Torque and mechanomyogram correlations during muscle relaxation: Effects of fatigue and time-course of recovery

To assess the validity and reliability of the mechanomyogram (MMG) as a tool to investigate the fatigue-induced changes in the muscle during relaxation, the torque and MMG signals from the gastrocnemius medialis muscle of 23 participants were recorded during tetanic electrically-elicited contractions before and immediately after fatigue, as well as at min 2 and 7 of recovery. The peak torque (pT), contraction time (CT) and relaxation time (RT), and the acceleration of force development (d2RFD) and relaxation (d2RFR) were calculated. The slope and τ of force relaxation were also determined. MMG peak-to-peak was assessed during contraction (MMG p–p) and relaxation (R-MMG p–p). After fatigue, pT, d2RFD, d2RFR, slope, MMG p–p and R-MMG p–p decreased significantly, while CT, RT and τ increased (P < 0.05 for all comparisons), remaining altered throughout the entire recovery period. R-MMG p–p correlated with pT, MMG p–p, slope, τ and d2RFR both before and after fatigue. Reliability measurements always ranged from high to very high. In conclusion, MMG may represent a valid and reliable index to monitor the fatigue-induced changes in muscle mechanical behavior, and could be therefore considered an effective alternative to the force signal, also during relaxation.     

Basic characteristics between mechanomyogram and muscle force during twitch and tetanic contractions in rat skeletal muscles

The mechanomyogram (MMG) is a signal measured by various vibration sensors for slight vibrations induced by muscle contraction, and it reflects the muscle force during electrically induced-contraction or until 60%–70% maximum voluntary contraction, so the MMG is considered an alternative and novel measurement tool for muscle strength. We simultaneously measured the MMG and muscle force in the gastrocnemius (GC), vastus intermedius (VI), and soleus (SOL) muscles of rats. The muscle force was measured by attaching a hook to the tendon using a load cell, and the MMG was measured using a charged-coupled device-type displacement sensor at the middle of the target muscle. The MMG-twitch waveform was very similar to that of the muscle force; however, the half relaxation time and relaxation time (10%), which are relaxation parameters, were prolonged compared to those of the muscle force. The MMG amplitude correlated with the muscle force. Since stimulation frequencies that are necessary to evoke tetanic progression have a significant correlation with the twitch parameter, there is a close relationship between twitch and tetanus in the MMG signal. Therefore, we suggest that the MMG, which is electrically induced and detected by a laser displacement sensor, may be an alternative tool for measuring muscle strength.     

Changes in contractile properties and neuromuscular propagation evaluated by simultaneous mechanomyogram and electromyogram during experimentally induced hypothermia.

The present study examined, whether or not mechanomyogram (MMG) amplitude and frequency component could reflect the contractile properties of the triceps surae muscles, composed of relatively slow soleus (SOL) and fast medial gastrocnemius (MG), during experimentally induced hypothermia condition. In eight male subjects, lying in prone position, supramaximal single twitch and repetitive electrical stimulations at 10 Hz were applied at the intramuscular temperatures of control (34 degrees C), 15, 20, and 25 degrees C, respectively. The hypothermia induced substantial reduction in muscle contractile properties, e.g. prolonged twitch contraction and half relaxation times, resulted in a highly significant reduction in the fluctuation of force signal during the repetitive stimulations. These changes were almost mirrored by the similar and significant reductions in the MMG amplitude in both SOL and MG. Power spectrum analysis revealed that peak frequency components of MMG and fluctuation of force were almost matched with the applied stimulation frequencies, independent of the temperature condition. These results strongly suggest that MMG analysis could be employed to study muscle contractile properties varying across different physiological conditions.     

Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms.

The focus of the present study is the long term element of muscle fatigue provoked by prolonged intermittent contractions at submaximal force levels and analysed by force, surface electromyography (EMG) and mechanomyogram (MMG). It was hypothesized that fatigue related changes in mechanical performance of the biceps muscle are more strongly reflected in low than in high force test contractions, more prominent in the MMG than in the EMG signal and less pronounced following contractions controlled by visual compared to proprioceptive feedback. Further, it was investigated if fatigue induced by 30 min intermittent contractions at 30% as well as 10% of maximal voluntary contraction (MVC) lasted more than 30 min recovery. In six male subjects the EMG and MMG were recorded from the biceps brachii muscle during three sessions with fatiguing exercise at 10% with visual feedback and at 30% MVC with visual and proprioceptive feedback. EMG, MMG, and force were evaluated during isometric test contractions at 5% and 80% MVC before prolonged contraction and after 10 and 30 min of recovery. MVC decreased significantly after the fatiguing exercise in all three sessions and was still decreased even after 30 min of recovery. In the time domain significant increases after the fatiguing exercise were found only in the 5% MVC tests and most pronounced for the MMG. No consistent changes were found for neither EMG nor MMG in the frequency domain and feedback mode did not modify the results. It is concluded that long term fatigue after intermittent contractions at low force levels can be detected even after 30 min of recovery in a low force test contraction. Since the response was most pronounced in the MMG this may be a valuable variable for detection of impairments in the excitation-contraction coupling.     

Mechanomyogram from the different heads of the quadriceps muscle during incremental knee extension

To investigate the time- and frequency-domain responses of mechanomyograms (MMGs) during the progressive fatigue induced by intermittent incremental contractions, a surface MMG was obtained from the three muscle heads of the quadriceps muscle in seven subjects while they performed isometric knee extensions lasting 7.6 min. Isometric intermittent incremental contractions started at 1% of the maximal voluntary contraction (MVC) for 3 s, with a 3-s relaxation period in between each contraction, and the contraction level was increased by 1% of MVC for every contraction (by 10% of MVC per min) up to exhaustion. Separate contractions with sufficient rest periods were also conducted to serve for the MMG characteristics without fatigue. The integrated MMG (iMMG) was linearly related to force in all of the muscles when fatigue was not involved. With regard to the incremental contractions, the relationship exhibited an ascending-descending shape, but the behavior was not the same for the individual muscle heads, especially for the rectus femoris muscle. A steep increase in the median frequency of MMG from around 60% of MVC corresponded to a decrease in iMMG. These results suggest that analysis of MMG in the time- and frequency-domain during an incremental protocol is a useful way of characterizing the motor unit recruitment strategy and fatigue properties of individual muscles. Accepted: 19 March 1998     

Relationship between mechanomyogram signals and changes in force of human forefinger flexor muscles during voluntary contraction

In previous studies on mechanomyogram (MMG) signals no analysis of these signals accompanying force generation has been performed. Therefore, we have recorded MMG signals (previously referred to as muscle sound or acoustomyographic signals) during voluntary contractions of forefinger flexor muscles in 31 young subjects. These subjects made contractions to produce force records of triangular or trapeziform shape. The peak target force amounted to 10, 20 or 40 N which represented less than 40% of maximal voluntary contraction. The MMG signals during the transient phases of force generation at three different rates were analysed. The MMG intensity level calculated for MMG records and the peak-to-peak amplitude of MMG signals correlated with both the velocity of force increase and the contraction force. The occurrence of the strongest MMG signals corresponded to changes in contractile force. Therefore, it is suggested that measurements of these parameters could be a useful tool in studies of changes in contractile force. Accepted: 11 March 1998     

Impaired sarcoplasmic reticulum Ca(2+) release rate after fatiguing stimulation in rat skeletal muscle.

The purpose of the study was to characterize the sarcoplasmic reticulum (SR) function and contractile properties before and during recovery from fatigue in the rat extensor digitorum longus muscle. Fatiguing contractions (60 Hz, 150 ms/s for 4 min) induced a reduction of the SR Ca(2+) release rate to 66% that persisted for 1 h, followed by a gradual recovery to 87% of prefatigue release rate at 3 h recovery. Tetanic force and rate of force development (+dF/dt) and relaxation (-dF/dt) were depressed by approximately 80% after stimulation. Recovery occurred in two phases: an initial phase, in which during the first 0.5-1 h the metabolic state recovered to resting levels, and a slow phase from 1-3 h characterized by a rather slow recovery of the mechanical properties. The recovery of SR Ca(2+) release rate was closely correlated to +dF/dt during the slow phase of recovery (r(2) = 0.51; P < 0.05). Despite a slowing of the relaxation rate, we did not find any significant alterations in the SR Ca(2+) uptake function. These data demonstrate that the Ca(2+) release mechanism of SR is sensitive to repetitive in vitro muscle contraction. Moreover, the results indicate that +dF/dt to some extent depends on the rate of Ca(2+) release during the slow phase of recovery.     

Experimental muscle pain increases mechanomyographic signal activity during sub-maximal isometric contractions.

This study was designed to investigate the local effect of experimental muscle pain on the MMG and the surface EMG during a range of sub-maximal isometric contractions. Muscle pain was induced by injections of hypertonic saline into the biceps brachii muscle in 12 subjects. Injections of isotonic saline served as a control. Pain intensity and location, MMG and surface EMG from the biceps brachii were assessed during static isometric (0%, 10%, 30%, 50% and, 70% of the maximal voluntary contraction) and ramp isometric (0-50% of the maximal voluntary contraction) elbow flexions. MMG and surface EMG signals were analyzed in the time and frequency domain. Experimentally induced muscle pain induced an increase in root mean square values of the MMG signal while no changes were observed in the surface EMG. Most likely this increase reflects changes in the mechanical contractile properties of the muscle and indicates compensatory mechanisms, i.e. decreased firing rate and increased twitch force to maintain a constant force output in presence of experimental muscle pain. Under well-controlled conditions, MMG recordings may be more sensitive than surface EMG recordings and clinically useful for detecting non-invasively increased muscle mechanical contributions during muscle pain conditions.     

Fatigue effect on cross-talk in mechanomyography signals of extensor and flexor forearm muscles during maximal voluntary isometric contractions

Objective:This paper presents the analyses of the fatigue effect on the cross-talk in mechanomyography (MMG) signals of extensor and flexor forearm muscles during pre- and post-fatigue maximum voluntary isometric contraction (MVIC).Methods:Twenty male participants performed repetitive submaximal (60% MVIC) grip muscle contractions to induce muscle fatigue and the results were analyzed during the pre- and post-fatigue MVIC. MMG signals were recorded on the extensor digitorum (ED), extensor carpi radialis longus (ECRL), flexor digitorum superficialis (FDS) and flexor carpi radialis (FCR) muscles. The cross-correlation coefficient was used to quantify the cross-talk values in forearm muscle pairs (MP1, MP2, MP3, MP4, MP5 and MP6). In addition, the MMG RMS and MMG MPF were calculated to determine force production and muscle fatigue level, respectively.Results:The fatigue effect significantly increased the cross-talk values in forearm muscle pairs except for MP2 and MP6. While the MMG RMS and MMG MPF significantly decreased (p<0.05) based on the examination of the mean differences from pre- and post-fatigue MVIC.Conclusion:The presented results can be used as a reference for further investigation of cross-talk on the fatigue assessment of extensor and flexor muscles’ mechanic.     

Characteristics of surface mechanomyogram are dependent on development of fusion of motor units in humans.

The purpose of this study was to test whether surface mechanomyogram (MMG) recorded on the skin reflects the contractile properties of individual motor units in humans. Eight motor units in the medial gastrocnemius muscle were identified, and trains of stimulation at 5, 10, 15, and 20 Hz were delivered to each isolated motor unit. There was a significant positive correlation between the duration of MMG and twitch duration. MMG amplitude decreased with increasing stimulation frequency. Reductions in MMG amplitude were in parallel with the reductions in force fluctuations, and the rate of change in both was positively correlated across the motor units. Rate of change in MMG amplitude against force was negatively correlated to half relaxation time and twitch duration. Similar negative correlations were found between force fluctuations and contractile properties. These results provide evidence supporting a direct relation between MMG and contractile properties of individual motor units within the gastrocnemius muscle, indicating that surface MMG is dependent on the contractile properties of the activated motor units in humans.     

Contractile and fatigue properties of the rat diaphragm musculature during the perinatal period.

The following two hypotheses regarding diaphragm contractile properties in the perinatal rat were tested. First, there is a major transformation of contractile and fatigue properties during the period between the inception of inspiratory drive transmission in utero and birth. Second, the diaphragm muscle properties develop to functionally match changes occurring in phrenic motoneuron electrophysiological properties. Muscle force recordings and intracellular recordings of end-plate potentials were measured by using phrenic nerve-diaphragm muscle in vitro preparations isolated from rats on embryonic day 18 and postnatal days 0-1. The following age-dependent changes occurred: 1) twitch contraction and half relaxation times decreased approximately two- and threefold, respectively; 2) the tetanic force levels increased approximately fivefold; 3) the ratio of peak twitch force to maximum tetanic force decreased 2.3-fold; 4) the range of forces generated by the diaphragm in response to graded nerve stimulation increased approximately twofold; 5) the force-frequency curve was shifted to the right; and 6) the propensity for neuromuscular transmission failure decreased. In conclusion, the diaphragm contractile and phrenic motoneuron repetitive firing properties develop in concert so that the full range of potential diaphragm force recruitment can be utilized and problems associated with diaphragm fatigue are minimized.     

Fatigue and recovery contractile properties of young and elderly men

The 24 h recovery pattern of contractile properties of the triceps surae muscle, following a period of muscle fatigue, was compared in physically active young (25 years, n = 10) and elderly (66 years, n = 7) men. The fatigue test protocol consisted of 10 min of intermittent submaximal 20 Hz tetani. The maximal twitch (Pt) and tetanic force at 3 frequencies (10, 20 and 50 Hz) were determined at baseline and at 15 min, 1, 4 and 24 h after fatiguing the muscle. Maximal voluntary contraction (MVC) and vertical jump (MVJ) were also assessed. The loss of force during the fatigue test was not significantly different between the young (18 +/- 13%) and elderly (22 +/- 15%). Both groups showed similar and significant reductions of Pt (15%), tetanic force (10 to 35%) and rate of force development (dp/dt) (20%) 15 min and 1 h into recovery. The loss of force was greater at the lower stimulation frequencies of 10 and 20 Hz. Time-to-peak tension was unchanged from baseline during recovery in either group. The average rate of relaxation of twitch force (-dPt/dt) was decreased (p less than 0.05) and half-relaxation time significantly increased at 15 min and 1 h in the elderly but not the young. The findings indicate that after fatiguing contractions, elderly muscle demonstrates a slower return to resting levels of the rate and time course of twitch relaxation compared to the young.     

Quantifying the history dependency of muscle recovery from a fatiguing intermittent task

Muscle fatigue and recovery are complex processes influencing muscle force generation capacity. While fatigue reduces this capacity, recovery acts to restore the unfatigued muscle state. Many factors can potentially affect muscle recovery, and among these may be a task dependency of recovery following an exercise. However, little has been reported regarding the history dependency of recovery after fatiguing contractions. We examined the dependency of muscle recovery subsequent to four different histories of fatiguing muscle contractions, imposed using two cycle times (30 and 60 s) during low to moderate levels (15% and 25% of maximum voluntary contraction (MVC)) of intermittent static exertions involving index finger abduction. MVC and low-frequency electrical stimulation (LFES) measures (i.e., magnitude, rise and relaxation rates) of muscle capacity were used, all of which indicated a dependency of muscle recovery on the muscle capacity state existing immediately after fatiguing exercise. This dependency did not appear to be modified by either the cycle time or exertion level leading to that state. These results imply that the post-exercise rate of recovery is primarily influenced by the immediate post-exercise muscle contractile status (estimated by MVC and LFES measures). Such results may help improve existing models of muscle recovery, facilitating more accurate predictions of localized muscle fatigue development and thereby helping to enhance muscle performance and reduce the risk of injury.     

Relationship between mechanomyogram and force during voluntary contractions reinvestigated using spectral decomposition

A mechanomyogram (MMG) is considered to represent the pressure waves resulting from the lateral expansion of contracting muscle fibers. However, the actual MMG recording appears not only to reflect lateral changes of active fibers, but also to include the effect of their longitudinal shortening, because the fiber orientation, particularly in pennate muscles, is not parallel with the MMG transducer attached at the skin surface. In the present investigation, a spectral decomposition method was developed to eliminate the interference due to fiber longitudinal movement from the MMG recording. The MMG was recorded over the belly of the rectus femoris muscle, which is a pennate muscle. Vibration over the tibial tuberosity (VTT) was used as a measure of the integrated longitudinal movement of the muscle fibers. The lateral and longitudinal components included in the MMG were separated by a spectral decomposition method that is based on the coherence function of the MMG and VTT. The MMG/force relationship was compared between the original and decomposed MMG. One-third of the 12 subjects demonstrated a curvilinear relationship between the original MMG and force throughout the range of force. In the other two-thirds, the MMG saturated or reduced beyond 70% of the maximum voluntary contraction (MVC). After decomposition, the MMG increased progressively with force up to 70% MVC, beyond which it decreased in all subjects. The spectral decomposition method described here is considered to be a useful tool with which to examine in more detail the MMG/force relationship of different pennate muscles.     

Does the frequency content of the surface mechanomyographic signal reflect motor unit firing rates? A brief review.

The purpose of this review is to examine the literature that has investigated the potential relationship between mechanomyographic (MMG) frequency and motor unit firing rates. Several different experimental designs/methodologies have been used to address this issue, including: repetitive electrical stimulation, voluntary muscle actions in muscles with different fiber type compositions, fatiguing and non-fatiguing isometric or dynamic muscle actions, and voluntary muscle actions in young versus elderly subjects and healthy individuals versus subjects with a neuromuscular disease(s). Generally speaking, the results from these investigations have suggested that MMG frequency is related to the rate of motor unit activation and the contractile properties (contraction and relaxation times) of the muscle fibers. Other studies, however, have reported that MMG mean power frequency (MPF) does not always follow the expected pattern of firing rate modulation (e.g. motor unit firing rates generally increase with torque during isometric muscle actions, but MMG MPF may remain stable or even decrease). In addition, there are several factors that may affect the frequency content of the MMG signal during a voluntary muscle action (i.e. muscle stiffness, intramuscular fluid pressure, etc.), independent of changes in motor unit firing rates. Despite the potential influences of these factors, most of the evidence has suggested that the frequency domain of the MMG signal contains some information regarding motor unit firing rates. It is likely, however, that this information is qualitative, rather than quantitative in nature, and reflects the global motor unit firing rate, rather than the firing rates of a particular group of motor units.     

Sarcolipin overexpression in rat slow twitch muscle inhibits sarcoplasmic reticulum Ca2+ uptake and impairs contractile function

Sarcolipin (SLN) is an inhibitor of sarco(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs) in vitro, but its function in vivo has not been defined. NF-SLN cDNA (SLN tagged N-terminally with a FLAG epitope) was introduced into rat soleus muscle in one hindlimb by plasmid injection and electrotransfer. Western blotting showed expression and co-immunoprecipitation showed physical interaction between NF-SLN and SERCA2a. Contractile properties and SERCA2a function were assessed and compared with vector-injected contralateral soleus muscles. NF-SLN reduced both peak twitch force (P(t)) (123.9 +/- 12.5 versus 69.8 +/- 8.9 millinewtons) and tetanic force (P(o)) (562.3 +/- 51.0 versus 300.7 +/- 56.9 millinewtons) and reduced both twitch and tetanic rates of contraction (+dF/dt) and relaxation (-dF/dt) significantly. Repetitive stimulation (750-ms trains at 50 Hz once every 2 s for 3 min) showed that NF-SLN increased susceptibility to fatigue. These changes in contractile function were observed in the absence of endogenous phospholamban, and NF-SLN had no effect on either SERCA2a or SERCA1a expression levels. NF-SLN also decreased maximal Ca(2+) transport activity at pCa 5 by 31% with no significant change in apparent Ca(2+) affinity (6.36 +/- 0.07 versus 6.39 +/- 0.08 pCa units). These results show that NF-SLN expression impairs muscle contractile function by inhibiting SERCA function and diminishing sarcoplasmic reticulum Ca(2+) stores.     

Muscle mechanism: The acceleration of the load

The load (force/cross-section) determines the response of muscle power output, force and speed of contraction). The force is the product of the mass by the acceleration, thus the same force is generated by an infinite number of mass and acceleration couples and each one of these couples displays different physical and biological effects. Therefore, the load must be defined both by the mass and by the acceleration. Early muscle investigators were well aware of this situation as it is indicated by the work of Hill on the flexion of the arm against the “heavy fly-wheel”. By making use of a model of sarcomere contraction we show here that the acceleration of the load is the first determinant of the time course of the process of generation of the isometric tension. We also propose that, in order to reproduce the rapid release, it is not necessary to invoke the presence of a distinct elastic element in the contractile machinery. It is sufficient to assume that the stiffness of the same machinery increases with the contractile force.     

Voluntary low-force contraction elicits prolonged low-frequency fatigue and changes in surface electromyography and mechanomyography.

Controversies exist regarding objective documentation of fatigue development with low-force contractions. We hypothesized that non-exhaustive, low-force muscle contraction may induce prolonged low-frequency fatigue (LFF) that in the subsequent recovery period is detectable by electromyography (EMG) and in particular mechanomyography (MMG) during low-force rather than high-force test contractions. Seven subjects performed static wrist extension at 10% maximal voluntary contraction (MVC) for 10 min (10%MVC10 min). Wrist force response to electrical stimulation of extensor carpi radialis muscle (ECR) quantified LFF. EMG and MMG were recorded from ECR during static test contractions at 5% and 80% MVC. Electrical stimulation, MVC, and test contractions were performed before 10%MVC10 min and at 10, 30, 90 and 150 min recovery. In spite of no changes in MVC, LFF persisted up to 150 min recovery but did not develop in a control experiment omitting 10%MVC10 min. In 5% MVC tests significant increase was found in time domain of EMG from 0.067+/-0.028 mV before 10%MVC10 min to 0.107+/-0.049 and 0.087+/-0.05 mV at 10 and 30 min recovery, respectively, and of the MMG from 0.054+/-0.039 ms(-2) to 0.133+/-0.104 and 0.127+/-0.099 ms(-2), respectively. No consistent changes were found in 80% MVC tests. In conclusion, non-exhaustive low-force muscle contraction resulted in prolonged LFF that in part was identified by the EMG and MMG signals.     

Time course of stretching-induced changes in mechanomyogram and force characteristics

To evaluate the time-course of stretching-induced changes in mechanical properties of the muscle-tendon unit (MTU), 11 participants (age 22 ± 1 yr; body mass 77 ± 5 kg; stature 1.78 ± 0.05 m; mean ± SD) underwent tetanic electrical stimulations of the medial gastrocnemius muscle before and after (up to 2 h) stretching administration. During contractions, surface electromyogram (EMG), mechanomyogram (MMG) and force were recorded simultaneously. From MMG, peak-to-peak (p–p) and root mean square (RMS) were calculated during the on-phase and plateau phase of tetanic contraction, respectively. After stretching: (i) no differences were found in EMG parameters; (ii) MMG p–p and slope decreased (−16% and −10%, respectively; P < 0.05) and remained depressed for the entire recovery period; (iii) MMG RMS increased (+20%; P < 0.05), returning to pre-stretching values within 15 min; and (iv) peak force (pF), with its first (dF/dt) and second (d²F/dt²) derivative, decreased significantly by 32%, 35% and 54%, respectively, and remained depressed for the entire recovery period. The lack of MMG p–p and pF recovery could be ascribable to a reduced muscle force generating capacity due to persisting changes in viscoelastic characteristics of series elastic components. The early return of MMG RMS to pre-stretching values suggests that changes in viscoelastic parallel components recovered after few minutes.     

Mechanomyogram amplitude vs. isometric ankle plantarflexion torque of human medial gastrocnemius muscle at different ankle joint angles

The purpose of this study was to investigate the influence of changes in ankle joint angle on the mechanomyogram (MMG) amplitude of the human medial gastrocnemius (MG) muscle during voluntary isometric plantarflexion contractions. Ten healthy individuals were asked to perform voluntary isometric contractions at six different contraction intensities (from 10% to 100%) and at three different ankle joint angles (plantarflexion of 26°; plantarflexion of 10°; dorsiflexion of 3°). MMG signals were recorded from the surface over the MG muscle, using a 3-axis accelerometer. The relations between root mean square (RMS) MMG and isometric plantarflexion torque at different ankle joint angles were characterized to evaluate the effects of altered muscle mechanical properties on RMS MMG.We found that the relation between RMS MMG and plantarflexion torque is changed at different ankle joint angles: RMS MMG increases monotonically with increasing the plantarflexion torque but decreases as the ankle joint became dorsiflexed. Moreover, RMS MMG shows a negative correlation with muscle length, with passive torque, and with maximum voluntary torque, which were all changed significantly at different ankle joint angles.Our findings demonstrate the potential effects of changing muscle mechanical properties on muscle vibration amplitude. Future studies are required to explore the major sources of this muscle vibration from the perspective of muscle mechanics and muscle activation level, attributable to changes in the neural command.