Changes in human alpha-motoneuron excitability during sustained maximum isometric contractions

Electromyography-driven musculoskeletal models with time-varying fatigue dynamics improve lumbosacral joint moments during lifting

Muscle fatigue is prevalent across different aspects of daily life. Tracking muscle fatigue is useful to understand muscle overuse and possible risk of injury leading to musculoskeletal disorders. Current fatigue models are not suitable for real-world settings as they are either validated using simulations or non-functional tasks. Moreover, models that capture the changes to muscle activity due to fatigue either assume a linear relationship between muscle activity and muscle force or utilize a simple muscle model. Personalised electromygraphy (EMG)-driven musculoskeletal models (pEMS) offer person-specific approaches to model muscle and joint kinetics during a wide repertoire of daily life tasks. These models utilize EMG, thus capturing central fatigue-dependent changes in multi-muscle bio-electrical activity. However, the peripheral muscle force decay is missing in these models. Thus, we studied the influence of fatigue on a large scale pEMS of the trunk. Eleven healthy participants performed functional asymmetric lifting task. Average peak body-weight normalized lumbosacral moments (BW-LM) were estimated to be 2.55 ± 0.26 Nm/kg by reference inverse dynamics. After complete exhaustion of the lower back, the pEMS overestimated the peak BW-LM by 0.64 ± 0.37 Nm/kg. Then, we developed a time-varying muscle force decay model resulting in a time-varying pEMS (t-pEMS). This reduced the difference between BW-LM estimated by the t-pEMS and reference to 0.49 ± 0.14 Nm/kg. We also showed that five fatiguing contractions are sufficient to calibrate the t-pEMS. Thus, this study presents a person and muscle specific model to track fatigue during functional tasks.

Torque steadiness and neuromuscular responses following fatiguing concentric exercise of the knee extensor and flexor muscles in young and older individuals

The purpose of this study was to investigate the age-related alterations in the ability to exert maximal and to sustain submaximal isometric muscle torques after a fatiguing concentric exercise conducted with knee extensor (KE) and flexor (KF) muscles. Sixteen young (aged 19-30 years; 8 women) and 17 older (aged 65-75 years; 9 women) volunteers participated. The following tasks were performed before and immediately after 22 maximal concentric efforts of the right KE and KF at 1.05 rad/s: (1) a maximal voluntary isometric contraction (MVIC) task involving both KE and KF; and (2) a KE torque-steadiness task at a submaximal target contraction intensity (20% MVIC). During the dynamometric tests, surface EMG was recorded simultaneously from the KE and KF muscles. Fatigue-induced reductions in knee extension MVIC were similar (~15%) between groups, but young participants showed more pronounced declines in agonist (i.e. quadriceps) EMG responses in both time (RMS amplitude; ~15% vs. ~10%, p < 0.001) and frequency (median frequency; ~14% vs. ~8%, p < 0.01) domains. Torque steadiness exhibited a similar post-fatigue decrease in the two age groups (p < 0.01), but interestingly agonist activation (~17%; p < 0.001) and antagonist (i.e. hamstrings) co-activation (~16%; p < 0.001) declined only in the older participants. These findings suggest that the fatiguing concentric KE and KF exercise results in similar relative reductions (%) in maximal torque and steadiness of the KE in young and older individuals, but they are sustained by different age-related neuromuscular strategies.

Spinal Cord Excitability and Sprint Performance Are Enhanced by Sensory Stimulation During Cycling

Spinal cord excitability, as assessed by modulation of Hoffmann (H-) reflexes, is reduced with fatiguing isometric contractions. Furthermore, spinal cord excitability is reduced during non-fatiguing arm and leg cycling. Presynaptic inhibition of Ia terminals is believed to contribute to this suppression of spinal cord excitability. Electrical stimulation to cutaneous nerves reduces Ia presynaptic inhibition, which facilitates spinal cord excitability, and this facilitation is present during arm cycling. Although it has been suggested that reducing presynaptic inhibition may prolong fatiguing contractions, it is unknown whether sensory stimulation can alter the effects of fatiguing exercise on performance or spinal cord excitability. Thus, the aim of this experiment was to determine if sensory stimulation can interfere with fatigue-related suppression of spinal cord excitability, and alter fatigue rates during cycling sprints. Thirteen participants randomly performed three experimental sessions that included: unloaded cycling with sensory stimulation (CONTROL + STIM), sprints with sensory stimulation (SPRINT + STIM) and sprints without stimulation (SPRINT). Seven participants also performed a fourth session (CONTROL), which consisted of unloaded cycling. During SPRINT and SPRINT + STIM, participants performed seven, 10 s cycling sprints interleaved with 3 min rest. For CONTROL and CONTROL + STIM, participants performed unloaded cycling for ~30 min. During SPRINT + STIM and CONTROL + STIM, participants received patterned sensory stimulation to nerves of the right foot. H-reflexes and M-waves of the right soleus were evoked by stimulation of the tibial nerve at multiple time points throughout exercise. Sensory stimulation facilitated soleus H-reflexes during unloaded cycling, whereas sprints suppressed soleus H-reflexes. While receiving sensory stimulation, there was less suppression of soleus H-reflexes and slowed reduction in average power output, compared to sprints without stimulation. These results demonstrate that sensory stimulation can substantially mitigate the fatiguing effects of sprints.

Assessment of Homonymous Recurrent Inhibition during Voluntary Contraction by Conditioning Nerve Stimulation

In humans, the amount of spinal homonymous recurrent inhibition during voluntary contraction is usually assessed by using a peripheral nerve stimulation paradigm. This method consists of conditioning the maximal M-wave (SM stimulus) with prior reflex stimulation (S1), with 10 ms inter-stimulus interval (ISI). The decrease observed between unconditioned (S1 only) and conditioned (S1+SM) reflex size is then attributed to recurrent inhibition. However, during a voluntary contraction, a superimposed SM stimulation leads to a maximal M-wave followed by a voluntary (V) wave at similar latency than the H-reflex. This wave can therefore interfere with the conditioned H-reflex when two different stimulation intensities are used (S1 and SM), leading to misinterpretation of the data. The aim of the present study was to assess if conditioning V-wave response instead of H-reflex, by applying SM for both stimuli (test and conditioning), can be used as an index of recurrent inhibition. Conditioned and unconditioned responses of soleus and medial gastrocnemius muscles were recorded in twelve subjects at 25% and at 50% of maximal voluntary contraction at the usual ISI of 10 ms and an optimal inter-stimulus of 15 ms determined upon M- and V-wave latencies. Conditioned H-reflex (obtained with S1+SM paradigm) was significantly lower than the unconditioned by ~30% on average, meaning that the amount of inhibition was 70%. This amount of recurrent inhibition was significantly lower at higher force level with both methods. Regardless of the level of force or the conditioning ISI, results obtained with V-wave conditioning (SM+SM) were similar at both force levels, linearly correlated and proportional to those obtained with H conditioning. Then, V-wave conditioning appears to be a reliable index of homonymous recurrent inhibition during voluntary contraction.

Physical and cognitive consequences of fatigue: A review

Fatigue is a common worrying complaint among people performing physical activities on the basis of training or rehabilitation. An enormous amount of research articles have been published on the topic of fatigue and its effect on physical and physiological functions. The goal of this review was to focus on the effect of fatigue on muscle activity, proprioception, and cognitive functions and to summarize the results to understand the influence of fatigue on these functions. Attaining this goal provides evidence and guidance when dealing with patients and/or healthy individuals in performing maximal or submaximal exercises.

Fatigue modulates synchronous but not asynchronous soleus activation during stimulation of paralyzed muscle

OBJECTIVE

Electrical stimulation over a motor nerve yields muscle force via a combination of direct and reflex-mediated activation. We determined the influence of fatigue on reflex-mediated responses induced during supra-maximal electrical stimulation in humans with complete paralysis.

METHODS

We analyzed soleus electromyographic (EMG) activity during repetitive stimulation (15 Hz, 125 contractions) in 22 individuals with complete paralysis. The bout of stimulation caused significant soleus muscle fatigue (53.1% torque decline).

RESULTS

Before fatigue, EMG at all latencies after the M-wave was less than 1% of the maximal M-wave amplitude (% MaxM). After fatigue there was a fourfold (p < 0.05) increase in EMG at the H-reflex latency; however, the overall magnitude remained low (< 2% change in % MaxM). There was no increase in "asynchronous" EMG ∼ 1 s after the stimulus train.

CONCLUSIONS

Fatigue enhanced the activation to the paralyzed soleus muscle, but primarily at the H-reflex latency. The overall influence of this reflex modulation was small. Soleus EMG was not elevated during fatigue at latencies consistent with asynchronous activation.

SIGNIFICANCE

These findings support synchronous reflex responses increase while random asynchronous reflex activation does not change during repetitive supra-maximal stimulation, offering a clinical strategy to consistently dose stress to paralyzed tissues.

Neuromuscular fatigue in healthy muscle: underlying factors and adaptation mechanisms

OBJECTIVES

This review aims to define the concept of neuromuscular fatigue and to present the current knowledge of the central and peripheral factors at the origin of this phenomenon. This review also addresses the literature that focuses on the mechanisms responsible for the adaption to neuromuscular fatigue.

METHOD

One hundred and eighty-two articles indexed in PubMed (1954-2010) have been considered.

RESULTS

Neuromuscular fatigue has central and peripheral origins. Central fatigue, preponderant during long-duration, low-intensity exercises, may involve a drop in the central command (motor, cortex, motoneurons) elicited by the activity of cerebral neurotransmitters and muscular afferent fibers. Peripheral fatigue, associated with an impairment of the mechanisms from excitation to muscle contraction, may be induced by a perturbation of the calcium ion movements, an accumulation of phosphate, and/or a decrease of the adenosine triphosphate stores. To compensate for the consequent drop in force production, the organism develops several adaptation mechanisms notably implicating motor units.

CONCLUSION

Fatigue onset is associated with an alteration of the mechanisms involved in force production. Then, the interaction between central and peripheral mechanisms leads to a series of events that ultimately contribute to the observed decrease in force production.

Assessment of H reflex sensitivity with M wave alternation consequent to fatiguing contractions

The objective of this study was to examine the changes in H reflex sensitivity after neuromuscular fatigue associated with fluctuations of the M wave. In the maximal and submaximal voluntary contraction (MVC and SMVC) paradigms, subjects performed voluntary plantarflexion at 100% MVC and 40% MVC respectively until the limit of torque maintenance was reached. In the submaximal electrical stimulation (SMES) paradigm, the tricep surae was exhausted with sustained electrical stimulation of 40% of the maximal tolerable intensity at a 40-Hz stimulus rate. The H reflexes and maximal M waves (M(max)) of the soleus were recorded before and after the three fatigue paradigms, and the H reflex was standardized with M(max) to minimize possible bias due to fatigue-induced M wave fluctuation. The results showed a significant increase in the standardized H reflex due to the SMES paradigm in spite of M(max) potentiation. The SMVC paradigm led to a reduction in size of the standardized H reflex without modification of M(max), whereas the standardized H reflex was not mediated by the MVC paradigm, which contributed to a noticeable M(max) potentiation. The present study underscored the fact that the H reflex sensitivity and M wave amplitude were not necessarily suppressed consequent to neuromuscular fatigue, but varied with the activation history of a muscle for size-dependent efficacy of the Ia transmission pathways and postactivation potentiation.

A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions

Magnetic and electrical stimulation at different levels of the neuraxis show that supraspinal and spinal factors limit force production in maximal isometric efforts ("central fatigue"). In sustained maximal contractions, motoneurons become less responsive to synaptic input and descending drive becomes suboptimal. Exercise-induced activity in group III and IV muscle afferents acts supraspinally to limit motor cortical output but does not alter motor cortical responses to transcranial magnetic stimulation. "Central" and "peripheral" fatigue develop more slowly during submaximal exercise. In sustained submaximal contractions, central fatigue occurs in brief maximal efforts even with a weak ongoing contraction (<15% maximum). The presence of central fatigue when much of the available motor pathway is not engaged suggests that afferent inputs contribute to reduce voluntary activation. Small-diameter muscle afferents are likely to be activated by local activity even in sustained weak contractions. During such contractions, it is difficult to measure central fatigue, which is best demonstrated in maximal efforts. To show central fatigue in submaximal contractions, changes in motor unit firing and force output need to be characterized simultaneously. Increasing central drive recruits new motor units, but the way this occurs is likely to depend on properties of the motoneurons and the inputs they receive in the task. It is unclear whether such factors impair force production for a set level of descending drive and thus represent central fatigue. The best indication that central fatigue is important during submaximal tasks is the disproportionate increase in subjects' perceived effort when maintaining a low target force.

Central fatigue affects plantar flexor strength after prolonged running

The primary aim of this study was to examine central fatigue of the plantar flexor muscle group after prolonged running using the twitch interpolation technique. Eight healthy, habitually active male subjects ran on a motorized treadmill for 2 h at a speed corresponding to 75% of peak oxygen uptake (VO(2peak)). Maximal voluntary isometric contraction (MVC) strength as well as the electrically induced twitch produced during MVC [interpolated twitch (IT)] and at rest [resting twitch (RT)] were measured before and after running. The level of activation (LOA) during each MVC was calculated as LOA (%)=100(1-IT/RT). Both MVC and LOA decreased (17+/-16% and 19+/-15%, respectively, P<0.05) after running, whereas RT did not change. The decrease in MVC was correlated with the decrease in LOA (r=0.87, P<0.05). The results demonstrate that after 2 h of treadmill running at an intensity of 75% of VO(2peak), there was a reduction in maximal voluntary plantar flexor muscle strength that was mainly related to central fatigue.

Neuromuscular and circulatory adaptation during combined arm and leg exercise with different maximal work loads

Cardiopulmonary kinetics and electromyographic activity (EMG) during exhausting exercise were measured in 8 males performing three maximal combined arm+leg exercises (cA+L). These exercises were performed at different rates of work (mean+/-SD; 373+/-48, 429+/-55 and 521+/-102 W) leading to different average exercise work times in all tests and subjects. VO2 reached a plateau versus work rate in every maximal cA+L exercise (range 6 min 33 s to 3 min 13 s). The three different exercise protocols gave a maximal oxygen consumption (VO2MAX) of 4.67+/-0.57, 4.58+/-0.52 and 4.66+/-0.53 l min(-1) (P=0.081), and a maximal heart rate (HRmax) of 190+/-6, 189+/-4 and 189+/-6 beats min(-1) (P=0.673), respectively. Root mean square EMG (EMGRMS) of the vastus lateralis and the triceps brachii muscles increased with increasing rate of work and time in all three cA+L protocols. The study demonstrates that despite different maximal rates of work, leading to different times to exhaustion, the circulatory adaptation to maximal exercise was almost identical in all three protocols that led to a VO2 plateau. The EMG(RMS) data showed increased muscle recruitment with increasing work rate, even though the HRmax and VO2MAX was the same in all three cA+L protocols. In conclusion, these findings do not support the theory of the existence of a central governor (CG) that regulates circulation and neuronal output of skeletal muscles during maximal exercise.

Neuromuscular fatigue during a prolonged intermittent exercise: Application to tennis

The time course of alteration in neuromuscular function of the knee extensor muscles was characterized during a prolonged intermittent exercise. Maximal voluntary contraction (MVC) and surface EMG activity of both vastii were measured during brief interruptions before (T(0)), during (30, 60, 90, 120, 150 and 180min: T(30), T(60), T(90), T(120), T(150), T(180)) and 30min after (T(+30)) a 3h tennis match in 12 trained players. M-wave and twitch contractile properties were analyzed following single stimuli. Short tetani at 20Hz and 80Hz were also applied to six subjects at T(0) and T(180). Significant reductions in MVC (P<0.05; -9%) and electromyographic activity normalized to the M wave for both vastii (P<0.01) occurred with fatigue at T(180). No significant changes in M-wave duration and amplitude nor in twitch contractile properties were observed. The ratio between the torques evoked by 20Hz and 80Hz stimulation declined significantly (P<0.001; -12%) after exercise. Central activation failure and alterations in excitation-contraction coupling are probable mechanisms contributing to the moderate impairment of the neuromuscular function during prolonged tennis playing.

Muscle proprioceptive feedback and spinal networks

This review revolves primarily around segmental feedback systems established by muscle spindle and Golgi tendon organ afferents, as well as spinal recurrent inhibition via Renshaw cells. These networks are considered as to their potential contributions to the following functions: (i) generation of anti-gravity thrust during quiet upright stance and the stance phase of locomotion; (ii) timing of locomotor phases; (iii) linearization and correction for muscle nonlinearities; (iv) compensation for muscle lever-arm variations; (v) stabilization of inherently unstable systems; (vi) compensation for muscle fatigue; (vii) synergy formation; (viii) selection of appropriate responses to perturbations; (ix) correction for intersegmental interaction forces; (x) sensory-motor transformations; (xi) plasticity and motor learning. The scope will at times extend beyond the narrow confines of spinal circuits in order to integrate them into wider contexts and concepts.

Fatigue Induced by a Cross-Country Skiing KO Sprint

PURPOSE

The aims of the present study were 1) to analyze whether the KO sprint simulation induced a phenomenon of fatigue of upper and lower limbs and 2) if there was any fatigue, to determine its origin.

METHODS

Seven elite male skiers were tested before and after a simulation of KO sprints consisting of three 1200-m laps separated by 12 min of recovery. Surface electromyographic activity and force obtained under voluntary and electrically evoked contractions (single twitch) on knee-extensor muscles were analyzed to distinguish neural adaptations from contractile changes. A maximal power output test of the upper limbs was also performed.

RESULTS

During the last lap, the final sprint velocity was significantly lower than during the first lap. After the KO sprint, knee-extensor voluntary (-9.8 +/- 9.5%) and evoked (-16.2 +/- 11.9%) isometric force and upper-limb power output (-11.0 +/- 9.3%) and force (-11.3 +/- 8.7%) significantly decreased, whereas the blood lactate concentration increased to 11.6 mM. On the other hand, no changes were seen in RMS measurement during maximal voluntary contractions, RMS normalized by M-wave amplitude, or M-wave characteristics.

CONCLUSION

Changes in performance, lactate concentration, knee-extensor strength, and upper-limb power indicated that the KO sprint test led the skiers to a state of fatigue. On lower-limb muscles, the decrease of knee-extensor strength was exclusively caused by peripheral fatigue, which was at least in part attributable to a failure of the excitation-contraction coupling.

Fatigue-sensitive afferents inhibit extensor but not flexor motoneurons in humans

The role of group III and IV muscle afferents in controlling the output from human muscles is poorly understood. We investigated the effects of these afferents from homonymous or antagonist muscles on motoneuron pools innervating extensor and flexor muscles of the elbow. In study 1, subjects (n = 8) performed brief maximal voluntary contractions (MVCs) of elbow extensors before and after a 2 min MVC of the extensors. During MVCs, electromyographic responses from triceps were evoked by stimulation of the corticospinal tracts [cervicomedullary motor evoked potentials (CMEPs)]. The same subjects repeated the protocol, but input from fatigue-sensitive afferents was prolonged after the fatiguing contraction by maintained muscle ischemia. In study 2, CMEPs were evoked in triceps during brief extensor MVCs before and after a 2 min sustained flexor MVC (n = 7) or in biceps during brief flexor MVCs before and after a sustained extensor MVC (n = 7). Again, ischemia was maintained after the sustained contractions. During sustained MVCs of the extensors, CMEPs in triceps decreased by approximately 35%. Without muscle ischemia, CMEPs recovered within 15 s, but with maintained ischemia, they remained depressed (by approximately 28%; p < 0.001). CMEPs in triceps were also depressed (by approximately 20%; p < 0.001) after fatiguing flexor contractions, whereas CMEPs in biceps were facilitated (by approximately 25%; p < 0.001) after fatiguing extensor contractions. During fatigue, inputs from group III and IV muscle afferents from homonymous or antagonist muscles depress extensor motoneurons but facilitate flexor motoneurons. The more pronounced inhibitory influence of these afferents on extensors suggests that these muscles may require greater cortical drive to generate force during fatigue.

Fatigue-related depression of the feline monosynaptic gastrocnemius-soleus reflex

In decerebrate cats, changes in the monosynaptic reflex (MSR) of gastrocnemius-soleus (G-S) motoneurones were studied after fatiguing stimulation (FST) of the G-S muscles. Monosynaptic reflexes were evoked by stimulation of Ia fibres in the G-S nerve and recorded from a filament of ventral root (VR) L7. FST (intermittent 40 s(-1) stimulation for 10-12 min) was applied to the distal part of the cut VR S1. FST reduced MSR amplitudes to 0.64 +/- 0.04 (mean +/-s.e.m.) of the prefatigue values. The suppression remained stable for approximately 25 min and then MSR amplitudes gradually returned towards the normal. To test for the involvement of presynaptic and recurrent inhibition, MSRs were conditioned by stimulation of the nerve to the posterior biceps and semitendinosus (PBSt) muscles or a filament of VR L7, respectively. The intensity of presynaptic inhibition (reduction of the normalized value of MSR amplitude during conditioning) increased from 0.19 +/- 0.02 in prefatigue to 0.44 +/- 0.04 within a 5.3-18.2 min interval after FST, followed by a recovery. In contrast, the intensity of recurrent inhibition first diminished from 0.23 +/- 0.02 in prefatigue to 0.15 +/- 0.01 within 15.6-30.1 min after FST and then gradually recovered. Both primary afferent depolarization and the intensity of antidromic discharges in primary afferents increased with the presynaptic inhibition intensity. These results demonstrate a fatigue-related suppression of Ia excitation of synergistic motoneurones, probably arising from the activation of group III and IV afferents. The effects could in part be due to increased presynaptic inhibition, while recurrent inhibition plays a minor role.

Effect of chemically activated fine muscle afferents on spinal recurrent inhibition in humans

OBJECTIVE

To test the hypothesis that 'metabolites released during fatiguing muscle contractions excite group III-IV muscle nociceptive afferents, inhibiting homonymous motoneurones via Renshaw cells,' by recording changes in recurrent inhibition of soleus motoneurones when high-threshold, small-diameter afferents (group III-IV fibres) from the same muscle were tonically activated.

METHODS

Experiments were performed in 7 healthy subjects at rest and during weak isometric voluntary contraction of the soleus muscle. Muscle nociceptive afferents were activated by local standardized injection of levo-ascorbic acid. Renshaw cells were orthodromically activated by a conditioning H reflex and the resulting recurrent inhibition of the soleus motoneurones was assessed by a subsequent test H reflex. An additional H reflex of the same size as the test reflex was used to assess motoneurone excitability.

RESULTS

At rest, muscle nociceptive stimulation produced transient facilitation of both test H and reference H reflexes. Under weak voluntary contraction, muscle nociceptive stimulation produced long-lasting extra-inhibition and extra-facilitation of the test reflex and reference reflex respectively, the time course of which closely resembled that of the subjective muscle pain curve.

CONCLUSIONS

Discharge of putative group III-IV muscle afferents facilitated homonymous recurrent inhibition. The filtering property of recurrent inhibition may contribute to limit motoneurone activity during muscle pain and/or adapt motoneurone firing rate to the modified contractile properties of motor units as muscle fatigue developed.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Transcranial magnetic stimulation and human muscle fatigue

During exercise, changes occur at many sites in the motor pathway, including the muscle fiber, motoneuron, motor cortex, and "upstream" of the motor cortex. Some of the changes result in fatigue, which can be defined as a decrease in ability to produce maximal muscle force voluntarily. Transcranial magnetic stimulation (TMS) over the human motor cortex reveals changes in both motor evoked potentials (MEPs) and the silent period during and after fatiguing voluntary contractions in normal subjects. The relationship of these changes to loss of force or fatigue is unclear. However, during a sustained maximal contraction TMS evokes extra force from the muscle and thus demonstrates the development of suboptimal output from the motor cortex, that is, fatigue at a supraspinal level. In some patients with symptoms of fatigue, the response to TMS after exercise is altered, but the changed MEP behavior is not yet linked to particular symptoms or pathology.

Fatigue after submaximal intensive stretch-shortening cycle exercise

OBJECTIVE

The aim of the present study was to examine some sites of neuromuscular fatigue after submaximal intensity stretch-shortening cycle exercise.

METHODS

Twelve male subjects performed consecutive sledge jumps at 60% of maximal height until exhaustion (mean duration 443.7 s +/- 304.9 s, mean +/- SD).

RESULTS

During the exercise, the blood lactate increased from 1.8 +/- 0.6 mmol x L(-1) (before exercise) to 6.1 +/- 1.7 mmol x L(-1) (P < 0.001) and serum creatine-kinase from 248 +/- 142 IU x L(-1) to 584 +/- 344 IU x L(-1) (P < 0.001). Electrical stimulation of the vastus lateralis and quadriceps femoris muscles to induce isometric knee extension resulted in decreased peak torque during single and double twitch after workout (from 22.1 +/- 6.3 Nm to 17.3 +/- 8.0 Nm, P < 0.05, and from 96.6 +/- 15.4 Nm to 76.2 x 19.8 Nm, P < 0.001, respectively), whereas there were no significant changes in contraction and relaxation times. Torque during 20-Hz stimulation decreased significantly (from 23.7 +/- 9.2 to 16.1 +/- 7.8 Nm, P < 0.01) but not at 100-Hz stimulation. During maximal voluntary isometric knee extensions, the rate of torque development was significantly (P < 0.01) more impaired than maximal torque (from 1619 +/- 390 Nm x s(-1) to 1,004 +/- 360 Nm x s(-1) and from 185 +/- 30.7 Nm to 151 +/- 32.3 Nm, respectively, both P < 0.001). At the same time, the muscle activation level increased by 15.8 +/- 24.1% (P < 0.05). The mean EMG amplitude of vastus lateralis during MVC increased by 34.9 +/- 39.2% (P > 0.05).

CONCLUSION

It was concluded that after submaximal stretch-shortening exercise, the low-frequency fatigue occurred, very likely caused by lower Ca2+ release per single action potential.

Ischaemia after exercise does not reduce responses of human motoneurones to cortical or corticospinal tract stimulation

Motor unit firing rates and voluntary activation of muscle decline during sustained isometric contractions. After exercise, the responses to motor cortical and corticospinal stimulation are reduced. These changes may reflect motoneuronal inhibition mediated by group III and IV muscle afferents. To determine whether the post-contraction depression of the responses to corticospinal or motor cortical stimulation could be maintained by continued firing of ischaemically sensitive group III and IV muscle afferents, we examined responses in muscles that were held ischaemic after exercise. Following a sustained maximal voluntary contraction (MVC) of the elbow flexors lasting 2 min, the response to stimulation of the corticospinal tract was reduced but the usual recovery (over approximately 2 min) was not delayed when the muscles were maintained ischaemic for 2 min after the contraction. Following a sustained MVC, the time course of the reduction in the response to motor cortical stimulation (a gradual decrease over approximately 2 min, maintained for > 10 min) was also not altered if the muscle was held ischaemic. Mean arterial blood pressure rose to 155 +/- 12 mmHg during the 2 min MVC, declined to 125 +/- 9 mmHg immediately after it, but remained at this level without returning to pre-exercise levels (102 +/- 10 mmHg) until circulation to the arm was restored. This confirms that the sustained MVC activated a reflex dependent on group III and IV muscle afferents. This study shows that ischaemically sensitive group III and IV muscle afferents do not mediate depression of responses to motor cortical or corticospinal stimulation after fatiguing exercise. It also suggests that firing of such afferents does not directly inhibit motoneurones or motor cortical output cells.

Changes in transmission in the pathway of heteronymous spinal recurrent inhibition from soleus to quadriceps motor neurons during movement in man

H reflexes were induced in the human quadriceps muscle by electrical stimulation of the femoral nerve. The reflexes were conditioned by prior stimulation of the inferior soleus nerve. The conditioning stimulus produced an inhibition of long duration (>20 ms). The threshold of this inhibition was at zero soleus motor discharge and the inhibition scaled with soleus motor discharge. It was concluded that the inhibition was a heteronymous recurrent inhibition of quadriceps motor neurons mediated by Renshaw cells which had been activated by soleus motor neuron discharge. This recurrent inhibition declined during voluntary tonic contraction of the quadriceps, falling to zero at around one-third of maximum voluntary contraction. Antagonist contraction and weak co-contraction of the quadriceps and its antagonists did not lead to any significant change in recurrent inhibition. It is concluded that motor commands descending from the brain reduce heteronymous recurrent inhibition during isolated quadriceps muscle contraction, but to a much lesser extent during co-contraction. No evidence was obtained for any descending facilitation of heteronymous recurrent inhibition.

Neural control in human muscle fatigue: changes in muscle afferents, motoneurones and motor cortical drive [corrected]

To understand the neural factors which contribute to fatigue, it is not satisfactory to regard fatigue as occurring only when a task can no longer be performed. Changes in muscle afferent feedback, motoneuronal discharge, motor cortical output, and perceived effort develop well before an endurance limit in limb muscles. During sustained maximal contractions the discharge of motoneurones declines, commonly to below the level required to produce maximal force from the muscle whose contractile speed is usually slowed. Thus, some 'central' fatigue develops. Recent findings using transcranial stimulation have revealed that the motor cortex is one site at which suboptimal output develops during human muscle fatigue. There is a need to study the reflex effects on motoneurones and the excitability of the motor cortex in experimental animals, as well as to apply rigorous methods to assess these processes in voluntary exercise in human subjects [corrected].

Fatigue characteristics following ankle fractures

The purpose of the study was to examine the effects of surgical and nonsurgical treatment of previously immobilized ankle fractures on voluntary and evoked contractile properties before and following fatigue. Twelve control and 12 previously immobilized (4-14 wk postfracture) internally fixated and nonfixated ankles were investigated before and following an isometric, intermittent, submaximal, fatigue protocol of the plantar flexors. Before fatigue, fracture groups had significantly lower force output (42.7 vs 78.8 Nm) and muscle activation (78.3 vs 98.7%) than controls. Decreased activation may be attributed to the inhibitory effects of injured muscle and swelling. All groups had similar force and muscle activation decreases (7-10%) following fatigue; however, the internally fixated group performed significantly fewer contractions during the fatigue test (19) than the nonfixated (71) and controls (61). In contrast to the other groups, internally fixated subjects experienced increased (13%) rather than decreased EMG activity (controls: 10.9%, nonfixated: 21.1%). M-waves and twitch torques potentiated to a similar extent in the fracture groups (4.5 and 5.7%) but decreased significantly in the control group (24.2 and 9.8%). The similar fatigue durations of non-fixated subjects compared with controls may be attributed to a lack of impairment in nonfixated neuromuscular propagation and contractile kinetics, while the increased fatigability of fixated subjects with a similar lack of evoked contractile property impairments suggested a greater intrinsic fatigability.

Central fatigue assessed by transcranial magnetic stimulation

Central fatigue is a subjective phenomenon which can be examined using transcranial magnetic stimulation (TMS). To assess central fatigue, we compared TMS and peripheral electrical stimulations in patients with central nervous system (CNS) lesions and controls before and after an exhaustive task. The recovery times of motor evoked potential (MEP) amplitudes were significantly prolonged in the patient group whereas the recovery of F waves and compound muscle action potentials showed no significant changes. The results indicate that fatigue cannot be attributed either to intramuscular processes or to reduced spinal excitability, but reflects a supraspinal, probably cortical phenomenon. The measurement of MEP recovery times proved to be a simple and objective tool for the assessment of fatigue and for the differentiation between healthy controls and patients with CNS lesions.

On the role of recurrent inhibitory feedback in motor control

This article reviews presumed roles of recurrent inhibition in motor control, that have been proposed over the past five decades. The discussion is structured in an order of increasing complexity. It starts out with the simplest and earliest circuit, that is recurrent self-inhibition of skeleto-motoneurons, and related functions. It soon becomes clear that in order to understand recurrent inhibition, we must look beyond the simple self-inhibitory CNS circuit. First, recurrent inhibition must be seen in the context of other neural circuits. Second, some quantitative features appear to be correlated with features of the neuromusculo-skeletal periphery. Third, the aspect of lateral inhibition between different members of a motoneuron pool as well as between different motoneuron pools points to the essential multiple input-multiple output structure of recurrent inhibition that again can be understood only by correlating it with features of the neuromusculo-skeletal periphery. Another extension results from the discovery that recurrent inhibition affects not only skeleto-motoneurons, but also gamma-motoneurons, Ia inhibitory interneurons mediating reciprocal inhibition between antagonist motoneurons, other Renshaw cells and cells of origin of the ventral spinocerebellar tract (VSCT). Then the view broadens again, investigating the potential role that recurrent inhibition plays in two far-ranging theories of motor control, the inverse-dynamics approach and the equilibrium-point hypothesis. Finally, the present author tries to formulate, in broad strokes, a personal functional interpretation of recurrent inhibition. All the functional considerations, right or wrong, should yield ideas for new experiments, and this then is the last objective of this review.

Recurrent inhibition of soleus alpha-motoneurons during a sustained submaximal plantar flexion

During 10 min of sustained isometric plantar flexion at 20% of maximal voluntary contraction, recurrent inhibition of soleus alpha-motoneurons was studied in 9 healthy subjects (age 22-37 years). Recurrent inhibition was brought about by a conditioning H-reflex and assessed by a test H-reflex delivered 10 ms later. The amplitude of the test H-reflex during the tenth minute of the contraction (16.9 +/- 13.2% of the maximal compound motor action potential) was significantly increased as compared to that during the first minute (9.8 +/- 7.6%), while the conditioning H-reflex remained unchanged. Concomitantly, muscle fatigue was evidenced by a significant increase in amplitude of the soleus electromyogram. The increase of the test-H-reflex amplitude implies that a decrease in recurrent inhibition occurred during the sustained submaximal contraction, which contrasts results from studies on maximal voluntary contractions. These results indicate a modulation of soleus Renshaw interneurons, which is likely to serve the purpose of optimising motor unit recruitment and firing rates of this muscle during a sustained submaximal contraction.

Excitatory drive to the alpha-motoneuron pool during a fatiguing submaximal contraction in man

1. This study was undertaken to examine changes of excitatory drive to the triceps surae alpha-motoneuron pool during fatiguing submaximal isometric contractions in man. Eight healthy subjects maintained isometric plantar flexions at 30 percent of maximum voluntary contraction (MVC) until the limit of endurance (range, 6-9 min). 2. Excitability of the alpha-motoneuron pool to Ia afferent stimulation (H reflex), electromyograms (EMG) and maximum compound motor unit action potentials (Mmax) from the lateral (LG) and medial heads (MG) of the gastrocnemius as well as from the soleus muscle (Sol) were recorded throughout the contraction. Superimposed maximum twitch torques (twitch occlusion) and isometric torque fluctuations (tremor) were also recorded as indirect measures of excitatory drive. 3. H reflexes were studied at different levels of underlying voluntary contraction to assess the relationship between H reflex amplitude and excitatory drive. With increasing levels of underlying contraction up to MVC, superimposed H reflex amplitude increased for LG in six subjects, for MG in all eight and for Sol in five. In the remaining cases, H reflex amplitude first increased and then plateaued between 30-50% of MVC. 4. H/Mmax ratios increased during fatigue in those muscles that showed an H reflex amplitude increase with high levels of underlying contraction. In these cases, LG and MG H/Mmax increased significantly after about 50 and 20% of endurance time onward, respectively, whereas Sol H/Mmax demonstrated a significant increase up to 40% of endurance time. 5. EMG root mean square (r.m.s.) increased linearly throughout the contraction for all three muscles, while tremor r.m.s. increased in a non-linear way, with a steeper increase from 60% of endurance time onward. Superimposed twitch amplitude decreased significantly from 25% of endurance time onward. 6. It is concluded that during fatiguing isometric contractions at 30% of MVC, the excitatory drive to the triceps surae alpha-motoneuron pool increases. This is thought to be a compensatory mechanism to facilitate recruitment of new, unfatigued motor units (MUs), and/or to increase MU firing rates. The facts that the twitch is not abolished at endurance limit and that the EMG does not attain its unfatigued MVC level are strong indications that central fatigue occurred during the sustained submaximal contraction.

Supraspinal factors in human muscle fatigue: evidence for suboptimal output from the motor cortex

1. Voluntary activation of elbow flexor muscles can be optimal during brief maximal voluntary contractions (MVCs), although central fatigue, a progressive decline in the ability to drive the muscle maximally, develops during sustained or repeated efforts. We stimulated the motor cortex and motor point in human subjects to investigate motor output during fatigue. 2. The increment in force (relative to the voluntary force) produced by stimulation of the motor point of biceps brachii increased during sustained isometric MVCs of the elbow flexors. Motoneuronal output became suboptimal during the contraction, i.e. central fatigue developed and accounted for a small but significant loss of maximal voluntary force. During 3 min MVCs, voluntary activation of biceps fell to an average of 90.7% from an average of > 99%. 3. The increment in force (relative to the voluntary force) produced by magnetic cortical stimulation was initially small (1.0%) but also increased during sustained MVCs to 9.8% (with a 2 min MVC). Thus, cortical output was not optimal at the time of stimulation nor were sites distal to the motor cortex already acting maximally. 4. A sphygmomanometer cuff around the upper arm blocked blood supply to brachioradialis near the end of a sustained MVC and throughout subsequent brief MVCs. Neither maximal voluntary force nor voluntary activation recovered during ischaemia after the sustained MVC. However, fatigue-induced changes in EMG responses to magnetic cortical stimulation recovered rapidly despite maintained ischaemia. 5. In conclusion, during sustained MVCs, voluntary activation becomes less than optimal so that force can be increased by stimulation of the motor cortex or the motor nerve. Complex changes in excitability of the motor cortex also occur with fatigue, but can be dissociated from the impairment of voluntary activation. We argue that inadequate neural drive effectively 'upstream' of the motor cortex must be one site involved in the genesis of central fatigue.

Overview: potential role of segmental motor circuitry in muscle fatigue

This chapter reviews several mechanisms that the CNS may use to mitigate muscle fatigue, including intrinsic motoneuron properties and feedback systems. The emphasis is on the effects of sensory inputs on spinal cord interneurons including: Renshaw cells; Ib inhibitory interneurons; interneurons mediating presynaptic inhibition; Ia inhibitory interneurons; and interneuronal networks constituting central pattern generators for locomotion. This exercise brings out how little is known about the operation of these circuits in dealing with muscle fatigue.

Behaviour of short and long latency reflexes in fatigued human muscles

1. The human abductor pollicis brevis (APB) and first dorsal interosseus (FDI) were fatigued by sustained maximal voluntary contractions and, in the case of the APB also by electrically induced (30 Hz) contractions, until the loss of force reached 50% of control. The short latency or Hoffmann reflex (H reflex) and the long latency reflex (LLR) were evoked during weak voluntary contractions by the electrical stimulation of the median nerve at the wrist in control, during and after the fatigue experiments. 2. As compared to control, the normalized H reflex amplitude in the two fatigue modalities was found to have decreased by 30% without any significant change in the LLR. This finding and the observation that the LLR was enhanced by 46% in simultaneous recordings, in which the APB remained at rest during FDI fatigue, could be explained by a stronger descending fatigue-induced central drive which spreads to neighbouring non-fatigued muscles. 3. A comparison of the H reflex and the LLR behaviour during fatigue indicates that motoneurone activation threshold is not affected but that changes in peripheral drive are present, which possibly induce presynaptic inhibition of Ia afferents and/or inhibition of interneurones in the oligosynaptic pathways. Our observation of a rather slow time course for the H reflex decrease during fatigue supports the point of view that these inhibitions are activated by metabolic and/or chemical changes in the fatigued muscle. 4. It is concluded from the results of this study that muscle fatigue induces an enhanced descending supraspinal drive which compensates for a loss of excitation from the peripheral afferents on motoneurones.

Impairment of neuromuscular propagation during human fatiguing contractions at submaximal forces

1. The purpose of the study was to examine the dependence of neuromuscular propagation impairment on the level of isometric force sustained to the endurance limit. The task involved human volunteers sustaining a submaximal abduction force with the index finger by activating the first dorsal interosseous muscle as long as possible. 2. The submaximal force was sustained at one of three levels (20, 35 or 65% of maximum) by increasing motor unit activity, as indicated by the electromyogram (EMG), during the fatiguing contraction. Although the EMG increased during the fatiguing contraction, the EMG was significantly less than maximum at the endurance limit for all subjects (deficit of 19-55% of maximum). This deficit was inversely related to the level of the sustained submaximal force. 3. The maximum voluntary contraction and twitch forces were significantly reduced following the fatiguing contraction. As with the EMG, the degree of force reduction was greatest for the subjects who sustained the low target forces. 4. The fatiguing contraction caused a 12-23% decline in M wave amplitude, a 33-51% increase in M wave duration, and no change in M wave area. The decline in M wave amplitude, which is an index of neuromuscular propagation impairment, was greatest among the subjects who sustained the low target forces. 5. The mean power frequency of the EMG decreased by a similar amount (50-57%) during the fatiguing contraction for all three groups of subjects. 6. A model representing the interaction of processes that enhance and impair force was developed to explain the recovery of twitch force following the sustained contractions at different target forces. 7. We conclude that the fatigue experienced by a subject when force is sustained at a submaximal value does involve an impairment of neuromuscular propagation. This impairment is one factor that limits muscle excitation during a submaximal, fatiguing contraction and contributes to the diminished force capability by the end of the fatigue task.

Effects of fatigue on the stretch reflex in a human muscle

The effects of fatigue on the electromyographic (EMG) reflex activities were compared during sustained voluntary contractions and contractions evoked by electrical stimulation (30 Hz) in the human first dorsal interosseus (FDI). Short latency (SL), medium latency (ML) and long latency (LL) reflex responses to a ramp-and-hold stretch of the muscle were recorded and analysed in 27 healthy subjects of both sexes. The amplitude of the reflex components was normalized as function of the amplitude of the surface action potential (SAP) recorded in response to the supramaximal stimulation of the motor nerve. The results indicate that for a similar reduction of force, SL and ML are significantly reduced after fatigue induced by voluntary contractions but they are not when the fatigue test is performed by electrical stimulation at the motor point. In voluntary fatigue experiments, the LL component showed no significant decrease below control values, but an enhancement was observed during electrically evoked contraction. This enhancement remained above control values for at least 15 min during the recovery period, whereas SL and ML decreases returned to control within 5 min after the fatigue tests. The electrical stimulation applied to the skin overlying the FDI at an intensity lower than the motor threshold did not affect SL and ML, but enhanced LL for about 15 min. On the contrary, the anaesthesia of the skin overlying the FDI induced a decrease in LL without significant change of SL and ML. It is concluded that muscle reflex fatigue is present during sustained voluntary contractions and decreases SL and ML responses to quick stretches.(ABSTRACT TRUNCATED AT 250 WORDS)

Presence of homonymous recurrent inhibition in motoneurones supplying different lower limb muscles in humans

The pattern of recurrent inhibition to motoneurones (MNs) innervating different lower limb muscles was investigated in ten healthy subjects. Three complementary experimental designs, all based on the electrophysiological method introduced by Bussel and Pierrot-Deseilligny in 1977, were used in Quadriceps (Qu), Pretibial (Pt), Soleus (Sol) and Abductor hallucis (Abh) motornuclei: 1) measurement of the relationship between the amplitude of conditioning H (H1) and test (H') reflexes; 2) measurement of the effect of the intravenous administration of L-Acetylcarnitine (L-Ac) on the amplitude of the test H' reflex after a constant H1 conditioning reflex; 3) measurement of the modifications of the test H' reflex in relation to a reference H (Ref H) reflex during a weak tonic voluntary contraction of the homonymous muscle. A complete agreement among results obtained with the different experimental paradigms was observed. Similarly to the Sol, both Pt and Qu MNs were found to be recurrently inhibited: a) the test H' reflex exhibited a progressive and consistent depression with increasing amplitude of the H1 conditioning reflex beyond a specific value; b) an additional decrement of the test H' reflex was obtained after intravenous administration of L-Ac; c) a decrease in the size of the test H' reflex, with respect to its value at rest, was observed during a weak tonic voluntary contraction, in spite of the enhanced MN excitability (as shown by the increase in the Ref H). By contrast, no evidence of recurrent inhibition to the Abh MNs was found.(ABSTRACT TRUNCATED AT 250 WORDS)

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.