Reflex gain of muscle spindle pathways during fatigue

Myoelectric Fatigue and Motor-Unit Firing Patterns During Sinusoidal Vibration Superimposed on Low-Intensity Isometric Contraction

Vibration exercise (VE) has shown promising results for improving muscle strength and power performance when superimposed on high-level muscle contraction. However, low-level contraction may be more preferable in many rehabilitation programs due to the weakness of the patients. Unfortunately, the effects and underlying physiological mechanisms of VE superimposed on low-level contraction are unclear. This study aims to investigate the fatiguing effects and motor unit (MU) firing patterns during VE with low-level muscle contraction. Twenty-one healthy participants performed 60-s isometric contraction of the upper limb under a baseline force at 30% maximum voluntary contraction and superimposed vibration with an amplitude of 50% baseline and different frequencies at 0 Hz (control), 15, 25, 35, and 45 Hz. High-density surface electromyography (EMG) was recorded on the biceps brachii. The decay in muscle fiber conduction velocity, calculated in 3-s sliding windows, was employed as an indicator of myoelectric fatigue. MU firing patterns were obtained by decomposing the high-density EMG into MU spike trains. VE, particularly at 25 Hz, produces increased myoelectric fatigue as compared to the control condition. Besides, synchronized MU discharges are observed at the vibration frequency for 15- and 25-Hz VE and the sub-harmonics for 35- and 45-Hz VE. Furthermore, VE-induced increase in MU synchronization (as compared to control) seems to decrease with myoelectric fatigue. Significance: Our findings suggest that VE may be a suitable modality for rehabilitation programs, providing useful insights for subscribing appropriate VE training protocols.

A Feeding-Related Mechanoreceptor Identified in the Crab Cancer borealis Shares Similarities and Differences with Homologs in Other Crustaceans

AbstractSensory feedback plays an essential role in shaping rhythmic animal movements. In the crustacean stomatogastric nervous system, which is responsible for grinding and filtering food particles in the animal's foregut, a number of mechanoreceptors whose activity affects motor output have been characterized. The hepatopancreas duct receptor neurons, which are located in the pyloric region of the foregut that is responsible for filtering, are among the less well understood groups of stomatogastric mechanoreceptors. Although they were first described decades ago in a number of decapod species, many questions remain about their role in shaping the movements produced by the stomatogastric nervous system. Here we provide the first anatomical and physiological evidence that there are also hepatopancreas duct receptors in the crab Cancer borealis, and we demonstrate that hepatopancreas duct receptor spiking produced by mechanical stimulation modifies the properties of an ongoing pyloric motor program.

Shaky hands are a part of motor neuron disease phenotype: clinical and electrophysiological study of 77 patients

BACKGROUND

In the sharp contrast with the existing literature, we frequently observe minipolymyoclonus, tremor and pseudodystonic thumb posturing in patients with motor neuron disease. We conducted a clinical and electrophysiological study to describe phenomenology, prevalence and pathophysiology of involuntary movements in motor neuron disease.

METHODS

We included 77 consecutive patients. Involuntary movements were assessed at rest and on action. Patients were videotaped. Arm muscle tone, power and deep tendon reflexes were evaluated. Accelerometry with electromyography was recorded in a subset of patients.

RESULTS

Involuntary movements were observed in 68.9% of patients and could be separated into rest minipolymyoclonus, thumb tremor, pseudodystonic thumb posture, action minipolymyoclonus, and action tremor. One-third of patients reported negative impact of involuntary movements on hand use. Logistic regression showed that rest minipolymyoclonus and thumb tremor were more likely to occur in patients with more prominent distal muscle weakness and less spasticity. Similarly, action involuntary movements were more likely to appear in weaker patients. Patients with brisk tendon reflexes were more likely to display action tremor than action minipolymyoclonus. Action tremor was characterized by accelerometer and corresponding electromyography peak frequency, which decreased with mass loading, suggesting a mechanical-reflex tremor.

CONCLUSIONS

Involuntary movements are common, but poorly recognized feature of motor neuron disease that may add to functional impairment. Results of our study suggest that involuntary movements are likely of peripheral origin, with a non-fused contraction of enlarged motor units being a common driving mechanism. Minipolymyoclonus appears if no synchronization of motor units occurs. When synchronization occurs via stretch reflex, mechanical-reflex tremor is generated.

Not all Forms of Muscle Hypertonia Worsen With Fatigue: A Pilot Study in Para Swimmers

In hypertonic muscles of patients with upper motor neuron syndrome (UMNS), investigation with surface electromyography (EMG) with the muscle in a shortened position and during passive muscle stretch allows to identify two patterns underlying hypertonia: spasticity and spastic dystonia. We recently observed in Para swimmers that the effect of fatigue on hypertonia can be different from subject to subject. Our goal was, therefore, to understand whether this divergent behavior may depend on the specific EMG pattern underlying hypertonia. We investigated eight UMNS Para swimmers (five men, mean age 23.25 ± 3.28 years), affected by cerebral palsy, who presented muscle hypertonia of knee flexors and extensors. Muscle tone was rated using the Modified Ashworth Scale (MAS). EMG patterns were investigated in rectus femoris (RF) and biceps femoris (BF) before and after two fatiguing motor tasks of increasing intensity. Before the fatiguing tasks, two subjects (#2 and 7) had spasticity and one subject (#5) had spastic dystonia in both RF and BF. Two subjects (#3 and 4) showed spasticity in RF and spastic dystonia in BF, whereas one subject (#1) had spasticity in RF and no EMG activity in BF. The remaining two subjects (#6 and 8) had spastic dystonia in RF and no EMG activity in BF. In all the 16 examined muscles, these EMG patterns persisted after the fatiguing tasks. Spastic dystonia increased (p < 0.05), while spasticity did not change (p > 0.05). MAS scores increased only in the muscles affected by spastic dystonia. Among the phenomena possibly underlying hypertonia, only spastic dystonia is fatigue-dependent. Technical staff and medical classifiers should be aware of this specificity, because, in athletes with spastic dystonia, intense and prolonged motor activity could negatively affect competitive performance, creating a situation of unfairness among Para athletes belonging to the same sports class.

Shoes with active insoles mitigate declines in balance after fatigue

Fatigue can induce postural instability and even lead to falls. However, most current methods to delay or reduce fatigue require long preparatory time, or large and expensive equipment. We propose a convenient method to alleviate postural instability due to fatigue. We paid attention to that fatigue and aging share similar neurophysiological deterioration of sensory-motor function. Considering that stochastic resonance via sub-sensory mechanical vibration increases postural stability in the elderly, we propose that sub-sensory insole vibration reduces the negative effect of fatigue on postural control. We performed experiments with 21 young and healthy adult participants, and demonstrated that insole vibration compensates for the loss of balance ability due to fatigue. The sub-sensory insole vibration restored both the area of center of pressure and the complexity of the time series of the motor output after fatigue to the pre-fatigue levels. The insole units generating the vibration were completely concealed in shoes and controlled by a smart phone. This compact implementation contrasts with the cumbersome procedure of current solutions to fatigue-induced postural instability.

Cardinal features of involuntary force variability can arise from the closed-loop control of viscoelastic afferented muscles

Involuntary force variability below 15 Hz arises from, and is influenced by, many factors including descending neural drive, proprioceptive feedback, and mechanical properties of muscles and tendons. However, their potential interactions that give rise to the well-structured spectrum of involuntary force variability are not well understood due to a lack of experimental techniques. Here, we investigated the generation, modulation, and interactions among different sources of force variability using a physiologically-grounded closed-loop simulation of an afferented muscle model. The closed-loop simulation included a musculotendon model, muscle spindle, Golgi tendon organ (GTO), and a tracking controller which enabled target-guided force tracking. We demonstrate that closed-loop control of an afferented musculotendon suffices to replicate and explain surprisingly many cardinal features of involuntary force variability. Specifically, we present 1) a potential origin of low-frequency force variability associated with co-modulation of motor unit firing rates (i.e.,'common drive'), 2) an in-depth characterization of how proprioceptive feedback pathways suffice to generate 5-12 Hz physiological tremor, and 3) evidence that modulation of those feedback pathways (i.e., presynaptic inhibition of Ia and Ib afferents, and spindle sensitivity via fusimotor drive) influence the full spectrum of force variability. These results highlight the previously underestimated importance of closed-loop neuromechanical interactions in explaining involuntary force variability during voluntary 'isometric' force control. Furthermore, these results provide the basis for a unifying theory that relates spinal circuitry to various manifestations of altered involuntary force variability in fatigue, aging and neurological disease.

Fatigue diminishes motoneuronal excitability during cycling exercise

Exercise-induced fatigue influences the excitability of the motor pathway during single-joint isometric contractions. This study sought to investigate the influence of fatigue on corticospinal excitability during cycling exercise. Eight men performed fatiguing constant-load (80% W_peak; 241 ± 13 W) cycling to exhaustion during which the percent increase in quadriceps electromyography (ΔEMG; vastus lateralis and rectus femoris) was quantified. During a separate trial, subjects performed two brief (∼45 s) nonfatiguing cycling bouts (244 ± 15 and 331 ± 23W) individually chosen to match the ΔEMG across bouts to that observed during fatiguing cycling. Corticospinal excitability during exercise was quantified by transcranial magnetic, electric transmastoid, and femoral nerve stimulation to elicit motor-evoked potentials (MEP), cervicomedullary evoked potentials (CMEP), and M waves in the quadriceps. Peripheral and central fatigue were expressed as pre- to postexercise reductions in quadriceps twitch force (ΔQ_tw) and voluntary quadriceps activation (ΔVA). Whereas nonfatiguing cycling caused no measureable fatigue, fatiguing cycling resulted in significant peripheral (ΔQ_tw: 42 ± 6%) and central (ΔVA: 4 ± 1%) fatigue. During nonfatiguing cycling, the area of MEPs and CMEPs, normalized to M waves, similarly increased in the quadriceps (∼40%; P < 0.05). In contrast, there was no change in normalized MEPs or CMEPs during fatiguing cycling. As a consequence, the ratio of MEP to CMEP was unchanged during both trials (P > 0.5). Therefore, although increases in muscle activation promote corticospinal excitability via motoneuronal facilitation during nonfatiguing cycling, this effect is abolished during fatigue. We conclude that the unaltered excitability of the corticospinal pathway from start of intense cycling exercise to exhaustion is, in part, determined by inhibitory influences on spinal motoneurons obscuring the facilitating effects of muscle activation.

Myelinated Afferents Are Involved in Pathology of the Spontaneous Electrical Activity and Mechanical Hyperalgesia of Myofascial Trigger Spots in Rats

Myofascial trigger points (MTrPs) are common causes for chronic pain. Myelinated afferents were considered to be related with muscular pain, and our clinical researches indicated they might participate in the pathology of MTrPs. Here, we applied myofascial trigger spots (MTrSs, equal to MTrPs in human) of rats to further investigate role of myelinated afferents. Modified pyridine-silver staining revealed more nerve endings at MTrSs than non-MTrSs (P < 0.01), and immunohistochemistry with Neurofilament 200 indicated more myelinated afferents existed in MTrSs (P < 0.01). Spontaneous electrical activity (SEA) recordings at MTrSs showed that specific block of myelinated afferents in sciatic nerve with tetrodotoxin (TTX) led to significantly decreased SEA (P < 0.05). Behavioral assessment showed that mechanical pain thresholds (MPTs) of MTrSs were lower than those of non-MTrSs (P < 0.01). Block of myelinated afferents by intramuscular TTX injection increased MPTs of MTrSs significantly (P < 0.01), while MPTs of non-MTrSs first decreased (P < 0.05) and then increased (P > 0.05). 30 min after the injection, MPTs at MTrSs were significantly lower than those of non-MTrSs (P < 0.01). Therefore, we concluded that proliferated myelinated afferents existed at MTrSs, which were closely related to pathology of SEA and mechanical hyperalgesia of MTrSs.

Hormonal and neuromuscular responses to mechanical vibration applied to upper extremity muscles

Objective

To investigate the acute residual hormonal and neuromuscular responses exhibited following a single session of mechanical vibration applied to the upper extremities among different acceleration loads.

Methods

Thirty male students were randomly assigned to a high vibration group (HVG), a low vibration group (LVG), or a control group (CG). A randomized double-blind, controlled-parallel study design was employed. The measurements and interventions were performed at the Laboratory of Biomechanics of the University of L'Aquila. The HVG and LVG participants were exposed to a series of 20 trials ×10 s of synchronous whole-body vibration (WBV) with a 10-s pause between each trial and a 4-min pause after the first 10 trials. The CG participants assumed an isometric push-up position without WBV. The outcome measures were growth hormone (GH), testosterone, maximal voluntary isometric contraction during bench-press, maximal voluntary isometric contraction during handgrip, and electromyography root-mean-square (EMG_rms) muscle activity (pectoralis major [PM], triceps brachii [TB], anterior deltoid [DE], and flexor carpi radialis [FCR]).

Results

The GH increased significantly over time only in the HVG (P = 0.003). Additionally, the testosterone levels changed significantly over time in the LVG (P = 0.011) and the HVG (P = 0.001). MVC during bench press decreased significantly in the LVG (P = 0.001) and the HVG (P = 0.002). In the HVG, the EMG_rms decreased significantly in the TB (P = 0.006) muscle. In the LVG, the EMG_rms decreased significantly in the DE (P = 0.009) and FCR (P = 0.006) muscles.

Conclusion

Synchronous WBV acutely increased GH and testosterone serum concentrations and decreased the MVC and their respective maximal EMG_rms activities, which indicated a possible central fatigue effect. Interestingly, only the GH response was dependent on the acceleration with respect to the subjects' responsiveness.

Tndon vibration does not alter recovery time following fatigue

PURPOSE

Tendon vibration has been shown to enhance muscle activity and to increase muscular endurance times. The impact of vibration on recovery from fatigue, however, is not known. This study aims to determine whether tendon vibration reduces recovery time following fatiguing contractions.

METHODS

Eight sedentary males (22 ± 2.8 yr) performed a fatiguing protocol of ankle dorsiflexor muscles on two separate days, with a minimum of 48 h between visits. Surface EMG was recorded from the tibialis anterior muscle while participants were performing 25 maximal voluntary contractions (MVCs), each lasting 5 s and separated by 2 s. Following the fatiguing protocol, recovery was assessed with 3-s MVC each minute over a 10-min period. Recovery time was defined as the time at which force had returned to 90% of baseline values. At one visit, vibration was applied to the distal tendon of the tibialis anterior muscle between MVCs (throughout recovery). The alternate visit involved a sham condition in which no vibration was applied.

RESULTS

MVC force (P = 0.48) and EMG amplitude (P = 0.26) were not significantly different across testing days. Both MVC force (P < 0.001) and EMG amplitude (P < 0.001) declined significantly at the end of the fatigue protocol. However, there were no significant interaction effects for MVC force (P = 0.82) or EMG amplitude (P = 0.09), indicating similar levels of fatigue across days. With tendon vibration, MVC force recovered within 4.0 ± 2.5 min, which was not different from the sham condition (5.3 ± 1.8 min; P = 0.42). Similarly, EMG recovery time was not different between vibration condition (3.9 ± 3.8 min) and sham condition (4.9 ± 2.5 min) (P = 0.41).

CONCLUSIONS

These results suggest that activation of excitatory group Ia afferents through tendon vibration does not substantially alter recovery time following fatigue.

Effects of quadriceps muscle fatigue on stiff-knee gait in patients with hemiparesis

The relationship between neuromuscular fatigue and locomotion has never been investigated in hemiparetic patients despite the fact that, in the clinical context, patients report to be more spastic or stiffer after walking a long distance or after a rehabilitation session. The aim of this study was to evaluate the effects of quadriceps muscle fatigue on the biomechanical gait parameters of patients with a stiff-knee gait (SKG). Thirteen patients and eleven healthy controls performed one gait analysis before a protocol of isokinetic quadriceps fatigue and two after (immediately after and after 10 minutes of rest). Spatiotemporal parameters, sagittal knee and hip kinematics, rectus femoris (RF) and vastus lateralis (VL) kinematics and electromyographic (EMG) activity were analyzed. The results showed that quadriceps muscle weakness, produced by repetitive concentric contractions of the knee extensors, induced an improvement of spatiotemporal parameters for patients and healthy subjects. For the patient group, the increase in gait velocity and step length was associated with i) an increase of sagittal hip and knee flexion during the swing phase, ii) an increase of the maximal normalized length of the RF and VL and of the maximal VL lengthening velocity during the pre-swing and swing phases, and iii) a decrease in EMG activity of the RF muscle during the initial pre-swing phase and during the latter 2/3 of the initial swing phase. These results suggest that quadriceps fatigue did not alter the gait of patients with hemiparesis walking with a SKG and that neuromuscular fatigue may play the same functional role as an anti-spastic treatment such as botulinum toxin-A injection. Strength training of knee extensors, although commonly performed in rehabilitation, does not seem to be a priority to improve gait of these patients.

Muscle spasticity associated with reduced whole-leg perfusion in persons with spinal cord injury

OBJECTIVE

To determine the association between peripheral blood flow and spasticity in individuals with spinal cord injury (SCI).

DESIGN

A cross-sectional study with measurements of muscle spasticity and whole-limb blood flow in individuals with SCI.

SETTING

University of Texas at Austin and Brain & Spine Recovery Center, Austin, TX, USA.

PARTICIPANTS

Eighteen individuals (14 males and 4 females) with SCI were classified into high (N = 7), low (N = 6), and no (N = 5) spasticity groups according to the spasticity levels determined by the modified Ashworth scale scores.

INTERVENTIONS

Whole-limb blood flow was measured in the femoral and brachial arteries using Doppler ultrasound and was normalized to lean limb mass obtained with dual-energy X-ray absorptiometry.

OUTCOME MEASURES

Limb blood flow and muscle spasticity.

RESULTS

Age, time post-SCI, and the American Spinal Injury Association impairment scale motor and sensory scores were not different among groups with different muscle spasticity. Femoral artery blood flow normalized to lean leg mass was different (P = 0.001) across the three spasticity groups (high 78.9 ± 16.7, low 98.3 ± 39.8, no 142.5 ± 24.3 ml/minute/kg). Total leg muscle spasticity scores were significantly and negatively correlated with femoral artery blood flow (r = -0.59, P < 0.01). There was no significant difference in brachial artery blood flow among the groups.

CONCLUSIONS

Whole-leg blood flow was lower in individuals with greater spasticity scores. These results suggest that a reduction in lower-limb perfusion may play a role, at least in part, in the pathogenesis leading to muscle spasticity after SCI.

Influence of back muscle fatigue on lumbar reflex adaptation during sudden external force perturbations

There is still conflicting evidence about the influence of fatigue on trunk reflex activity. The aim of this study was to measure response latency and amplitude changes of lumbar and abdominal muscles after heavy external force perturbation applied to the trunk in the sagittal plane before and after back muscle fatigue, in expected and unexpected conditions. Ten healthy subjects in a semi-seated position, torso upright in a specific apparatus performed an intermittent back muscle fatigue protocol. EMG reflex activity of erector spinae (ES) and external oblique muscles were recorded in unexpected and in expected (self pre-activation) conditions. After fatigue, the normalized reflex amplitude of ES increased in expected and unexpected conditions (P<0.05) while ES response latency was slightly decreased. Reflexes latencies for ES were systematically shorter (P<0.05) of 25% in expected compared to unexpected conditions. These findings suggest that a large external force perturbation would elicit higher paraspinal magnitude responses and possible earlier activation in order to compensate the loss of muscular force after fatigue. Because of the seated position the postural adjustments were probably not triggered and thus explain the lack of abdominal activation. The self-anticipated pre-activation in order to counteract perturbations was not affected by fatigue illustrating the natural muscular activation to maintain trunk stability.

Ischemic compression block attenuates mechanical hyperalgesia evoked from latent myofascial trigger points

The aim of the present study is to test the hypothesis that large-diameter myelinated muscle afferents contribute to the pathophysiology of myofascial trigger points (MTrPs). The ischemic compression blockage (ICB) of large-diameter myelinated muscle afferents was obtained with a 7-cm-wide tourniquet applied around the upper arm proximal to the brachioradialis muscle in 20 healthy subjects. This study consisted of two randomized sessions with an interval of 1 week in between each session. In one session, pressure pain threshold (PPT) and pressure threshold for eliciting referred pain (PTRP) were measured at an MTrP region in the brachioradialis muscle in one forearm. In another session, PPT was measured at a non-MTrP region in the brachioradialis muscle of the contralateral forearm at the time of pre-compression, 20 min following compression, and 10 min after decompression. The results showed that ICB, which mainly blocks large-diameter myelinated muscle afferents, was associated with an increase in PPT and PTRP (all P < 0.001) at MTrP regions but not at non-MTrP regions. These results suggest that large-diameter muscle afferents may be involved in pain and mechanical hyperalgesia at MTrPs.

Influence of activity-induced axonal hypoexcitability on transmission of descending and segmental signals

In this experiment, the changes in excitability of motor axons produced after natural activity were measured in nine healthy subjects using 1 min of maximal voluntary contractions (MVC) of the abductor digiti minimi (ADM) by studying the relationship between stimulus intensity applied to the ulnar nerve and the size of the ADM compound muscle action potential (CMAP). On cessation of the contraction, there was a prominent right-shift of the input-output curve: the intensity required to produce a control CMAP approximately 60% of maximum, generated a post-contraction response approximately 25% of maximum. Similar changes occurred in the input-output curves obtained by recording the ulnar nerve volley evoked by same test stimulus for CMAP. Motor-evoked potential (MEP) and F-waves (and H-reflex in one subject) were recorded from ADM before and after 1 min of MVC. On cessation of contraction, the MEP input-output curves exhibited a significant right-shift: the stimulus required to evoke a pre-contraction maximum MEP ( approximately 60% of maximum CMAP) generated a post-contraction response approximately 65% of initial values. One minute of MVC produced similar decreases of F ( approximately 35%)- and H ( approximately 30%)-ADM responses. All responses recovered their control value in 15-20 min after the end of contraction. The almost identical depressive effect produced by 1 min of MVC on peripherally and centrally generated muscle responses suggests a common conditioning factor. These findings are discussed within the context of activity-induced motor axonal hyperpolarizion.

High frequency tendon reflexes in the human soleus muscle

Tendon reflexes have been often used in studies of the human nervous system in health and disease. They have been investigated either in response to single tendon taps or to long duration vibrations. Tendon reflexes are described here in response to a high frequency vibration burst (3 cycles of a 100 Hz sine wave) applied to the Achilles tendon of standing subjects, either in quiet stance or during a forward leaning posture. The electromyogram from the soleus muscle usually showed three components separated by 10 ms which were interpreted as being three reflexes, each reflex induced by each of the three cycles in a burst. This result indicates that soleus tendon reflexes can respond in fast succession in a phasic manner when a brief high frequency vibration is applied to the Achilles tendon. This occurs in spite of possible depression of the Ia to motoneuron synapses and the long after hyperpolarization of the motoneurons. An interpretation of the results is that motoneurons from different subsets of the motoneuron pool respond to different cycles of the sinusoidal vibratory burst.

Combined effects of fatigue and decision making on female lower limb landing postures: central and peripheral contributions to ACL injury risk

BACKGROUND

In spite of ongoing prevention developments, anterior cruciate ligament injury rates and the associated sex-disparity have remained, suggesting an incomplete understanding of the injury mechanism. While both fatigue and decision making are known in isolation to directly impact anterior cruciate ligament injury risk, their combined manifestations remain unknown. We thus examined the combined effects of fatigue and decision making on lower limb kinematics during sports relevant landings.

METHODS

Twenty five female National College Athletic Association athletes had initial contact and peak stance phase 3D lower limb joint kinematics quantified during anticipated and unanticipated single (left and right) leg landings, both before and during the accumulation of fatigue. Jump direction was governed by light stimuli activated prior to and during the pre-land phase of respective anticipated and unanticipated trials. To induce fatigue, subjects performed repetitive squat (n=5) and randomly ordered jump sequences, until squats were no longer possible. Subject-based measures of each dependent factor were then calculated across pre-fatigue trials, and for those denoting 100% and 50% fatigue, and submitted to a 3-way mixed design analysis of covariance to test for the main effects of fatigue time, decision and leg.

FINDINGS

Fatigue caused significant increases in initial contact hip extension and internal rotation, and in peak stance knee abduction and internal rotation and ankle supination angles. Fatigue-induced increases in initial contact hip rotations and in peak knee abduction angle were also significantly more pronounced during unanticipated compared to anticipated landings.

INTERPRETATION

The integrative effects of fatigue and decision making may represent a worst case scenario in terms of anterior cruciate ligament injury risk during dynamic single leg landings, by perpetuating substantial degradation and overload of central control mechanisms.

Effect of random interpulse interval modulation on neuromuscular fatigue

Neuromuscular endurance during electrical stimulation may be enhanced if naturally occurring motor unit firing patterns are used. Variability in the interpulse interval (IPI) distribution may enable brief periods of rest and optimization of force output. Nine individuals participated in three 3-minute fatigue protocols of the thenar muscles elicited by supramaximal stimulation of the median nerve. All protocols consisted of a mean IPI of 33.3 ms and differed only in the type of IPI modulation, which was constant (0%), random (+/-20%), or ramped from 0% to +/-20%. M-wave amplitude declined following all protocols and the reduction was smallest following the ramp protocol. There was no significant difference among the starting or final forces or between the overall force-time integrals for the three protocols. Thus, IPI variability did not improve endurance time during electrical stimulation and the M-wave amplitude was not a reliable indicator of muscle force output.