The influence of torque and velocity on erector spinae muscle fatigue and its relationship to changes of electromyogram spectrum density

The influence of contraction force and velocity during isokinetic contractions on the development of fatigue in the erector spinae muscle was studied. Seven male subjects performed a series of 250 contractions at 25% and 50% of their isometric maximal voluntary contraction (MVC) at 40 and 80°·s^–1. Fatigue defined as a decrease of the contractile capacity of the muscles was studied by means of a 15-s maximal test-contraction following the exercise. Both the initial force and the force decrement during the test-contraction were studied. Surface electromyogram (EMG) signals of the main tracts of the erector spinae muscle were recorded. The frequency content was studied by calculating the zero-crossing rate for the signals obtained during dynamic contractions and by means of fast Fourier transformation for the test contraction. After the 50% MVC dynamic contractions the initial force during the postexercise test-contraction was significantly lower than after the 25% MVC contractions. No significant influence of contraction velocity on fatigue development was found. The force decrement during the test-contraction did not depend on the experimental conditions. The EMG amplitude indicated that the subjects were better able to relax their muscles during the counter movement (flexion) at high forces and high velocities compared to the other experimental conditions. The frequency content of the EMG signals during the dynamic contractions and the postexercise test-contraction showed only very weak relationships with fatigue. Therefore, spectrum EMG parameters as determined in the present study do not seem suitable as indicators of muscle fatigue as a consequence of dynamic contractions of trunk extensor muscles.     

Electromyography and fatigue during prolonged,low-level static contractions

Summary Findings from five separate studies of EMG changes and muscle fatigue during prolonged low-level static contractions are summarized, and the possible mechanisms behind the changes are briefly discussed. Sustained static contractions (10%, 7% and 5% MVC) of up to 1 h duration were performed by finger flexors, elbow flexors and extensors, and knee extensors. In one experiment, intermittent static arm pulling (triceps) (10 s contraction and 5 s rest, average work load 14% and 10% MVC) was performed for 7 h. The endurance time for thesustained contractions was around one hour for 10% MVC, and it was shown — all in all — that the concept of indefinite endurance times at contractions below 15–20% MVC cannot be maintained. After 5% MVCsustained contractions for one hour a 12% reduction in MVC was seen, and significant increases in EMG amplitude and decreases in the mean spectral frequency of the EMG-power spectrum were found. Marked differences were also seen in the EMG changes in the elbow flexors and extensors, and transcutanous electrical stimulation of the knee extensors showed that low frequency fatigue was present after the contraction. Withintermittent contractions similar changes in the EMG parameters were seen after 2–3 h of contractions at 14% MVC. On average, during contractions of 10% MVC no EMG changes were detected. Increased extracellular potassium concentration in the contracting muscles is suggested as a possible explanation of these findings.     

Characteristics of corticomotoneuronal postspike facilitation and reciprocal suppression of EMG activity in the monkey

In this study we present further evidence supporting the reciprocal nature of output effects on forearm flexor and extensor muscles from single corticomotoneuronal (CM) cells. Spike-triggered averaging of rectified EMG activity was used to test the output effects of 105 motor cortex cells in two rhesus monkeys (Macaca mulatta) trained to perform alternating wrist movements and power grip. The electromyographic (EMG) activity was recorded from six forearm flexor and six forearm extensor muscles through pairs of percutaneously inserted intramuscular stainless steel wires. CM cells were identified by their characteristic postspike facilitation (PSF) in spike-triggered averages of agonist muscle EMG activity. Agonist muscles are those which coactivate with the cortical cell during movement. Of 105 motor cortex cells tested, 56 (53%) had no effect on either agonist or antagonist muscles. Of 49 cells that produced PSF of the agonist muscles, 14 (29%) also produced clear postspike suppression (PSS) of the antagonist muscles. Reproducibility of postspike effects was demonstrated by comparing spike-triggered averages of full-wave rectified EMG with averages of the same EMG activity triggered from randomly generated pulses. Consecutive averages from random triggers never showed consistent postspike effects. As a further test that our postspike effects were real, we computed averages of simulated EMG activity from the spikes of CM cells with reciprocal output effects. None of these averages showed consistent postspike effects. The mean onset latency of PSF calculated from 14 reciprocal CM cells yielding 51 PSF effects was 6.3 ms compared with 10.1 ms for 28 PSS effects from the same cells. PSS effects from a particular CM cell were nearly always longer in latency than the cell's PSF effects; only 2 of 28 PSS onset latencies were shorter than the longest latency PSF onset from the same cell. Average peak latencies for PSF and PSS were 8.6 and 11.6 ms, respectively. The magnitude of postspike effects was expressed as the percent of peak facilitation above the base-line mean for PSF or peak suppression below the base-line mean for PSS. With this measure, the average magnitude of PSF was 7.0% compared with 4.1% for reciprocal PSS. There was no correlation between onset latency and magnitude of PSF or PSS, although strong PSFs tended to have shorter latencies. Concerning the distribution of postspike effects, the average reciprocal CM cell facilitated 3.8 agonist muscles and suppressed 2.1 antagonist muscles. EDC was facilitated by all extension-related reciprocal CM cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Human motor unit activity during the onset of muscle fatigue in submaximal isometric isotonic contraction

Summary The onset of fatigue has been studied by relating motor unit activity and surface myoelectric activity during constant force isometric contraction in man.The surface (global) EMG of the biceps brachii was recorded using bipolar electrodes. The global EMGs of the other elbow flexors were also recorded. Motor unit activity was recorded simultaneously at three points of the biceps brachii by using wire electrodes.The time course of the integrated surface EMG showed that fatigue occurred right at the beginning of the contraction. The increase of the integrated EMG was especially important when the value of the force maintenance was high. The time course of this increase was close to that of the output of the spikes recorded by the wire electrodes.No matter what the value of the force maintenance, MU recruitment occurred throughout the contraction. The existence of an increase in the MU firing frequency depended on the value of this force.Contractile element fatigue is believed to provide the explanation of these results, while neuromuscular junction fatigue may perhaps be more important near the time limit.     

Differences in activation patterns in elbow flexor muscles during isometric,concentric and eccentric contractions

Summary To investigate the relative activation of the synergistic muscles during three different types of muscle contraction, the electromyograms (EMG) of two elbow flexor muscles, the biceps brachii (BB) and the brachioradialis (BR), have been compared. To accomplish this eight healthy human subjects performed the following elbow flexions against the same load — concentric, eccentric and isometric contractions. The isometric contractions were performed at three elbow angles: 10, 45 and 90° (0° equal to full expension). The EMG were recorded by bipolar surface electrodes, and the relative activation between the two muscles was evaluated as the quotient of mean EMG activities (BR/BB). For the isotonic elbow flexions, BR/BB were calculated at three angle divisions: 0–30°, 30–60° and 60–90°. Results indicated that the relative activation of the BR during the concentric contractions was higher than that of the eccentric contraction, particularly at the extended elbow angles, i.e. the BR/BB of the concentric contractions for the elbow joint angles ranging from 0–30° and 30–60° were significantly greater (P<0.05) than those of the eccentric contractions. During the isometric and eccentric contractions, the BR/BB at the flexed joint angles tended to be greater than those at the extended angles. In contrast, there were no angle-dependent BR/BB variations during the concentric elbow flexions. Further, changing patterns in the EMG power spectra due to the type of contraction were different between BB and BR. These results indicated that the activation pattern in the two elbow flexor muscles varied with the muscle contraction pattern.     

Physiological modifications of local haemodynamic conditions during bilateral isometric contractions

Summary Nine subjects (five women and four men) simultaneously performed two isometric contractions sustained until exhaustion at different relative forces: 40% of maximum voluntary contraction (MVC) for the right elbow flexors; 50% MVC for the left elbow flexors. Contraction of the left elbow flexors commenced at 50% of the limit time (maximum maintenance time) of isometric contraction of the right elbow flexors. Increase in heart rate during concomitant contraction of the left elbow flexors led to an increase in blood flow to the right elbow flexors. Under these conditions, the limit time of isometric contraction of the right elbow flexors was prolonged with respect to the limit time obtained for an isolated isometric contraction at the same relative tension. The difference was more significant in the female (+40%,P<0.05) than in the male subjects (+20%,P>0.05).     

Trunk extensor endurance and its relationship to electromyogram parameters

Summary The present study was designed to investigate the relationship between muscle performance and electromyogram (EMG) parameters of the trunk extensor muscles in the development of fatigue. Nine subjects performed continuous isometric trunk extensions at 25% and 40% maximal voluntary contraction. The EMG signals of the longissimus thoracis, iliocostalis lumborum, multifidus and latissimus dorsi muscles were recorded. The EMG amplitude (RA-EMG) appeared to increase consistently during the contractions in all muscles, whereas the mean power frequency (MPF) showed a fairly consistent decrease during the contractions. The time constants of the exponential change of the RA-EMG and of the MPF were related to the endurance time. The prediction of endurance based on both EMG parameters appeared to yield better results than the prediction based on the relative force. In particular the time constants of the MPF changes of the multifidus and longissimus muscles appeared to be good predictors of endurance time. The consistency of the spectrum shift of EMG appeared to coincide with a reduced variability of the activation of the muscle involved.     

Do corticomotoneuronal cells predict target muscle EMG activity?

Data from two rhesus macaques were used to investigate the pattern of cortical cell activation during reach-to-grasp movements in relation to the corresponding activation pattern of the cell's facilitated target muscles. The presence of postspike facilitation (PSpF) in spike-triggered averages (SpTAs) of electromyographic (EMG) activity was used to identify cortical neurons with excitatory synaptic linkages with motoneurons. EMG activity from 22 to 24 muscles of the forelimb was recorded together with the activity of M1 cortical neurons. The extent of covariation was characterized by 1) identifying the task segment containing the cell and target muscle activity peaks, 2) quantifying the timing and overlap between corticomotoneuronal (CM) cell and EMG peaks, and 3) applying Pearson correlation analysis to plots of CM cell firing rate versus EMG activity of the cell's facilitated muscles. At least one firing rate peak, for nearly all (95%) CM cells tested, matched a corresponding peak in the EMG activity of the cell's target muscles. Although some individual CM cells had very strong correlations with target muscles, overall, substantial disparities were common. We also investigated correlations for ensembles of CM cells sharing the same target muscle. The ensemble population activity of even a small number of CM cells influencing the same target muscle produced a relatively good match (r >/= 0.8) to target muscle EMG activity. Our results provide evidence in support of the notion that corticomotoneuronal output from primary motor cortex encodes movement in a framework of muscle-based parameters, specifically muscle-activation patterns as reflected in EMG activity.     

EMG power spectrum as a measure of muscular fatigue at different levels of contraction

The shift in the power spectrum resulting from a 5-7 min fatigue-inducing effort followed by a 1-2 min recovery period of two elbow flexors, the biceps brachii (BB) and the brachio-radialis (BR), was assessed using two variables, the mean frequency Fm and the median or central frequency Fmd. These two variables were calculated in pre- and post-fatigue conditions and following a brief recovery, at four levels, namely 20, 40, 60 and 80 per cent of maximum voluntary contraction (MVC). These were taken from a ramped isometric effort that is from 0 to 100 per cent MVC. The EMG activity of the two flexors was recorded with bipolar surface electrodes from a group of ten volunteers. Following muscle fatigue, induced with a maintained 60 per cent MVC isometric contraction, a statistically significant (p less than 0.05) shift towards the lower frequencies was observed for both Fm and Fmd for both muscles. Following a brief recovery, a shift towards the pre-fatigue higher frequencies was statistically significant (p less than 0.05). These two synergists responded to muscle fatigue and recovery similarly, as they both demonstrated parallel shifts in power spectrum. The power spectrum is consequently a reliable measure of muscular fatigue. It is also complementary to the net articular moment results.     

Myo-electric signals from two extrinsic hand muscles and force tremor during isometric handgrip

Summary The objective of the present study was to investigate the myo-electric signs of muscle fatigue and the isometric force tremor of two extrinsic hand muscles, extensor digitorum communis and flexor digitorum superficialis, during isometric power grip. In addition, the synergy between flexor and extensor muscles and hand differences in a right-handed population have been studied. During isometric hand-dynamometry the myo-electric signal was recorded using surface electrodes and isometric force tremor was recorded using a special load cell. Eight subjects participated in this study and contractions were performed at 20%, 40%, 60% and 80% of maximal voluntary contraction (MVC) with left and right hands. The decrease of mean power frequency (MPF) with duration of contraction was greater in the left extensor as compared to the ipsilateral flexor muscle. No differences in the decrease in MPF with the duration of the contraction were found between the right extensor and flexor muscles. Isometric force tremor root mean square did not change during contractions at a given contraction level. Isometric tremor amplitude increased from 20% to 60% MVC and decreased at higher contraction levels. Tremor amplitude was higher in the left hand at all contraction levels but 60% MVC. These data would suggest differences in fatiguability and muscle fibre composition between the dominant and nondominant hand, which may be due to preferred use. The significance of force tremor for the evaluation of recruitment order and muscle fatigue is discussed.     

Oxygen availability and motor unit activity in humans

Summary 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.     

Changes in soleus motoneuron pool reflex excitability and surface EMG parameters during fatiguing low- vs. high-intensity isometric contractions

The fatigue-related changes in soleus motoneuron pool reflex excitability and surface electromyography (EMG) parameters, and maximal voluntary contraction (MVC) force of the plantarflexor (PF) muscles during repeatedly sustained low- (30% MVC) vs. high-intensity (70% MVC) isometric contractions were evaluated Twelve young men with mean (+/- SE) age of 22.4 +/- 0.3 years participated in two fatigue tasks on separate days with at least 1-week interval. The fatigue task consisted of three sustained isometric contractions of PF muscles at a target force level until exhaustion separated with 2-min pause between contractions. M-wave (muscle compound action potential) amplitude (M(max)), Hoffmann reflex maximal amplitude (H(max)) to M-wave amplitude ratio (H(max)/M(max)), and root mean square amplitude (RMS) and median frequency (MF) of EMG power spectrum were recorded from the soleus muscle. The M(max) remained constant immediately post-fatigue and during recovery for low- and high-intensity fatigue tasks, whereas H(max)/M(max) was significantly (p < 0.05) reduced only after high-intensity fatigue task. The increase in RMS and decrease in MF during isometric contractions, and reduction in MVC force immediately after the exercise was greater (p < 0.05) for low-intensity fatigue task. We conclude that low-intensity isometric contractions, repeatedly sustained to fatigue, resulted in a marked increase in the EMG amplitude and spectral compression without a significant post-fatigue reflex inhibition of soleus motoneuron pool. High-intensity contractions, however, resulted in post-fatigue reflex inhibition of soleus motoneuron pool and less pronounced EMG spectral compression during fatiguing contractions. A failure of neuromuscular transmission-propagation was not evident after repetitive fatiguing isometric contractions.     

EMG differences between concentric and eccentric maximum voluntary contractions are evident prior to movement onset

This investigation addressed the question of whether the muscle activation signal prior to movement onset, as measured by surface EMG, differs if the contraction to be performed is concentric (shortening) or eccentric (lengthening). Specifically, the purpose was to determine if differences in knee extensor muscle EMG prior to voluntary maximum concentric and eccentric contractions and initiated from the same knee joint angle are evident at a time before muscle length changes could be influential. A protocol was designed using isokinetic knee extensions. The EMG of the vastus lateralis, vastus medialis, rectus femoris, and hamstrings muscles and the associated knee extension moment were measured during the isometric phase preceding the onset of dynamometer motion. During this isometric phase the muscles initially contracted under identical conditions, irrespective of whether the contraction was to be concentric or eccentric. The EMG of the eccentric contractions was significantly smaller than that of the concentric contractions. However, the rate of change of knee extension moment generally did not differ between the two conditions. This was found for both the monoarticular and multiarticular knee extensor muscles. The results suggest that initial differences between the EMG of maximum voluntary concentric and eccentric knee extensor contractions are selected a priori and support the contention that the central nervous system distinguishes between maximum eccentric and concentric contractions. The emergence of differences in activation prior to muscle length changes suggests supraspinal influences.     

Recovery of the human biceps electromyogram after heavy eccentric,concentric or isometric exercise

Summary Five men performed submaximal isometric, concentric or eccentric contractions until exhaustion with the left arm elbow flexors at respectively 50%, 40% and 40% of the prefatigued maximal voluntary contraction force (MVC). Subsequently, and at regular intervals, the surface electromyogram (EMG) during 30-s isometric test contractions at 40% of the prefatigued MVC and the muscle performance parameters (MVC and the endurance time of an isometric endurance test at 40% prefatigued MVC) were recorded. Large differences in the surface EMG response were found after isometric or concentric exercise on the one hand and eccentric exercise on the other. Eccentric exercise evoked in two of the three EMG parameters [the EMG amplitude (root mean square) and the rate of shift of the EMG mean power frequency (MPF)] the greatest (P<0.001) and longest lasting (up to 7 days) response. The EMG response after isometric or concentric exercise was smaller and of shorter duration (1–2 days). The third EMG parameter, the initial MPF, had already returned to its prefatigued value at the time of the first measurement, 0.75 h after exercise. The responses of EMG amplitude and of rate of MPF shift were similar to the responses observed in the muscle performance parameters (MVC and the endurance time). Complaints of muscle soreness were most frequent and severe after the eccentric contractions. Thus, eccentric exercise evoked the greatest and longest lasting response both in the surface EMG signal and in the muscle performance parameters.     

Surface EMG and motor unit activity of partially denervated human muscle during fatiguing submaximal isometric contraction

The extent to which the normal fatigue compensatory mechanisms are disturbed in partially denervated muscles was investigated in human patients. Surface EMG, as well as motor unit electrical and mechanical activity, were analyzed from the partially denervated first interosseous muscle, during fatiguing isometric submaximal contraction. The EMG power and frequency changes which reveal the local fatigue process of healthy muscle have not been systematically found. Motor unit firing rate changes were rather normal and twitch contraction time did not increase during the fatiguing exercise. Differences between normal and partially denervated muscles could be explain by the occurrence of a central fatigue process more or less important in neurogenic lesions.     

Local muscle blood flow and sustained contractions of human arm and back muscles

The endurance during sustained contraction of elbow, flexors, elbow extensors, and back extensors was tested in 3 human subjects. The force level used was varied between ca. 15 and ca. 75% of maximal isometric strength (IS). The clearance of¹³³Xe from contracting muscles was registered during and after the endurance test. In this way it was possible to determine whether muscle blood flow (MBF) was increased or had stopped during the contraction. Experiments with artificial ischaemia of the upper arm together with MBF measurements showed that MBF was of no importance for continuing sustained contractions above a certain force level, which was 50, 25, and 40% of IS for elbow flexors, elbow extensors and back extensors, respectively. However, the level where longer lasting (>15 min) sustained contraction is possible is directly related to MBF. These levels were 22, 15, and 20% IS for elbow flexors, elbow extensors, and back extensors, respectively.     

Group III and IV muscle afferents differentially affect the motor cortex and motoneurones in humans

The influence of group III and IV muscle afferents on human motor pathways is poorly understood. We used experimental muscle pain to investigate their effects at cortical and spinal levels. In two studies, electromyographic (EMG) responses in elbow flexors and extensors to stimulation of the motor cortex (MEPs) and corticospinal tract (CMEPs) were evoked before, during, and after infusion of hypertonic saline into biceps brachii to evoke deep pain. In study 1, MEPs and CMEPs were evoked in relaxed muscles and during contractions to a constant elbow flexion force . In study 2, responses were evoked during elbow flexion and extension to a constant level of biceps or triceps brachii EMG , respectively. During pain, the size of CMEPs in relaxed biceps and triceps increased (by ∼47% and ∼56%, respectively; P < 0.05). MEPs did not change with pain, but relative to CMEPs, they decreased in biceps (by ∼34%) and triceps (by ∼43%; P < 0.05). During flexion with constant force, ongoing background EMG and MEPs decreased for biceps during pain (by ∼14% and 15%; P < 0.05). During flexion with a constant EMG level, CMEPs in biceps and triceps increased during pain (by ∼30% and ∼26%, respectively; P < 0.05) and relative to CMEPs, MEPs decreased for both muscles (by ∼20% and ∼17%; P < 0.05). For extension, CMEPs in triceps increased during pain (by ∼22%) whereas MEPs decreased (by ∼15%; P < 0.05). Activity in group III and IV muscle afferents produced by hypertonic saline facilitates motoneurones innervating elbow flexor and extensor muscles but depresses motor cortical cells projecting to these muscles.     

Facilitation and suppression of wrist and digit muscles from single rubromotoneuronal cells in the awake monkey.

1. The output effects of 214 cells in the magnocellular red nuclei of two rhesus monkeys (Macaca mulatta) were tested with spike-triggered averaging of electromyogram (EMG) activity from six forearm extensor and six flexor muscles. The monkeys performed an alternating wrist movement task (auxotonic paradigm) or generated wrist torque trajectories alternating between flexion and extension (isometric paradigm). 2. Sixty-five cells (30%) were identified as rubromotoneuronal (RM) cells on the basis of their postpike effects on forearm flexor and extensor muscles. Three major types of RM cell output organization were identified: 1) pure facilitation (28 cells), 2) reciprocal (18 cells), and 3) cofacilitation (16 cells). 3. RM cell output showed a strong preference for facilitation of extensor forearm muscles. This preference was reflected in the fact that 69% (43 of 62) of RM cells facilitated extensors exclusively or most strongly; 27% facilitated flexors exclusively or most strongly; and 5% facilitated flexors and extensors equally. Postspike facilitation (PSpF) was observed in 45% of the extensor muscles and 20% of the flexors tested. In contrast, postpike suppression (PSpS) was observed in 3% of the extensors and 7% of the flexors. 4. The mean number of extensors facilitated per RM cell was 3.1 (53% of tested) compared with 2.8 (51% of tested) flexors facilitated per cell. The extensor and flexor PSpS muscle field sizes were both 2.0 (35% of extensors and 36% of flexors tested). The mean number of muscles facilitated by cofacilitation cells was 5.8 (48%) per cell. No clear preference was found for facilitation of particular combinations of synergist muscles. 5. PSpF magnitude was assessed by measuring both the percent change of facilitation or suppression from baseline and the signal-to-noise ratio of effects. The overall average magnitudes of RM PSpF and PSpS were 4.1 +/- 2.0 and 4.0 +/- 2.3% change from baseline, respectively. The average magnitude of PSpF in flexors was not significantly different from that of extensors; neither was there a difference in the average magnitude of PSpS in flexors and extensors. 6. The mean onset latency of RM cell PSpS was greater than PSpF (9.2 +/- 3.0 vs. 5.7 +/- 1.8 ms; P less than or equal to 0.05). This can be attributed to an underlying minimal disynaptic linkage to motoneurons for suppression effects, whereas most PSpFs are probably mediated by underlying monosynaptic connections. The mean onset latency of flexor PSpFs was greater than that of extensors (6.4 +/- 2.3 vs. 5.4 +/- 1.5 ms; P less than or equal to 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)     

Muscular fatigue and rate of tension development

Summary The effects of long-term fatigue upon maximal force and peak rate of tension development (PRTD) (dF/dt max) are studied in man (elbow flexors), in the rat (pseudo-isolated gastrocnemius muscle) and in the frog (isolated sartorius muscle). The muscles are fatigued by voluntary anisometric anisotonic contractions against an elastic resistance in man, and by maximal tetanic contractions in the frog and the rat. In man, the excitation level of the muscle is controlled by the integrated surface EMG of the biceps brachii. In the animals, the muscles are stimulated by a neurostimulator. The PRTD and the maximal isometric force are measured during fatigue tests.In man, frog and rat, the maximal voluntary isometric torque or the maximal force and the PRTD decrease initially more or less rapidly according to the power developed during the fatigue process, and then less rapidly. The relationship between PRTD and maximal force is linear in the animals and curvilinear in man.The variations of maximal force and PRTD are discussed in relation to the level of excitation of the muscles and of the composition in different motor units types and their spatio-temporal recruitment. From a biomechanical point of view, it seems necessary to study the behavior of the series elastic component during the evolution of long term fatigue.     

Motor unit firing behavior in human arm flexor muscles during sinusoidal isometric contractions and movements

Simultaneous recordings of action potentials (APs) of multiple single motor units (MUs) were obtained in brachialis and biceps (caput breve) muscles during sinusoidally modulated isometric contractions of elbow flexor muscles and during sinusoidal flexion/extension movements in the elbow against a preload in the extension direction. The results show that MUs typically fire in one short burst for each sinusoidal cycle. The mean phase lead of the bursts of APs relative to a sinusoidally modulated isometric torque in the elbow joint or relative to sinusoidal movements in the elbow increases gradually with frequency. The increase of the mean phase lead during isometric contractions was very similar for all MUs and could be explained well by modeling the force production of MUs with a second-order linear low-pass system. For sinusoidal flexion/extension movements each MU reveals a specific, reproducible phase lead as a function of frequency. However, there is a large variability in phase behavior between MUs. Also, the modulation of the firing rate for sinusoidal isometric contractions versus sinusoidal movements appeared to be different for various MUs. In simultaneous recordings some MUs clearly revealed a larger firing rate in each burst for movements relative to isometric contractions, whereas other MUs revealed a smaller firing rate. This suggests that some MUs are preferentially activated during movements whereas others are preferably activated during isometric contractions. The results demonstrate task-dependent changes in the relative activation of MUs within a single muscle for sinusoidal isometric contractions and movements. Received: 18 November 1996 / Accepted: 3 April 1997