratory setting.

PATIENTS

Ten young, 10 elderly healthy subjects, and 12 volunteers with longstanding hemiparesis following stroke were tested. The hemiplegic group had good functional balance scores on the Berg Balance Scale (BBS).

MAIN OUTCOME MEASURES

The peak arm acceleration, center of pressure (CP) excursion speed, and the electromyographic activity in the posterior leg muscles (bilateral hamstrings and triceps surae) were monitored during self-initiated rapid unilateral arm flexion and during quiet stance.

RESULTS

During unilateral arm flexion, the young group showed the highest arm accelerations and lowest CP excursion speeds. The variability of postural responses was largest in the elderly and hemiplegic groups with hemiplegic subjects showing the greatest CP excursion speeds for the lowest arm accelerations. The first burst of activity in ipsilateral hamstrings muscle was the same in all subjects. However, the hemiplegic group had less activation (latency and amplitude) of other posterior leg muscles.

CONCLUSION

The elderly subjects had more variable responses to perturbation than the young subjects despite similar BBS scores. This may indicate that the BBS fails to detect differences in balance at the high end of the scale. Although the hemiplegic subjects demonstrated some anticipatory control of standing balance, they consistently performed poorer than elderly and young controls. The failure to coordinate postural leg muscle activity with focal movement may contribute to the instability of subjects with hemiplegia.

Control of motor units in human flexor digitorum profundus under different proprioceptive conditions

1. Changing the posture of the human fingers can functionally 'disengage' the deep finger flexor muscle from its normal action on the terminal phalanx of the fourth (or third) finger. This enables the activity of the muscle to be studied both with and without its normal proprioceptive inputs. 2. Spike trains of long duration from pairs of concurrently active motor units in this muscle were recorded in both the engaged and disengaged hand postures. Subjects voluntarily kept one of the motor units (the 'controlled' unit) discharging at the same target frequency in both postures. The strength of short-term synchrony, the strength of common drive, and the variability of discharge of these pairs of motor units were determined in both postures. 3. All subjects reported that the effort required to activate the motor units in the disengaged hand posture was substantially greater than in the normal engaged posture. 4. Short-term synchrony, which is a function of common corticospinal inputs to pairs of motor units, was similar in both hand postures. However, the strength of common drive was significantly decreased when the muscle was disengaged. Although the neural substrate for common drive is not known, this observation suggests that proprioceptive feedback is involved either directly or indirectly. 5. Although the discharge rate of the 'uncontrolled' motor units increased when the muscle was disengaged, the variability of discharge of these and the 'controlled' motor units increased significantly. This supports the idea that the precision with which fine motor tasks can be performed is improved when proprioceptive feedback is intact.

Motor unit recruitment and discharge behavior in movements and isometric contractions

The purpose of this study was to contrast the discharge patterns of the same motor units during movements and during isometric contractions that were produced with comparable torque-time characteristics. Subjects performed elbow flexion and extension movements with predetermined acceleration characteristics. The average acceleration and deceleration profiles for the movements were reproduced in the isometric setting by presenting the kinematic profiles as templates for torque production. Trained subjects were able to match the first agonist (AG1) and antagonist (ANT) electromyographic (EMG) bursts, but tended to produce a smaller second agonist burst (AG2) in the isometric contraction. Twenty-five motor units from triceps brachii were studied. The same motor units (with one exception) were recruited and subsequently discharged in a similar fashion in both the isometric and movement tasks in the AG1 and ANT EMG bursts, with fewer motor unit discharges in the AG2 burst in the isometric contraction. The central control mechanisms appear to be the same for the acceleration phase of movement and isometric contraction, but differ during the deceleration phase.

Responses of human single motor units to transcranial magnetic stimulation

Transcranial electromagnetic brain stimuli elicit a complex response in the electromyogram of active human hand muscles. Relatively weak stimuli evoke a short-latency primary response via a presumably monosynaptic corticospinal path. This is followed by a silent period that is terminated by a second peak at a latency of 50-80 ms. The responses evoked in single motor units in flexor digitorum profundus (FDP) were recorded. Responses were elicited at the second-peak latency only in trials in which no primary response was elicited in that unit, and only when the stimulus was given during the first half of the interspike interval (ISI). When given during the second half of the ISI, the same stimulus evoked a primary response but no second peak response. Stronger stimuli suppressed the second peak by evoking a primary response in more trials. Having discharged at about 20 ms latency, the parent motoneurone was unable to discharge again at second-peak latency, 30-60 ms later. The response at second-peak latency was not modified by disengaging both FDP and the extensors of the distal interphalangeal joint. Hence, this response is not secondary to a stretch reflex provoked by activation of the finger extensors, nor is it the result of a cutaneous signal resulting from movement of the finger. The latencies suggest that the corticospinal volley evokes a beta-motoneurone-mediated twitch in FDP muscle spindles, which elicits an afferent volley that activates the motoneurone reflexly. The first 100 ms or so of the silent period is due to the realignment of the first post-stimulus spike in most trials to corticospinal latency; i.e. this is not necessarily the result of an inhibitory or disfacilitatory process. Still stronger stimuli increase the duration of the ISI in which the stimulus is given, indicating the presence of an inhibitory/disfacilitatory process.

Motor unit activity during human single joint movements

1. To explore the neural control of single joint movements in humans, the activity of 47 motor units in triceps brachii muscle was recorded during elbow flexion and extension movements. Movements were performed with different but changing deceleration magnitudes, while the acceleration magnitude was kept constant, to determine the relationship between motor unit activity and the acceleration and deceleration characteristics of single joint movements. 2. The number of motor unit action potentials was found to vary with the magnitude of the movement deceleration. In addition the duration of the discharge of a motor unit was found to parallel the duration of the acceleration phase of the movement, when the acceleration duration was varied while acceleration magnitude was kept constant. 3. Approximately half of the recorded motor units in triceps brachii were active both in the initiation and in the termination of the extension movements. However, motor units were identified that participated in specific phases of the movement (i.e., either during the 1st agonist or 2nd agonist burst of muscle activity) depending on the magnitude of the acceleration or deceleration. 4. During flexion movements, when the triceps muscle served as an antagonist, approximately half of the motor units were recruited only when the magnitude of the flexion deceleration was large. Further, this deceleration magnitude was larger than that evident during the extension movements in which the motor unit discharged. 5. The findings of this study demonstrate that the nervous system activates the same motor units whether the muscle is functioning as an agonist or antagonist so as to control the characteristics of acceleration and deceleration of single joint movements.

Standing balance during internally produced perturbations in subjects with hemiplegia: validation of the balance scale

OBJECTIVE

To examine the concurrent validity of the Berg Balance Scale (BBS) in subjects with hemiparesis after stroke by correlating functional balance performance on the BBS with laboratory measures of balance (center of pressure [CP] and electromyographic [EMG] activity of leg musculature during rapid arm flexion and during quiet stance).

DESIGN

Concurrent validation study. Consistency of performance was determined through repeat measures obtained from 13 subjects.

SETTING

Laboratory.

PATIENTS

Twenty-four volunteers with longstanding hemiparesis from cerebrovascular accident (CVA) were tested.

MAIN OUTCOME MEASURES

BBS scores were compared with CP excursion speed during (1) self-initiated rapid unilateral arm flexion with the nonhemiplegic arm and (2) quiet stance. The peak tangential arm acceleration and the sequence of EMG events in the posterior leg muscles were monitored during the arm flexion movement.

RESULTS

In the arm flexion condition, BBS performance was significantly correlated with arm acceleration and CP excursion speed (multiple regression R = .81). BBS performance was also related to the presence of anticipatory activation of ipsilateral hamstrings during the rapid arm flexion movement. BBS performance and CP excursion speed during quiet stance were also significantly related (r = .76). The initial and repeat measures were not significantly different in magnitude (paired tests of difference, p < .01).

CONCLUSION

The BBS appears to reflect differing abilities to tolerate the internally produced perturbation to standing balance associated with forward arm flexion in individuals with hemiparesis from CVA.

Motor-unit behavior in humans during fatiguing arm movements

1. The activity of 40 triceps brachii motor units was recorded from the dominant arms of 9 healthy adult volunteers (age 27.8 +/- 4.4 yr, mean +/- SD) during a fatigue task that included both isometric and anisometric contractions. The fatigue task lasted 8.3 min and consisted of 50 extension and 50 flexion movements of the elbow. Each movement (40 degrees in 0.8s) was separated by an isometric contraction. A constant load resisting extension of 17.7 +/- 3.0% of maximal voluntary contractions (MVC) was applied throughout the task. This paradigm enabled the direct contrast of motor-unit discharge behavior during the different types of fatiguing contractions. 2. Motor-unit behavior was examined to determine the relative contribution of two mechanisms for optimizing force production under fatiguing conditions: recruitment of motor units and modulation of motor-unit discharge following recruitment. Threshold torques for motor-unit recruitment thresholds were determined by ramp-and-hold isometric contractions. Motor-unit discharge was evaluated during the fatigue task by contrasting the number of motor-unit potentials (spikes) per contraction for concentric eccentric, and isometric contractions. 3. The fatigue task resulted in a 30 +/- 12% decline in the mean MVC of elbow extension. Recruitment of nine new motor units (23%) was evident during the fatiguing extension movements, often within five to seven movements (i.e., within 25-35 s). Each newly recruited motor unit had the largest recruitment threshold torque in that experiment. 4. Analysis of the motor units that were active from the beginning of the fatigue task revealed that the mean number of motor-unit spikes per contraction increased, or remained constant as fatigue ensued, yet for the majority of motor units it increased or remained constant. None of the newly recruited motor units demonstrated decreased number of mean spikes per contraction after recruitment. Further, concurrently active motor units displayed different discharge behavior in two-thirds of the subjects. It is proposed that if the neural drive to the muscle is distributed uniformly upon the motoneuron pool, peripheral feedback from the exercising muscle may modulate specific motoneuron discharge levels during fatigue.

Task-dependent factors in fatigue of human voluntary contractions

This chapter explores the hypothesis that fatigue is not caused uniquely by any common set of factors, but rather that the amount of stress placed on each site depends on the type of exercise from which fatigue develops. Evidence supporting this idea is presented by comparing results from various studies in which fatigue was caused by different exercise protocols. However, the way in which human endurance capacity changes with the type or intensity of the task performed suggest a unitary process. Thus, perhaps the neuromuscular system as a whole is so well adjusted that any task-related additional impairment at one site is compensated by corresponding functional improvements at others. We suggest that nature has had a long time in which to "get it right".

Role of muscle afferents in the inhibition of motoneurons during fatigue

In conscious humans, fatiguing muscular contractions are accompanied by a decrease in the discharge rate of alpha motoneurons. The association between alpha motoneuron discharge rate and the generation of force by skeletal muscle has been called "muscle wisdom" (Marsden et al., 1983). Its purpose is believed to ensure that central neural drive to skeletal muscle, which is fatigued, matches that needed to generate the required force. In addition, muscle wisdom may be one mechanism that functions either to decrease or to postpone central neural fatigue (Enoka & Stuart, 1992). Bigland-Ritchie and colleagues (1986) have suggested that a reflex arising from fatigued skeletal muscle is responsible, at least in part, for muscle wisdom. This chapter has two purposes. The first is to evaluate the evidence that a reflex arising from fatigued skeletal muscle causes muscle wisdom, and the second is to examine the discharge properties of muscle afferents to determine which ones are most likely to initiate reflexly this phenomenon.

Behavior of motor units in human biceps brachii during a submaximal fatiguing contraction

The activity of 50 single motor units was recorded in the biceps brachii muscle of human subjects while they performed submaximal isometric elbow flexion contractions that were sustained to induce fatigue. The purposes of this study were to examine the influence of fatigue on motor unit threshold force and to determine the relationship between the threshold force of recruitment and the initial interimpulse interval on the discharge rates of single motor units during a fatiguing contraction. The discharge rate of most motor units that were active from the beginning of the contraction declined during the fatiguing contraction, whereas the discharge rates of most newly recruited units were either constant or increased slightly. The absolute threshold forces of recruitment and derecruitment decreased, and the variability of interimpulse intervals increased after the fatigue task. The change in motor unit discharge rate during the fatigue task was related to the initial rate, but the direction of the change in discharge rate could not be predicted from the threshold force of recruitment or the variability in the interimpulse intervals. The discharge rate of most motor units declined despite an increase in the excitatory drive to the motoneuron pool during the fatigue task.

Association between muscle architecture and quadriceps femoris H-reflex

The purpose of the present study was to establish the relationship between muscle architecture and H-reflex recordings in quadriceps femoris muscle. H-reflexes were elicited in human quadriceps femoris muscle over a broad area of skin to document the shape and amplitude of the H-potentials. This, in combination with recording monopolar and bipolar H-potentials, was performed to determine the location and method for measuring maximum-amplitude H-reflexes. The influence of neural and peripheral factors on the H-potential during passive length changes was studied by comparing the amplitude of H-potentials to motor unit action potentials. Monopolar recordings of the H-potential were found to be preferable to bipolar recordings because of the reproducibility of shape and easier distinction between the M- and H-potentials. The location for recording maximum H-potentials was in the distal one third of the quadriceps femoris muscle, over the border between vastus lateralis and rectus femoris. The inferred relationship between H-potential amplitude and reflex excitability must be made with caution in quadriceps femoris muscle because the amplitude of both the motor unit potential and H-potential change as a function of muscle length.

Lack of task-related motor unit activity in human triceps brachii muscle during elbow movements

Surface electromyographic (EMG) recordings have been associated with the acceleration and deceleration characteristics of single joint elbow movements [J. Neurophysiol., 63 (1990) 465-472]. To explore further the neural control of single joint movements, we investigated the behavior of motor units in triceps brachii muscle during elbow flexion and extension movements. In this communication, we provide evidence that individual motor units can contribute to each of the surface EMG bursts. This finding suggests that the nervous system can use the same motor units during movement regardless of whether the muscle is acting as an agonist or antagonist or functioning to initiate or terminate the movement.

Prolonged depression of force developed by single motor units after their intermittent activation in adult cats

The fatigue of fast-twitch, glycolytic mammalian motor units [i.e., type FF; nomenclature of (3)] is dependent, in part, on the stimulation regimen (total number of stimuli, frequency, duty cycle, temporal patterning of stimuli, etc.) used to induce fatigue. To study the effect of the temporal pattern of the stimulus train on the rate and extend of fatigue in single FF units, one theoretically acceptable approach would be to use each motor unit as its own control: i.e., a sequential testing with two fatigue tests that differ only in the temporal organization of their stimuli. The purpose of this communication is to provide evidence that such an approach is not feasible when studying FF units, due to the delayed recovery of force following their repetitive activation. It was shown that 1/s activation of single FF units for only 15 or 45 s with intermittent 40-Hz, 300-ms duration trains significantly reduced their force response to a double-pulse shock for several hours. This finding suggests that in studies designed to test for the effects of different stimulation patterns on the fatigue of single motor units, deeply anaesthetized, reduced animal preparations are not appropriate models for the sequential application of different stimulation regimens to fast-twitch, glycolytic, mammalian motor units.

Role of small diameter afferents in reflex inhibition during human muscle fatigue

1. Previous work has shown that the H reflex excitability of the human soleus motoneurones is reduced during fatigue and is accompanied by a corresponding decrease in electromyographic (EMG) activity during maximal voluntary contractions. These findings were consistent with the existence of a reflex whereby alpha-motoneurones are inhibited by sensory input from the fatigued muscle. 2. To elucidate the contribution of different-sized afferents in such reflex inhibition, compression of the sciatic nerve was used in an attempt to block large myelinated afferents prior to fatigue. 3. Fatigue of the soleus muscle was induced under ischaemic conditions by intermittent electrical stimulation at 15 Hz in ten healthy subjects. These subjects also participated in a control test in which the compression block was followed by ischaemia without fatigue. 4. Following nerve compression alone, both the mean maximal plantarflexion torque and the associated EMG for all ten subjects declined by 18.8 +/- 16.2% (S.D.) and 13.4 +/- 17.2%, respectively. 5. Following fatigue, there were five subjects in whom the large afferents remained blocked and the experimental findings were consistent with the existence of reflex inhibition during fatigue. The mean maximal plantarflexion torque decreased further by 36.2 +/- 7.6% from the value following the compression block compared to a decrease of 5.0 +/- 9.9% in the ischaemia control. The mean EMG associated with these contractions also decreased from post-block values by 56.8 +/- 19.6% following fatigue and by only 6.4 +/- 8.0% following ischaemia alone. 6. The peripheral excitability of the neuromuscular junction and muscle fibre membrane was adequate following fatigue as evidenced by only modest changes in the M wave (muscle compound action potential). The descending motor drive was deemed sufficient because of the absence of any large interpolated twitches superimposed upon the maximal voluntary contraction in all but two subjects. 7. The declines in maximal plantarflexion torque and the associated EMG activity were very similar to those found in a previous study in which the sensory input was unaltered. The findings demonstrated that any reflex inhibition of the alpha-motoneurone pool during fatigue was probably not mediated by large diameter afferents. Rather, it is suggested that the reflex is mediated by smaller diameter afferents originating from the fatigued muscle.

Reflex inhibition of human soleus muscle during fatigue

1. Human soleus muscles were fatigued under ischaemic conditions by intermittent stimulation at 15 Hz. When maximal voluntary plantarflexion was then attempted, the loss of torque was found to be associated with a reduction in voluntary EMG activity. 2. The decrease in EMG activity could not have been due to 'exhaustion' of descending motor drive in the central nervous system since fatigue had been induced by electrical stimulation of peripheral nerve fibres. Similarly, the decrease could not be explained by changes at the neuromuscular junction or muscle fibre membrane, since changes in the M wave (evoked muscle compound action potential) were relatively modest. 3. When the excitability of the soleus motoneurones was tested during fatigue, using the H (Hoffmann) reflex, it was found to be significantly reduced. Control experiments with ischaemia or electrical stimulation, but without fatigue, failed to demonstrate any significant effects on reflex excitability. 4. The findings in this study favour the concept of reflex inhibition of alpha-motoneurones during fatigue.

Reduced voluntary electromyographic activity after fatiguing stimulation of human muscle

1. After ischaemic ankle dorsiflexor muscles had been fatigued by repetitive stimulation of the peroneal nerve at 15 Hz, there was a reduction in voluntary EMG activity which persisted as long as the arterial cuff remained inflated. 2. The reduction in voluntary EMG activity could not have been due to loss of excitability at the neuromuscular junctions or muscle fibre membranes since the M-waves (muscle compound action potentials) evoked by peroneal nerve stimulation were well maintained. 3. The preceding observations were consistent with the view that the reduction in EMG activity was due to reflex inhibition of motoneurones by afferents from the fatigued muscle. 4. The absence of responses to stimuli interpolated among the voluntary activity indicated that any motor units which could not be recruited in the fatigued muscle were no longer capable of generating tension.

Relationship between numbers and frequencies of stimuli in human muscle fatigue

The effect of stimulus frequency on the rate of muscle fatigue has been studied on dorsiflexor muscles of the human ankle. It was found that significantly fewer stimuli were required to abolish twitch and tetanic torque when the stimuli were delivered at 15 Hz rather than 30 Hz. At both stimulus frequencies twitch torque disappeared before tetanic torque. The difference in numbers of stimuli required for fatigue was not due to impaired excitation of muscle fibers at either of the two frequencies. At both stimulating frequencies, twitch fatigue appeared to be due to a defect in excitation-contraction coupling and/or the contractile machinery.