Neural compensation for muscular fatigue: evidence for significant force regulation in man

Data-Driven Approach for Upper Limb Fatigue Estimation Based on Wearable Sensors

Muscle fatigue is defined as a reduced ability to maintain maximal strength during voluntary contraction. It is associated with musculoskeletal disorders that affect workers performing repetitive activities, affecting their performance and well-being. Although electromyography remains the gold standard for measuring muscle fatigue, its limitations in long-term work motivate the use of wearable devices. This article proposes a computational model for estimating muscle fatigue using wearable and non-invasive devices, such as Optical Fiber Sensors (OFSs) and Inertial Measurement Units (IMUs) along the subjective Borg scale. Electromyography (EMG) sensors are used to observe their importance in estimating muscle fatigue and comparing performance in different sensor combinations. This study involves 30 subjects performing a repetitive lifting activity with their dominant arm until reaching muscle fatigue. Muscle activity, elbow angles, and angular and linear velocities, among others, are measured to extract multiple features. Different machine learning algorithms obtain a model that estimates three fatigue states (low, moderate and high). Results showed that between the machine learning classifiers, the LightGBM presented an accuracy of 96.2% in the classification task using all of the sensors with 33 features and 95.4% using only OFS and IMU sensors with 13 features. This demonstrates that elbow angles, wrist velocities, acceleration variations, and compensatory neck movements are essential for estimating muscle fatigue. In conclusion, the resulting model can be used to estimate fatigue during heavy lifting in work environments, having the potential to monitor and prevent muscle fatigue during long working shifts.

Enhanced selectivity of transcutaneous spinal cord stimulation by multielectrode configuration

Objective.Transcutaneous spinal cord stimulation (tSCS) has been gaining momentum as a non-invasive rehabilitation approach to restore movement to paralyzed muscles after spinal cord injury (SCI). However, its low selectivity limits the types of movements that can be enabled and, thus, its potential applications in rehabilitation.Approach.In this cross-over study design, we investigated whether muscle recruitment selectivity of individual muscles could be enhanced by multielectrode configurations of tSCS in 16 neurologically intact individuals. We hypothesized that due to the segmental innervation of lower limb muscles, we could identify muscle-specific optimal stimulation locations that would enable improved recruitment selectivity over conventional tSCS. We elicited leg muscle responses by delivering biphasic pulses of electrical stimulation to the lumbosacral enlargement using conventional and multielectrode tSCS.Results.Analysis of recruitment curve responses confirmed that multielectrode configurations could improve the rostrocaudal and lateral selectivity of tSCS. To investigate whether motor responses elicited by spatially selective tSCS were mediated by posterior root-muscle reflexes, each stimulation event was a paired pulse with a conditioning-test interval of 33.3 ms. Muscle responses to the second stimulation pulse were significantly suppressed, a characteristic of post-activation depression suggesting that spatially selective tSCS recruits proprioceptive fibers that reflexively activate muscle-specific motor neurons in the spinal cord. Moreover, the combination of leg muscle recruitment probability and segmental innervation maps revealed a stereotypical spinal activation map in congruence with each electrode's position.Significance. Improvements in muscle recruitment selectivity could be essential for the effective translation into stimulation protocols that selectively enhance single-joint movements in neurorehabilitation.

Enhanced selectivity of transcutaneous spinal cord stimulation by multielectrode configuration

Objective

Transcutaneous spinal cord stimulation (tSCS) has been gaining momentum as a non-invasive rehabilitation approach to restore movement to paralyzed muscles after spinal cord injury (SCI). However, its low selectivity limits the types of movements that can be enabled and, thus, its potential applications in rehabilitation.

Approach

In this cross-over study design, we investigated whether muscle recruitment selectivity of individual muscles could be enhanced by multielectrode configurations of tSCS in 16 neurologically intact individuals. We hypothesized that due to the segmental innervation of lower limb muscles, we could identify muscle-specific optimal stimulation locations that would enable improved recruitment selectivity over conventional tSCS. We elicited leg muscle responses by delivering biphasic pulses of electrical stimulation to the lumbosacral enlargement using conventional and multielectrode tSCS.

Results

Analysis of recruitment curve responses confirmed that multielectrode configurations could improve the rostrocaudal and lateral selectivity of tSCS. To investigate whether motor responses elicited by spatially selective tSCS were mediated by posterior root-muscle reflexes, each stimulation event was a paired pulse with a conditioning-test interval of 33.3 ms. Muscle responses to the second stimulation pulse were significantly suppressed, a characteristic of post-activation depression suggesting that spatially selective tSCS recruits proprioceptive fibers that reflexively activate muscle-specific motor neurons in the spinal cord. Moreover, the combination of leg muscle recruitment probability and segmental innervation maps revealed a stereotypical spinal activation map in congruence with each electrode's position.

Significance

Improvements in muscle recruitment selectivity could be essential for the effective translation into stimulation protocols that selectively enhance single-joint movements in neurorehabilitation.

Epidural electrical stimulation of the cervical dorsal roots restores voluntary upper limb control in paralyzed monkeys

Regaining arm control is a top priority for people with paralysis. Unfortunately, the complexity of the neural mechanisms underlying arm control has limited the effectiveness of neurotechnology approaches. Here, we exploited the neural function of surviving spinal circuits to restore voluntary arm and hand control in three monkeys with spinal cord injury, using spinal cord stimulation. Our neural interface leverages the functional organization of the dorsal roots to convey artificial excitation via electrical stimulation to relevant spinal segments at appropriate movement phases. Stimulation bursts targeting specific spinal segments produced sustained arm movements, enabling monkeys with arm paralysis to perform an unconstrained reach-and-grasp task. Stimulation specifically improved strength, task performances and movement quality. Electrophysiology suggested that residual descending inputs were necessary to produce coordinated movements. The efficacy and reliability of our approach hold realistic promises of clinical translation.

Change of bio-electric interferential currents of acute fatigue and recovery in male sprinters

We studied the muscle fatigue and recovery of thirty male sprinters (aged 18–22 years) using the Frequency Analysis Method (FAM). The interferential currents (ICs) with different thresholds for sensory, motor and pain responses, the maximal voluntary contraction (MVC), and the amplitude of the surface EMG (aEMG, sEMG) were assessed prior to and immediately after an acute explosive fatigue training session, and during one-week recovery. We found that IC increased on average from 32.3 ± 8.9 mA to 37.5 ± 7.5 mA in sensory response at 10 Hz immediately post training (p = 0.004) but decreased at 24-hr post training (p = 0.008) and returned to pre-levels thereafter. Motor and pain response patterns at 10 Hz were similar (motor: p = 0.033 and 0.040; pain: p = 0.022 and 0.019, respectively). The change patterns of ICs were similar to but prior to the changes of sEMG. The agreement between IC assessment and amplitude of sEMG (aEMG)/MVC ratio was good (>95%). The present study suggested that the changes in ICs were prior to the changes in both the aEMG and force during fatigue. These changes may reflect the physiological sensory change due to peripheral fatigue. FAM may be useful as an effective early detection and simple tool for monitoring muscle fatigue during training and recovery in athletes.

Changes in motor unit behavior following isometric fatigue of the first dorsal interosseous muscle

The neuromuscular strategies employed to compensate for fatigue-induced muscle force deficits are not clearly understood. This study utilizes surface electromyography (sEMG) together with recordings of a population of individual motor unit action potentials (MUAPs) to investigate potential compensatory alterations in motor unit (MU) behavior immediately following a sustained fatiguing contraction and after a recovery period. EMG activity was recorded during abduction of the first dorsal interosseous in 12 subjects at 20% maximum voluntary contraction (MVC), before and directly after a 30% MVC fatiguing contraction to task failure, with additional 20% MVC contractions following a 10-min rest. The amplitude, duration and mean firing rate (MFR) of MUAPs extracted with a sEMG decomposition system were analyzed, together with sEMG root-mean-square (RMS) amplitude and median frequency (MPF). MUAP duration and amplitude increased immediately postfatigue and were correlated with changes to sEMG MPF and RMS, respectively. After 10 min, MUAP duration and sEMG MPF recovered to prefatigue values but MUAP amplitude and sEMG RMS remained elevated. MU MFR and recruitment thresholds decreased postfatigue and recovered following rest. The increase in MUAP and sEMG amplitude likely reflects recruitment of larger MUs, while recruitment compression is an additional compensatory strategy directly postfatigue. Recovery of MU MFR in parallel with MUAP duration suggests a possible role for metabolically sensitive afferents in MFR depression postfatigue. This study provides insight into fatigue-induced neuromuscular changes by examining the properties of a large population of concurrently recorded single MUs and outlines possible compensatory strategies involving alterations in MU recruitment and MFR.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

Effects of fatigue on trunk stability in elite gymnasts

The aim of the present study was to test the hypothesis that fatigue due to exercises performed in training leads to a decrement of trunk stability in elite, female gymnasts. Nine female gymnasts participated in the study. To fatigue trunk muscles, four series of five dump handstands on the uneven bar were performed. Before and after the fatigue protocol, participants performed three trials of a balancing task while sitting on a seat fixed over a hemisphere to create an unstable surface. A force plate tracked the location of the center of pressure (CoP). In addition, nine trials were performed in which the seat was backward inclined over a set angle and suddenly released after which the subject had to regain balance. Sway amplitude and frequency in unperturbed sitting were determined from the CoP time series and averaged over trials. The maximum displacement and rate of recovery of the CoP location after the sudden release were determined and averaged over trials. After the fatigue protocol, sway amplitude in the fore-aft direction was significantly increased (p = 0.03), while sway frequency was decreased (p = 0.005). In addition, the maximum displacement after the sudden release was increased (p = 0.009), while the rate of recovery after the perturbation was decreased (p = 0.05). Fatigue induced by series of exercises representing a realistic training load caused a measurable decrement in dynamic stability of the trunk in elite gymnasts.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

Quadriceps fatigue alters human muscle performance during a novel weight bearing task

Limited information is currently available regarding muscle synergistic patterns and triggered reflex responses during dynamic weight-bearing activities in the presence of muscle fatigue.

PURPOSE

The purpose of this study was to examine the effects of quadriceps muscle fatigue on patterns of muscle activation and performance in response to sudden, unexpected perturbations during a weight-bearing task.

METHODS

Motion of the knee was measured as subjects were asked to track a visual target as accurately as possible while performing a resisted single leg squat task. Random perturbations were delivered in 20% of the trials by unexpectedly releasing the resistance during the flexion phase of the exercise. Absolute and constant errors were calculated to evaluate target tracking performance. Quadriceps and hamstring muscle activity was recorded during both perturbed and unperturbed trials. Twelve healthy women were tested before and after completing a repetitive submaximal eccentric quadriceps fatigue protocol. A second group of 12 women served as controls. Unexpected perturbations elicited long-latency responses characterized by facilitation of the quadriceps and inhibition of the hamstrings.

RESULTS

Muscle fatigue increased the amplitude of the long-latency response in vastus lateralis by 4.3% maximum voluntary isometric contraction (P = 0.004). Changes in tracking error occurred in response to perturbations after fatigue in spite of significantly increased quadriceps muscle activity, especially during the extension phase of the exercise.

CONCLUSION

Quadriceps muscle fatigue alters the patterns of coordinated muscle activity and may render subjects less able to cope with unexpected perturbations during weight-bearing tasks.

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.

The implications of force feedback for the lambda model

It is argued here that length and force feedback play important but distinct roles in motor coordination. Length feedback compensates for several nonlinear properties of muscle and therefore simplifies its behavior, but in addition promotes the nonlinear relationship between force and stiffness that is essential to the mechanism for modulating joint stiffness. Excitatory force feedback is also primarily autogenic. Under conditions of level treadmill stepping in cat walking, positive force feedback is restricted in the distal hindlimb to a few and perhaps only one ankle extensor, the gastrocnemius muscle group. Based on the anatomy of this group, positive force feedback provides a stiff linkage that reinforces proportional coordination between ankle and knee joints. In terms of the lambda model, excitatory force feedback can reinforce muscular force generation and stiffness, but should have no significant effect on activation threshold. Inhibitory force feedback projects mainly to muscles that span different joints and axes of rotation than the parent muscle. This heterogenic force feedback is thought to promote interjoint coordination and thought to influence stiffness of the joints and limbs. During locomotion, the inhibitory influences appear to be focused on the distal musculature. Since the inhibitory force feedback is heterogenic, it also influences the threshold for activation of relevant musculature. Threshold is therefore not entirely a control variable and independent of feedback. It is proposed that the actuators for movement consist of systems of muscles or motor units that are linked by feedback and that receive control signals from elsewhere in the nervous system.

Effect of Physical Fatigue on Motor Control at Different Skill Levels

The purpose of this experiment was to explore the effect of fatigue on motor coordination, and of prospective adjustment strategies to compensate for fatigue in a multijoint movement. Two male groups ( N = 8) participated in the experiment: Highly skilled table tennis players ( M age = 27 yr., SD = 2.3, n = 4) and Recreational table tennis players ( M age = 25.9 yr., SD = 0.04, n = 4). The task was an attacking forehand drive towards a scaled target on the opposite side of the net. The Highly skilled players adjusted their movement patterns and preserved the task requirements in terms of spatial accuracy under the condition of fatigue by using opportunistic movement coordination. The Recreational players did not adjust their forehand drive, and spatial accuracy deteriorated. The current results support the notion that expertise enhances potential to adjust motor coordination strategies as a reaction to induced physical fatigue.

Self-rescue swimming in cold water: the latest advice

According to the 2006 Canadian Red Cross Drowning Report, 2007 persons died of cold-water immersion in Canada between 1991 and 2000. These statistics indicate that prevention of cold-water immersion fatalities is a significant public health issue for Canadians. What should a person do after accidental immersion in cold water? For a long time, aquatic safety organizations and government agencies stated that swimming should not be attempted, even when a personal flotation device (PFD) is worn. The objective of the present paper is to present the recent scientific evidence making swimming a viable option for self-rescue during accidental cold-water immersion. Early studies in the 1960s and 1970s led to a general conclusion that "people are better off if they float still in lifejackets or hang on to wreckage and do not swim about to try to keep warm". Recent evidence from the literature shows that the initial factors identified as being responsible for swimming failure can be either easily overcome or are not likely the primary contributors to swimming failure. Studies over the last decade reported that swimming failure might primarily be related not to general hypothermia, but rather to muscle fatigue of the arms as a consequence of arm cooling. This is based on the general observation that swimming failure developed earlier than did systemic hypothermia, and can be related to low temperature of the arm muscles following swimming in cold water. All of the above studies conducted in water between 10 and 14 degrees C indicate that people can swim in cold water for a distance ranging between about 800 and 1500 m before being incapacitated by the cold. The average swimming duration for the studies was about 47 min before incapacitation, regardless of the swimming ability of the subjects. Recent evidence shows that people have a very accurate idea about how long it will take them to achieve a given swimming goal despite a 3-fold overestimation of the absolute distance to swim. The subjects were quite astute at deciding their swimming strategy early in the immersion with 86% success, but after about 30 min of swimming or passive cooling, their decision-making ability became impaired. It would therefore seem wise to make one's accidental immersion survival plan early during the immersion, directly after cessation of the cold shock responses. Additional recommendations for self-rescue are provided based on recent scientific evidence.

Effects of fatigue and gender on peroneal reflexes elicited by sudden ankle inversion

The purpose of this study was to investigate the effects of neuromuscular fatigue and gender on peroneus brevis muscle (PB) and peroneus longus muscle (PL) reflexes elicited by sudden ankle inversion. Sixteen males and fifteen females stood on a "trap-door" platform that suddenly inverted the ankle 20 degrees when released. The resulting movement elicited peroneal reflexes. Reflex measurements were collected both before and after fatiguing the ankle evertors. Reflex delay in the PB and PL was not affected by fatigue, gender, or their interaction. Reflex amplitude in the PL exhibited no main effect of fatigue or gender, but their interaction was significant. Pairwise comparisons revealed PL reflex amplitude decreased by 11.3% in males (p=0.008) and increased 22.1% in females (p=0.003) with fatigue. A similar trend was found in the PB, but was not statistically significant. This opposite effect in females and males may be due to differences in how males and females compensate for fatigue, and may be related to the gender difference in ankle injury rate.

Does motoneuron adaptation contribute to muscle fatigue?

To help reduce the gap between the cellular physiology of motoneurons (MNs) as studied "bottom-up" in animal preparations and the "top-down" study of the firing patterns of human motor units (MUs), this article addresses the question of whether motoneuron adaptation contributes to muscle fatigue. Findings are reviewed on the intracellularly recorded electrophysiology of spinal MNs as studied in vivo and in vitro using animal preparations, and the extracellularly recorded discharge of MUs as studied in conscious humans. The latter "top-down" approach, combined with kinetic measurements, has provided most of what is currently known about the neurobiology of muscle fatigue, including its task and context dependencies. It is argued that although the question addressed is still open, it should now be possible to design new "bottom-up" research paradigms using animal preparations that take advantage of what has been learned with the use of relatively noninvasive quantitative procedures in conscious humans.

Localized muscle pain causes prolonged recovery after fatiguing isometric contractions

The purpose of this study was to investigate the force and electromyographic (EMG) signal recorded from the muscles immediately after a sustained fatiguing contraction with or without muscle pain. Ten subjects performed sustained dorsi- and plantarflexions at two contraction levels (50 and 80% of maximum voluntary contraction) until exhaustion with or without muscle pain induced by injection of 6% hypertonic saline in one of the torque producing muscles. The muscle pain intensity was scored on a visual analogue scale (VAS, 0-10 cm). The root mean square (RMS) of the surface EMG signal from plantarflexors and dorsiflexors were estimated during maximum voluntary contractions (MVC) and ramp contractions before and after the fatiguing task at 0, 5, 10 and 15 min during the recovery phase. VAS scores immediately after the contractions with hypertonic saline (on average 3.2 +/- 1.1 cm) progressively decreased during recovery and no pain was experienced 15 min after the contraction. After the painful contraction the RMS-EMG during MVC was on average decreased (23.4 +/- 7.4%) compared to the non-painful condition both in muscles where pain was previously induced and in non-painful synergists. During recovery, the slope of the torque-EMG curve during ramp contraction was significantly decreased (28.4 +/- 8.1%) after the painful contraction compared to the control contraction both for the muscle previously exposed to pain and also the other active synergists. The decreased EMG during recovery after painful contractions compared with control was not accompanied by significant reductions in force during MVC indicating a change in the strategy for motor unit recruitment. This study shows that localized muscle pain inhibits muscle activation and increases the effects of fatigue on EMG recovery curves both for painful and non-painful synergists probably by a central effect. These effects can modify the normal patterns of synergistic activation and can also generate overload problems in muscle pain patients if compensatory motor control strategies are applied.

Reflex gain of muscle spindle pathways during fatigue

There are conflicting observations of the effects of fatigue on the sensitivity of large diameter Ia afferents. Our goal was to characterize any fatigue-related changes in the spinal reflex pathways during fatigue. Manipulation of the Ia afferent response by vibration and tendon tap, in which the motor neuron pool is modulated by both short- and long-loop activation from muscle spindles, were elicited before and after a fatigue task. The fatigue task consisted of intermittent submaximal and maximal voluntary contractions (MVCs). Percent voluntary activation fell from 98.75% MVC to 80.92% MVC following the fatigue task as measured by the twitch interpolation technique. Voluntary contractions of the same force profile as the force produced by 30 s of vibration were produced by having participants (n = 10) follow the trajectory on a computer monitor, before and after the fatigue task. Recruitment thresholds (RTs) of voluntarily activated units showed no change during fatigue; however, units activated via the reflex pathway were recruited approximately 30% sooner during fatigue (P < 0.05). The ratio of the electrical-to-mechanical response of the tendon tap increased significantly with fatigue. Our findings of decreased RTs in response to vibration and increased EMG activity during the tendon tap following the fatigue task indicate that Ia afferent input to the motoneuron pool was increased. The decrease in MVC force indicates that during this time the descending drive was compromised. These results provide evidence that the gain of the gamma loop is increased during fatigue, indicating possible peripheral neural compensation to the motor neuron pool in order to preserve force output.

Effect of lumbar extensor fatigue on paraspinal muscle reflexes

Low back disorders are a frequent medical problem. Altered neuromuscular control of the spine has been associated with low back pain, and may contribute to its occurrence. The purpose of this study was to investigate the effect of lumbar extensor fatigue on reflex delay and amplitude in the paraspinal muscles. Ten healthy males (20-22 years of age) were subjected to an anteriorly-directed perturbation applied at the inferior margin of the scapulae while standing quietly before and after a lumbar extensor fatiguing protocol. The fatiguing protocol consisted of multiple sets of back extensions and intermittent isometric maximum voluntary contraction on a Roman chair for 14 min until 60% of unfatigued lumbar extensor MVC was reached. Reflexes were recorded from the paraspinal muscles at the level of L4. Results indicated the mean reflex delay was 60+/-18 ms and was not affected by fatigue (p=0.278). Reflex amplitude increased 36+/-32% with fatigue (p=0.017). The increase in reflex amplitude may reflect an attempt to compensate for losses in muscle force capacity with fatigue in order to maintain sufficient spinal stability. However, additional studies are necessary to investigate the mechanisms of this fatigue-related change in paraspinal reflex.

Evidence for force-feedback inhibition in chronic stroke

The presence of force-feedback inhibition was explored during reflex responses in five subjects with known incidence of stroke. Using constant velocity stretches, it was previously found that after movement onset, active reflex force progressively increases with increasing joint angle, at a rate proportional to a fractional exponent of the speed of stretch. However, after the reflex force magnitude exceeds a particular level, it begins rolling off until maintaining a steady-state value. The magnitudes of these force plateaus are correlated with the speed of stretch, such that higher movement speeds result in higher steady-state forces. Based upon these previous studies, we hypothesized that force plateau behavior could be explained by a force-feedback inhibitory pathway. To help facilitate an understanding of this stretch reflex force roll off, a simple model representing the elbow reflex pathways was developed. This model contained two separate feedback pathways, one representing the monosynaptic stretch reflex originating from muscle spindle excitation, and another representing force-feedback inhibition arising from force sensitive receptors. It was found that force-feedback inhibition altered the stretch reflex response, resulting in a force response that followed a sigmoidal shape similar to that observed experimentally. Furthermore, simulated reflex responses were highly dependent on force-feedback gain, where predicted reflex force began plateauing at decreasing levels with increases in this force-feedback gain. The parameters from the model fits indicate that the force threshold for force-sensitive receptors is relatively high, suggesting that the inhibition may arise from muscle free nerve endings rather than Golgi tendon organs. The experimental results coupled with the simulations of elbow reflex responses suggest the possibility that after stroke, the effectiveness of force-feedback inhibition may increase to a level that has functional significance. Practical implications of these findings are discussed in relation to muscle weakness commonly associated with stroke.

Bifurcation and stability analysis in musculoskeletal systems: a study in human stance

Reflexes are important in the control of such daily activities as standing and walking. The goal of this study is to establish how reflexive feedback of muscle length, velocity, and force can lead to stable equilibria (i.e., posture) and limit cycles (e.g., ankle clonus and gait). The influence of stretch reflexes on the behavior and stability of musculoskeletal systems was examined using a model of human stance. We computed branches of fold and Hopf bifurcations by numerical bifurcation analysis of the model. These fold and Hopf branches divide the parameter space, constructed by the reflexive feedback gains, into regions of different behavior: unstable posture, stable posture, and stable limit cycles. These limit cycles correspond to a neural deficiency, termed ankle clonus. We also linked bifurcation analysis to known biomechanical concepts by linearizing the model: the fold branch corresponds to zero ankle stiffness and defines the minimal muscle length feedback necessary for stable posture; the Hopf branch is related to unstable reflex loops. Crossing the Hopf branch can lead to the above-mentioned stable limit cycles. The Hopf branch reduces with increasing time delays, making the subject's posture more susceptible to unstable reflex loops. This might be one of the reasons why elderly people, or those with injuries to the central nervous system, often have trouble with standing and other posture tasks. The influence of cocontraction and force feedback on the behavior of the posture model was also investigated. An increase in cocontraction leads to an increase in ankle stiffness (i.e., intrinsic muscle stiffness) and a decrease in the effective reflex loop gain. On the one hand, positive force feedback increases the ankle stiffness (i.e., intrinsic and reflexive muscle stiffness); on the other hand it makes the posture more susceptible to unstable reflex loops. For negative force feedback, the opposite is true. Finally, we calculated areas of reflex gains for perturbed stance and quiet stance in healthy subjects by fitting the model to data from the literature. The overlap of these areas of reflex gains could indicate that stretch reflexes are the major control mechanisms in both quiet and perturbed stance. In conclusion, this study has successfully combined bifurcation analysis with the more common biomechanical concepts and tools to determine the influence of reflexes on the stability and quality of stance. In the future, we will develop this line of research to look at rhythmic tasks, such as walking.

Fatigue-related changes in the coordination of lifting and their effect on low back load

In this study, changes in movement coordination caused by fatigue that developed during repetitive lifting were examined. Five men performed 6 times a 5-min bout of lifting an 8-kg barbell at 15 lifts/min, using two lifting techniques; one minimized trunk rotation (squat lift), and the other minimized rotation in the knee joint (stoop lift). Kinematics and dynamics were studied by means of movement analysis and inverse dynamics, using a two-dimensional linked segment model. Within-subject variation over repetitive lifts of the time course of joint angles was smaller than between-subjects variation on the first analyzed lift. Relative timing between joint rotations did not change significantly across repetitive lifts, except between knee and hip in the squat lift. No change of the lumbosacral torque over repetitive lifts was found. The adaptability of the neural control appeared to be sufficient to accommodate the strong changes of the input-output characteristics of the muscles caused by fatigue so that an essentially constant performance of the movement act was maintained.

Combined effect of repetitive work and cold on muscle function and fatigue

This study compared the effect of repetitive work in thermoneutral and cold conditions on forearm muscle electromyogram (EMG) and fatigue. We hypothesize that cold and repetitive work together cause higher EMG activity and fatigue than repetitive work only, thus creating a higher risk for overuse injuries. Eight men performed six 20-min work bouts at 25 degrees C (W-25) and at 5 degrees C while exposed to systemic (C-5) and local cooling (LC-5). The work was wrist flexion-extension exercise at 10% maximal voluntary contraction. The EMG activity of the forearm flexors and extensors was higher during C-5 (31 and 30%, respectively) and LC-5 (25 and 28%, respectively) than during W-25 (P < 0.05). On the basis of fatigue index (calculated from changes in maximal flexor force and flexor EMG activity), the fatigue in the forearm flexors at the end of W-25 was 15%. The corresponding values at the end of C-5 and LC-5 were 37% (P < 0.05 in relation to W-25) and 20%, respectively. Thus repetitive work in the cold causes higher EMG activity and fatigue than repetitive work in thermoneutral conditions.

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.

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.

Reflex and intrinsic changes induced by fatigue of human elbow extensor muscles

Fatigue-induced changes in intrinsic and reflex properties of human elbow extensor muscles and the underlying mechanisms for fatigue compensation were investigated. The elbow joint was perturbed using small-amplitude and pseudorandom movement patterns while subjects maintained steady levels of mean joint extension torque. Intrinsic and reflex properties were identified simultaneously using a nonlinear delay differential equation model. Intrinsic joint properties were characterized by measures of joint stiffness, viscous damping, and limb inertia and reflex properties characterized by measures of dynamic and static reflex gains. Fatigue was induced using 15 min of intermittent voluntary isometric (submaximal) exercise, and a rest period of 10 min was taken to allow the fatigued muscles to recover from acute fatigue effects. Identical experimental and data analysis procedures were used before and after fatigue. Our findings were that after fatigue, joint stiffness was significantly reduced at higher torque levels, presumably reflecting the reduced force-generating capacity of fatigued muscles. Conversely, joint viscosity was increased after fatigue potentially because of the reduced crossbridge detachment rate and prolonged relaxation associated with intracellular acidosis accompanying fatigue. Static stretch reflex gain decreased significantly at higher torque levels after fatigue, indicating that the isometric fatiguing exercise might be associated with a preferential change in properties of spindle chain fibers and bag(2) fibers. For matched pre- and postfatigue torque levels, dynamic reflexes contributed relatively more torque after fatigue, displaying higher dynamic reflex gains and larger dynamic electromyographic responses elicited by the controlled small-amplitude position perturbations. These changes appear to counteract the fatigue-induced reductions in joint stiffness and static reflex gain. The compensatory responses could be partly due to the effects of increasing the number of active motoneurons innervating the fatiguing muscles. This shift in operating point gave rise to significant compensation for the loss of contractile force. The compensation could also be due to fusimotor adjustment, which could make the dynamic reflex gain much less sensitive to fatigue than intrinsic stiffness. In short, the reduced contribution from intrinsic stiffness to joint torque was compensated by increased contribution from dynamic stretch reflexes after fatigue.

Influence of fatigue on EMG/force ratio and cocontraction in cycling

PURPOSE

The purpose of the present study was to observe force and power losses and electromyographic manifestations of fatigue during repeated sprints performed on a friction-loaded cycle ergometer.

METHODS

Ten subjects performed 15 maximal 5-s sprints with 25-s rests between them. Power, velocity, and torque were measured during sprints 1 and 13 and during two submaximal constant-velocity (50 rpm) periods of cycling performed before and after the sprints. The EMG signals of five leg muscles were stored to determine the EMG/force ratio of power producer muscles and the coactivation of antagonist muscles. The power producer muscles were activated to the same level during sprints 1 and 13, despite a loss of force, whereas the vastus lateralis muscle was recruited more during the submaximal cycling period under fatigue conditions.

RESULTS

This led to an increased EMG/force ratio for the power producer muscles, indicating the peripheral fatigue status of these muscles. Antagonist muscles were less activated during the sprints after fatigue; whereas they stayed unchanged during the last submaximal cycling period.

CONCLUSIONS

This suggests that there is a decrease in coactivation as agonist force is lost. This decrease in coactivation under fatigue conditions has not been previously reported and is probably due to the training status of the subjects. Subjects may have learned to better use their antagonist muscles to efficiently transfer force and power to the rotating pedal. This coordination can be adapted to cope with fatigue of the power producer muscles.

1998 ISEK Congress Keynote Lecture: The use of electromyography in applied physiology. International Society of Electrophysiology and Kinesiology

Electromyogram (EMG) analyses (surface, intramuscular and evoked potentials) in studies of muscle function have attracted increasing attention during recent years and have been applied to assess muscle endurance capacity, anaerobic and lactate thresholds, muscle biomechanics, motor learning, neuromuscular relaxation, optimal walking and pedalling speeds, muscle soreness, neuromuscular diseases, motor unit (MU) activities (MU recruitment and rate coding), and skeletal muscle fatigue. This paper deals with the use of EMG analyses employed in the area of applied physiology and is divided into three sections: surface EMG analyses; intramuscular EMG analyses; and evoked potential analyses.

Aftereffects of resisted muscle contractions on the accuracy of joint position sense in elite male athletes

OBJECTIVE

To examine the effects of quadriceps and hamstring muscle strength testing on the results of subsequent knee joint position sense tests.

DESIGN

A case-control study of 40 elite male athletes divided equally into two groups, experimental and control.

METHODS

Both groups underwent position sense tests of both knees, with eyes closed, with isometric maintenance of the knee in each test position, return of the limb to the starting position, then active replication of the perceived test position using the same limb. In the experimental group, joint sense testing commenced 5 to 25 min after strength testing of quadriceps and hamstring muscle strength in both legs using maximum isokinetic contractions at plateau speeds of 60 degrees/sec and 120 degrees/sec. The control group did not undergo pretest muscle strength testing.

RESULTS

The difference in the mean absolute (signless) and relative (signed) position sense errors between the experimental and control groups was 0.8 degrees and 1.4 degrees, respectively. The difference between the standard deviation of the relative errors was 1.2 degrees. These results were not statistically significant (analysis of variance p = .24, .12, and .13, respectively).

CONCLUSION

In elite male athletes knee joint position sense is unaffected by nonfatiguing strength tests conducted 5 to 25 min before position sense testing.

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].

A neural model of cerebellar learning for arm movement control: cortico-spino-cerebellar dynamics

A neural network model of opponent cerebellar learning for arm movement control is proposed. The model illustrates how a central pattern generator in cortex and basal ganglia, a neuromuscular force controller in spinal cord, and an adaptive cerebellum cooperate to reduce motor variability during multijoint arm movements using mono- and bi-articular muscles. Cerebellar learning modifies velocity commands to produce phasic antagonist bursts at interpositus nucleus cells whose feed-forward action overcomes inherent limitations of spinal feedback control of tracking. Excitation of alpha motoneuron pools, combined with inhibition of their Renshaw cells by the cerebellum, facilitate movement initiation and optimal execution. Transcerebellar pathways are opened by learning through long-term depression (LTD) of parallel fiber-Purkinje cell synapses in response to conjunctive stimulation of parallel fibers and climbing fiber discharges that signal muscle stretch errors. The cerebellar circuitry also learns to control opponent muscles pairs, allowing cocontraction and reciprocal inhibition of muscles. Learning is stable, exhibits load compensation properties, and generalizes better across movement speeds if motoneuron pools obey the size principle. The intermittency of climbing fiber discharges maintains stable learning. Long-term potentiation (LTP) in response to uncorrelated parallel fiber signals enables previously weakened synapses to recover. Loss of climbing fibers, in the presence of LTP, can erode normal opponent signal processing. Simulated lesions of the cerebellar network reproduce symptoms of cerebellar disease, including sluggish movement onsets, poor execution of multijoint plans, and abnormally prolonged endpoint oscillations.

Reduced servo-control of fatigued human finger extensor and flexor muscles

1. In healthy human subjects holding the index finger semi-extended at the metacarpophalangeal joint against a moderate load, electromyographic (EMG) activity was recorded from the finger extensor and flexor muscles during different stages of muscle fatigue. The aim was to study the effect of muscle fatigue on the level of background EMG activity and on the reflex responses to torque pulses causing sudden extensor unloadings. Paired comparisons were made between the averaged EMG and finger deflection responses under two conditions: (1) at a stage of fatigue (following a sustained co-contraction) when great effort was required to maintain the finger position, and (2) under non-fatigue conditions while the subject tried to produce similar background EMG levels to those in the corresponding fatigue trials. 2. Both the unloading reflex in the extensor and the concurrent stretch reflex in the flexor were significantly less pronounced and had a longer latency in the fatigue trials. Consequently, the finger deflections had a larger amplitude and were arrested later in the fatigue trials. 3. It is concluded that--with avoidance of 'automatic gain compensation', i.e. reflex modifications attributable to differences in background EMG levels--the servo-like action of the unloading and stretch reflexes is reduced in fatigued finger extensor and flexor muscles.

Independent control of reflex and volitional EMG modulation during sinusoidal pursuit tracking in humans

It is well known that during volitional sinusoidal tracking the long-latency reflex modulates in parallel with the volitional EMG activity. In this study, a series of experiments are reported demonstrating several conditions in which an uncoupling of reflex from volitional activity occurs. The paradigm consists of a visually guided task in which the subject tracked a sinusoid with the wrist. The movement was perturbed by constant torque or controlled velocity perturbations at 45 degrees intervals of the tracking phase. Volitional and reflex-evoked EMG and wrist displacement as functions of the tracking phase were recorded. The relationship of both short-latency (30-60 ms) and longer-latency (60-100 ms) reflex components to the volitional EMG was evaluated. In reflex tracking, the peak reflex amplitude occurs at phases of tracking which correspond to a maximum of wrist joint angular velocity in the direction of homonymous muscle shortening and a minimum of wrist compliance. Uncoupling of the reflex and volitional EMG was observed in three situations. First, during passive movement of the wrist through the sinusoidal tracking cycle perturbation-evoked long-latency stretch reflex peak is modulated as for normal, volitional tracking. However, with passive joint movement the volitional EMG modulation is undetectable. Second, a subset of subjects demonstrate a normally modulated and positioned long-latency reflex with a single peak. However, these subjects have distinct bimodal peaks of volitional EMG. Third, the imposition of an anti-elastic load (positive position feedback) shifts the volitional EMG envelope by as much as 180 degrees along the tracking phase when compared with conventional elastic loading. Yet the long-latency reflex peak remains at its usual phase in the tracking cycle, corresponding to the maximal velocity in the direction of muscle shortening. Furthermore, comparison of the results from elastic and anti-elastic loads reveals a dissociation of short- and long-latency reflex activity, with the short-latency reflex shifting with the volitional EMG envelope. Comparable results were also obtained for controlled velocity perturbations used to control for changes in joint compliance. The uncoupling of the reflex and volitional EMG activity in the present series of experiments points to a flexible relationship between reflex and volitional control systems, altered by peripheral input and external load.

Quantitative analysis of four EMG amplifiers

Four typical EMG amplifiers were tested quantitatively to observe the diversity and specificity of available equipment. Gain, phase, common mode rejection ratio (CMRR) and noise characteristics were measured for each device. Various gain and phase responses were observed, each best suited to specific application areas. For all amplifiers, the CMRR was shown to decrease dramatically in the presence of input impedance mismatches of more than 10 k omega between the two electrodes. Because such impedance mismatches are common on the skin surface, these results indicate that proper skin preparation is required to maximize the noise rejection capabilities of the tested amplifiers.

Oxygen availability and motor unit activity in humans

Six men were studied to determine the interrelationships among blood supply, motor unit (MU) activity and lactate concentrations during intermittent isometric contractions of the hand grip muscles. The subjects performed repeated contractions at 20% of maximal voluntary contraction (MVC) for 2 s followed by 2-s rest for 4 min with either unhindered blood circulation or arterial occlusion given between the 1st and 2nd min. The simultaneously recorded intramuscular MU spikes and surface electromyogram (EMG) data indicated that mean MU spike amplitude, firing frequency and the parameters of surface EMG power spectra (mean power frequency and root mean square amplitude) remained constant during the experiment with unhindered circulation, providing no electrophysiological signs of muscle fatigue. Significant increases in mean MU spike amplitude and frequency were, however, evident during the contractions with arterial occlusion. Similar patterns of significant changes in the surface EMG spectra parameters and venous lactate concentration were also observed, while the integrated force-time curves remained constant. These data would suggest that the metabolic state of the active muscles may have played an important role in the regulation of MU recruitment and rate coding patterns during exercise.

Muscle fatigue: conduction or mechanical failure?

It is well documented that repeated voluntary activity or electrical stimulation of skeletal muscle results in a decline in force production or power output. However, the precise physiological causes of "muscle fatigue" are not yet well understood. It is conceivable that the mechanism(s) may lie either in the conduction of action potentials in the central and peripheral nervous systems or in the transformation of the electrical event into mechanical force production by the muscle itself. In fact, none of the components of the electrical pathway from generation of impulses in the brain to their conduction over the neuron and the excitable membranes of the muscle can as yet be ruled out as potential contributors to the fatigue process. Relative to that on conduction failure, more information exists concerning the possibility that a defect in the excitation contraction coupling process in skeletal muscle, e.g., intracellular acidosis, inadequate supply of energy for contraction, or a disruption in Ca2+ homeostasis may also be significant in compromising force production following sustained activity. Despite this, the amount of conflicting data derived from these experiments has hindered the resolution of this question. In the future more attention must be given to such issues as the type of activity used to elicit fatigue and the fiber composition of the muscles studied. This is imperative as these factors clearly impact the nature of correlations between the biochemical and physiological events in muscle that are required to support prospective fatigue mechanisms.

Autogenetic inhibition from contraction receptors in the decerebrate cat

1. Autogenetic inhibition from contraction receptors was measured by eliciting contractions of the soleus muscle in the decerebrate cat. Inhibitory feedback was detected when the tension increment f, produced by stimulating motor fibres in the presence of a background reflex contraction, was smaller than the tension d elicited by the same stimulus in the absence of reflex action. Tendon vibration was applied throughout to clamp primary spindle afferents at a constant firing rate, thereby preventing spindle unloading from disfacilitating the reflex contraction. 2. The reduction in tension d--f varied roughly linearly with the size of the tension stimulus f. Feedback gain was proportional to d--f/f, i.e. the ratio of inhibited tension to stimulus tension. It was computed by averaging over several measurements obtained with stimuli of different sizes, and ranged between 0 and 0.88 in ten animals. The average gain, 0.39, implies that voluntary muscle force is reduced by approximately 27% through the direct inhibition of alpha-motoneurones from homonymous contraction receptors. 3. Inhibitory feedback gain did not appear to co-vary with the background reflex contraction. When measured without vibration, however, a positive covariance did emerge, suggesting that this is due to unloading of muscle spindles, either by extrafusal muscle shortening or by inhibition of fusimotor neurones. 4. Inhibited tension varied linearly with the estimated increment in Ib afferent firing. On the assumption that group Ib afferents carried the entire inhibitory signal, inhibitory feedback gain measured with vibration was used to predict the size of the gain if vibration had not been applied. Feedback gain calculated in this way was reduced by still did not vary with reflex tension. 5. In one animal with signs of brain stem trauma, feedback gain was increased to around six. It is argued that inhibitory feedback in the intact animal can rise to comparable values, as a result both of convergence of signals from different muscles and of supraspinal facilitation.

The fatigability of two agonistic muscles in human isometric voluntary submaximal contraction: an EMG study. II. Motor unit firing rate and recruitment

The recruitment and firing rate of biceps brachii (BB) and brachioradialis (BR) motor units (MUs) were studied in the course of fatiguing isometric contractions at 20%-30% of maximal voluntary contraction (MVC). MU recruitment generally occurred throughout the maintained contraction and was similar for BB and BR muscles. Newly recruited MUs started to discharge in the form of bursts, the duration of which increased until a continuous rhythmical firing was achieved. Within each burst, the first interval between two consecutive discharges was usually the shortest. MU threshold was lowered just after the limit time of the maintained contraction. The MU's firing rate either increased or remained stable as a function of the elapsed time. It is concluded that (1) in fatiguing isometric contractions at 20%-30% MVC contractile failure is mainly compensated for by MU recruitment and a lowered MU threshold and (2) differences between in surface changes in the electromyogram of BB and BR muscles cannot easily be explained by related differences in MU firing rate and recruitment.