Robust estimation of average twitch contraction forces of populations of motor units in humans

The characteristics of motor unit force twitch profiles provide important information for the understanding of the muscle force generation. The twitch force is commonly estimated with the spike-triggered averaging technique, which, despite the many limitations, has been important for clarifying central issues in force generation. In this study, we propose a new technique for the estimation of the average twitch profile of populations of motor units with uniform contractile properties. The method encompasses a model-based deconvolution of the force signal using the identified discharge times of a population of motor units. The proposed technique was validated using simulations and tested on signals recorded during voluntary activation. The results of the simulations showed that the proposed method provides accurate estimates (relative error <25%) of the main parameters of the average twitch force when the number of identified motor units is between 5% and 15% of the total number of active motor units. It is discussed that current detection and decomposition methods of multi-channel surface EMG signals allow decoding this relative sample of the active motor unit pool. However, even when this condition is not met, our results show that the estimates provided by the new method are anyway always superior to those obtained by the spike triggered average approach, especially for high motor unit synchronization levels and when a relatively small number of triggers is available. In conclusion, we present a new method that overcome the main limitations of the spike-triggered average for the study of contractile properties of individual motor units. The method provides a new reliable tool for the investigation of the determinants of muscle force.

A motor unit-based model of muscle fatigue

Muscle fatigue is a temporary decline in the force and power capacity of skeletal muscle resulting from muscle activity. Because control of muscle is realized at the level of the motor unit (MU), it seems important to consider the physiological properties of motor units when attempting to understand and predict muscle fatigue. Therefore, we developed a phenomenological model of motor unit fatigue as a tractable means to predict muscle fatigue for a variety of tasks and to illustrate the individual contractile responses of MUs whose collective action determines the trajectory of changes in muscle force capacity during prolonged activity. An existing MU population model was used to simulate MU firing rates and isometric muscle forces and, to that model, we added fatigue-related changes in MU force, contraction time, and firing rate associated with sustained voluntary contractions. The model accurately estimated endurance times for sustained isometric contractions across a wide range of target levels. In addition, simulations were run for situations that have little experimental precedent to demonstrate the potential utility of the model to predict motor unit fatigue for more complicated, real-world applications. Moreover, the model provided insight into the complex orchestration of MU force contributions during fatigue, that would be unattainable with current experimental approaches.

Muscle- and Mode-Specific Responses of the Forearm Flexors to Fatiguing, Concentric Muscle Actions

BACKGROUND

Electromyographic (EMG) and mechanomyographic (MMG) studies of fatigue have generally utilized maximal isometric or dynamic muscle actions, but sport- and work-related activities involve predominately submaximal movements. Therefore, the purpose of the present investigation was to examine the torque, EMG, and MMG responses as a result of submaximal, concentric, isokinetic, forearm flexion muscle actions.

METHODS

Twelve men performed concentric peak torque (PT) and isometric PT trials before (pretest) and after (posttest) performing 50 submaximal (65% of concentric PT), concentric, isokinetic (60°·s^-1), forearm flexion muscle actions. Surface EMG and MMG signals were simultaneously recorded from the biceps brachii and brachioradialis muscles.

RESULTS

The results of the present study indicated similar decreases during both the concentric PT and isometric PT measurements for torque, EMG mean power frequency (MPF), and MMG MPF following the fatiguing workbout, but no changes in EMG amplitude (AMP) or MMG AMP.

CONCLUSIONS

These findings suggest that decreases in torque as a result of fatiguing, dynamic muscle actions may have been due to the effects of metabolic byproducts on excitation⁻contraction coupling as indicated by the decreases in EMG MPF and MMG MPF, but lack of changes in EMG AMP and MMG AMP from both the biceps brachii and brachioradialis muscles.

Adaptations to fatigue of a single digit violate the principle of superposition in a multi-finger static prehension task

We investigated the effects of exercise-induced fatigue of a digit on the biomechanics of a static prehension task. The participants were divided into two groups. One group performed the fatiguing exercise using the thumb (group-thumb) and the second group performed the exercise using the index finger (group-index). We analyzed the prehensile action as being based on a two-level hierarchy. Our first hypothesis was that fatigue of the thumb would have stronger effects at the upper level (action shared between the thumb and all four fingers combined-virtual finger) and fatigue of the index finger would have stronger effects at the lower level of the hierarchy (action of the virtual finger shared among actual fingers). We also hypothesized that fatigue would cause a decrease in the normal force applied by the exercised digit and correspondingly lead to a decrease in the normal force applied by the opposing digit(s). Our third hypothesis was that fatigue would leave the tangential forces unaffected. Fatigue led to a significant drop in the normal force of both exercised and non-exercised (opposing) digits. The tangential forces of the exercised digits increased after fatigue. This led to a drop in the safety margin in the group-thumb, but not group-index. As such, the results supported the first two hypotheses but not the third hypothesis. Overall, the results suggested that fatigue triggered a chain reaction that involved both forces and moments of force produced by individual digits leading to a violation of the principle of superposition. The findings are interpreted within the framework of the referent configuration hypothesis.

Mechanomyographic and electromyographic responses to repeated concentric muscle actions of the quadriceps femoris

In comparison to isometric muscle action models, little is known about the electromyographic (EMG) and mechanomyographic (MMG) amplitude and mean power frequency (MPF) responses to fatiguing dynamic muscle actions. Simultaneous examination of the EMG and MMG amplitude and MPF may provide additional insight with regard to the motor control strategies utilized by the superficial muscles of the quadriceps femoris during a concentric fatiguing task. Thus, the purpose of this study was to examine the EMG and MMG amplitude and MPF responses of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) during repeated, concentric muscle actions of the dominant leg. Seventeen adults (21.8+/-1.7 yr) performed 50 consecutive, maximal concentric muscle actions of the dominant leg extensors on a Biodex System 3 Dynamometer at velocities of 60 degrees s(-1) and 300 degrees s(-1). Bipolar surface electrode arrangements were placed over the mid portion of the VL, RF, and VM muscles with a MMG contact sensor placed adjacent to the superior EMG electrode on each muscle. Torque, MMG and EMG amplitude and MPF values were calculated for each of the 50 repetitions. All values were normalized to the value recorded during the first repetition and then averaged across all subjects. The cubic decreases in torque at 60 degrees s(-1) (R2 = 0.972) and 300 degrees s(-1) (R2 = 0.931) was associated with a decline in torque of 59+/-24% and 53+/-11%, respectively. The muscle and velocity specific responses for the MMG amplitude and MPF demonstrated that each of the superficial muscles of the quadriceps femoris uniquely contributed to the control of force output across the 50 repetitions. These results suggested that the MMG responses for the VL, RF, VM during a fatiguing task may be influenced by a number of factors such as fiber type differences, alterations in activation strategy including motor unit recruitment and firing rate and possibly muscle wisdom.

Re-evaluation of muscle wisdom in the human adductor pollicis using physiological rates of stimulation

Motor unit discharge rates decline by about 50 % over 60 s of a sustained maximum voluntary contraction (MVC). It has been suggested that this decline in discharge rate serves to maintain force by protecting against conduction failure and by optimizing the input to motor units as their contractile properties change. This hypothesis, known as muscle wisdom, is based in part on studies in which muscle force was shown to decline more rapidly when stimulation was maintained at a high rate than when stimulus rate was reduced over time. The stimulus rates used in those studies, however, were higher than those normally encountered during MVCs. The purpose of this study was to compare force loss under constant and declining stimulus rate conditions using rates similar to those that occur during voluntary effort. Isometric force and surface EMG signals were recorded from human adductor pollicis muscles in response to supramaximal stimuli delivered to the ulnar nerve at the elbow. Three fatigue protocols, each 60 s in duration, were carried out on separate days on each of 10 subjects: (1) continuous stimulation at 30 Hz, (2) stimulation at progressively decreasing rates from 30 to 15 Hz and (3) sustained MVC. The relative force-time integral (endurance index) was significantly smaller for the sustained MVC (0.75 +/- 0.08) and decreasing stimulus rate conditions (0.76 +/- 0.16) compared to the condition in which stimulus rate was maintained at 30 Hz (0.90 +/- 0.13). These findings suggest that decreases in discharge rate may contribute to force decline during a sustained MVC.

MMG and EMG responses during fatiguing isokinetic muscle contractions at different velocities

The purpose of this study was to determine mechanomyographic (MMG) and electromyographic (EMG) responses of the superficial quadriceps muscles during repeated isokinetic contractions in order to provide information about motor control strategies during such activity, and to assess uniformity in mechanical activity (MMG) between the investigated muscles. Ten adults performed 50 maximal concentric muscle contractions at three randomly selected contraction velocities (60, 180, and 300 degrees.s(-1)) on different days. Surface electrodes and an MMG sensor were placed on the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). EMG and MMG amplitude and peak torque (PT) were calculated for each contraction, normalized, and averaged across all subjects. The results demonstrated that MMG amplitude more closely tracked the fatigue-induced decline in torque production at each velocity than did EMG amplitude. This indicates that MMG amplitude may be useful for estimating force production during fatiguing dynamic contractions when a direct measure is not available, such as during certain rehabilitative exercises. MMG amplitude responses of the VL, RF, and VM were not uniform for each velocity or across velocities, indicating that it may be possible to detect the individual contribution of each muscle to force production during repeated dynamic contractions. Therefore, MMG amplitude may be clinically useful for detecting abnormal force contributions of individual muscles during dynamic contractions, and determining whether various treatments are successful at correcting such abnormalities.

Discharge behaviour of single motor units during maximal voluntary contractions of a human toe extensor

1. While it is known that the average firing rate of a population of motoneurones declines with time during a maximal voluntary contraction, at least for many muscles, it is not known how the firing patterns of individual motoneurones adapt with fatigue. To address this issue we used tungsten microelectrodes to record spike trains (mean +/- s.e.m., 183 +/- 27 spikes per train; range, 100-782 spikes) from 26 single motor units in extensor hallucis longus during sustained (60-180 s) maximal dorsiflexions of the big toe in seven human subjects. 2. Long spike trains were recorded from 13 units during the first 30 s of a maximal voluntary contraction (mean train duration, 9.6 +/- 1.2 s; range, 3.6-21.9 s) and from 13 units after 30 s (mean train duration, 16.6 +/- 3.7 s; range, 7.1-58.1 s). Maximal isometric force generated by the big toe declined to 78.3 +/- 6.3 % of its control level by 60-90 s and to 39.5 +/- 1.4 % of control by 120-150 s. Despite this substantial fatigue, mean firing rates did not change significantly over time, declining only slightly from 15.8 +/- 0.7 Hz in the first 30 s to 14.0 +/- 0.5 Hz by 60-90 s and 13.6 +/- 0.3 Hz by 120-150 s. 3. To assess fatigue-related adaptation in discharge frequency and variability of individual motor units, each spike train was divided into 2-15 equal segments containing at least 50 interspike intervals. Discharge variability was measured from the coefficient of variation (s.d. /mean) in the interspike intervals, with the s.d. being calculated using a floating mean of 19 consecutive intervals. Adaptation was computed as the average change in firing rate or variability that would occur for each 1 s of activity. There were no systematic changes in either firing rate or variability with time. 4. We conclude that single motoneurones supplying the extensor hallucis longus, a muscle comprised primarily of slow twitch muscle units, show little adaptation in firing with fatigue, suggesting that a progressive reduction in firing rate is not an invariable consequence of the fatigue associated with sustained maximal voluntary contractions.

The relationship between isometric force requirement and forelimb tremor in the rat

To explore the effects of isometric force of rodent forelimb contraction on forelimb tremor, rats were trained to press downward on an isometric force transducer to raise a water-filled dipper cup and maintain force to keep the dipper in the raised position while licking. Force requirements were then manipulated parametrically to measure the effects of escalating force output on forelimb tremor and other variables. In the Peak-Force greater than Hold-Force (PF > HF) manipulation, the forces required to raise the dipper were 20, 40, and 60 g (each condition for about 2 weeks), while the force required to maintain the dipper in the raised position remained 6.7 g for all three conditions. In the Peak-Force equal to the Hold-Force (PF = HF) manipulation, rats were required to maintain the "dipper-raising" force throughout the response. The forces required were 20 g, 40 g, and 60 g (each for 2 weeks). For all force requirement manipulations, data were analyzed within and across conditions. As expected, force output increased with increased force requirements. Spectral analysis of force-time records revealed that during all manipulations, high-frequency (>10 Hz) forelimb tremor increased with increased force output, an effect that is consistent with human studies, and that may reflect increases in the number of motor units firing at higher rates. Additionally, with the exception of the 60-g PF = HF condition, there were within-condition decreases in tremor and increases in task engagement, evidence suggesting increased muscle strength as a function of experience (i.e., "physical training"). Taken together, the results suggest that the rodent-based method may provide a valuable, noninvasive functional assay for animal models of disorders that affect skeletal muscle control in humans.

Force-frequency and fatigue properties of motor units in muscles that control digits of the human hand

Modulation of motor unit activation rate is a fundamental process by which the mammalian nervous system encodes muscle force. To identify how rate coding of force may change as a consequence of fatigue, intraneural microstimulation of motor axons was used to elicit twitch and force-frequency responses before and after 2 min of intermittent stimulation (40-Hz train for 330 ms, 1 train/s) in single motor units of human long finger flexor muscles and intrinsic hand muscles. Before fatigue, two groups of units could be distinguished based on the stimulus frequency needed to elicit half-maximal force; group 1 (n = 8) required 9.1 +/- 0.5 Hz (means +/- SD), and group 2 (n = 5) required 15.5 +/- 1.1 Hz. Twitch contraction times were significantly different between these two groups (group 1 = 66. 5 ms; group 2 = 45.9 ms). Overall 18% of the units were fatigue resistant [fatigue index (FI) > 0.75], 64% had intermediate fatigue sensitivity (0.25 </= FI </= 0.75), and 18% were fatigable (FI < 0. 25). However, fatigability and tetanic force were not significantly different among groups. Therefore unlike findings in some other mammals, fast-contracting motor units were neither stronger nor more susceptible to fatigue than slowly contracting units. Fatigue, however, was found to be greatest in those units that initially exerted the largest forces. Despite significant slowing of contractile responses, fatigue caused the force-frequency relation to become displaced toward higher frequencies (44 +/- 41% increase in frequency for half-maximal force). Moreover, the greatest shift in the force-frequency relation occurred among those units exhibiting the largest force loss. A selective deficit in force at low frequencies of stimulation persisted for several minutes after the fatigue task. Overall, these findings suggest that with fatigue higher activation rates must be delivered to motor units to maintain the same relative level of force. Questions regarding classification of motor units and possible mechanisms by which fatigue-related slowing might coexist with a shift in the force-frequency curve toward higher frequencies are discussed.

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].