Motor-unit synchronization is not responsible for larger motor-unit forces in old adults

Motor-unit synchronization, which is a measure of the near simultaneous discharge of action potentials by motor units, has the potential to influence spike-triggered average force and the steadiness of a low-force isometric contraction. The purpose of the study was to estimate the contribution of motor-unit synchronization to the larger spike-triggered average forces and the decreased steadiness exhibited by old adults. Eleven young (age 19-30 yr) and 14 old (age 63-81 yr) adults participated in the study. Motor-unit activity was recorded with two fine-wire intramuscular electrodes in the first dorsal interosseus muscle during isometric contractions that caused the index finger to exert an abduction force. In a separate session, steadiness measurements were obtained during constant-force isometric contractions at target forces of 2.5, 5, 7. 5, and 10% of the maximum voluntary contraction (MVC) force. Mean (+/-SD) motor-unit forces measured by spike-triggered averaging were larger in old (15.5 +/- 12.1 mN) compared with young (7.3 +/- 5.7 mN) adults, and the differences were more pronounced between young (8.7 +/- 6.4 mN) and old (19.9 +/- 12.2 mN) men. Furthermore, the old adults had a reduced ability to maintain a steady force during an isometric contraction, particularly at low target forces (2.5 and 5% MVC). Mean (+/-SD) motor-unit synchronization, expressed as the frequency of extra synchronous discharges above chance in the cross-correlogram, was similar in young [0.66 +/- 0.4 impulses/s (imp/s); range, 0.35-1.51 imp/s; 53 pairs) and old adults (0.72 +/- 0.5 imp/s; range, 0.27-1.38 imp/s; 56 pairs). The duration of synchronous peaks in the cross-correlogram was similar for each group (approximately 16 ms). These data suggest that motor-unit synchronization is not responsible for larger spike-triggered average forces in old adults and that motor-unit synchronization does not contribute to the decreased steadiness of low-force isometric contractions observed in old adults.     

Discharge characteristics of biceps brachii motor units at recruitment when older adults sustained an isometric contraction

The purpose of this study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit. The discharge times of 27 single motor units were recorded from the biceps brachii in 11 old adults (78.8 ± 5.9 yr). The target force was set at either a relatively small (6.6 ± 3.7% maximum) or large (11.4 ± 4.5% maximum) difference below the recruitment threshold force and the contraction was sustained until the motor unit was recruited and discharged action potentials for about 60 s. The time to recruitment was longer for the large target-force difference (P = 0.001). At recruitment, the motor units discharged repetitively for both target-force differences, which contrasts with data from young adults when motor units discharged intermittently at recruitment for the large difference between recruitment threshold force and target force. The coefficient of variation (CV) for the first five interspike intervals (ISIs) increased from the small (18.7 ± 7.9) to large difference (35.0 ± 10.2%, P = 0.008) for the young adults, but did not differ for the two target force differences for the old adults (26.3 ± 14.7 to 24.0 ± 13.1%, P = 0.610). When analyzed across the discharge duration, the average CV for the ISI decreased similarly for the two target-force differences (P = 0.618) in old adults. These findings contrast with those of young adults and indicate that the integration of synaptic input during sustained contractions differs between young and old adults.     

Discharge rate variability influences the variation in force fluctuations across the working range of a hand muscle

The goal of this study was to improve the ability of a motor unit model to predict experimentally measured force variability across a wide range of forces. Motor unit discharge characteristics were obtained from 38 motor units of the first dorsal interosseus muscle. Motor unit discharges were recorded in separate isometric contractions that ranged from 4 to 85% of the maximal voluntary contraction (MVC) force above recruitment threshold. High-threshold motor units exhibited both greater minimal and peak discharge rates compared with low-threshold units (P < 0.01). Minimal discharge rate increased from 7 to 23 pps, and peak discharge rate increased from 14 to 38 pps with an increase in recruitment threshold. Relative discharge rate variability (CV) decreased exponentially for each motor unit from an average of 30 to 13% as index finger force increased above recruitment threshold. In separate experiments, force variability was assessed at eight force levels from 2 to 95% MVC. The CV for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC (P < 0.01) and remained constant at higher forces (1.2-1.9%; P = 0.14). When the motor unit model was revised using these experimental findings, discharge rate variability was the critical factor that resulted in no significant difference between simulated and experimental force variability (P = 0.22) at all force levels. These results support the hypothesis that discharge rate variability is a major determinant of the trends in isometric force variability across the working range of a muscle.     

Diminished task-related adjustments of common inputs to hand muscle motor neurons in older adults

The purpose of this study was to quantify correlated motor unit activity during isometric, shortening and lengthening contractions of a hand muscle in older adults. Thirteen old subjects (69.6±5.9 years, six women) lifted and lowered a light load with abduction–adduction movements of the index finger over 10° using 6-s shortening and lengthening contractions of the first dorsal interosseus muscle. The task was repeated 10–20 times while activity in 23 pairs of motor units was recorded with intramuscular electrodes. The data were compared with 23 motor-unit pairs in 15 young (25.9±4.6 years, five women) subjects obtained using a similar protocol in a previous study. Correlated motor unit activity was quantified using time-domain (synchronization index; Common Input Strength) and frequency-domain (coherence) analyses for the same motor-unit pairs. For all contractions, there was no difference with age for the strength of motor-unit synchronization, although age-related differences were observed for synchronous peak widths (young, 17.6±7.4 ms; old, 13.7±4.9 ms) and motor-unit coherence at 6–9 Hz (z score for young, 3.0±1.8; old, 2.2±1.5). Despite increased synchrony during lengthening contractions and narrower peak widths for shortening contractions in young subjects, there was no difference in the strength of motor unit synchronization (CIS ~0.8 imp/s), or the width of the synchronous peak (~14 ms) during the three tasks in old subjects. Furthermore, no significant differences in motor-unit coherence were observed between tasks at any frequency for old adults. These data suggest that the strategy used by the central nervous system to control isometric, shortening, and lengthening contractions varies in young adults, but not old adults. The diminished task-related adjustments of common inputs to motor neurons are a likely consequence of the neural adaptations that occur with advancing age.     

Motor-unit activity differs with load type during a fatiguing contraction

Despite a similar rate of change in average electromyographic (EMG) activity, previous studies have observed different rates of change in mean arterial pressure, heart rate, perceived exertion, and fluctuations in motor output during the performance of fatiguing contractions that involved different types of loads. To obtain a more direct measure of the motor output from the spinal cord, the purpose of this study was to compare the discharge characteristics of the same motor unit in biceps brachii during the performance of two types of fatiguing contractions. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task) for a prescribed duration. The force and position tasks were performed in random order at a target force equal to 3.5 +/- 2.1% (mean +/- SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of the isolated motor unit. Each subject maintained the two tasks for an identical duration (161 +/- 96 s) at a mean target force of 22.2 +/- 13.4% MVC (range: 3-49% MVC). The dependent variables included the discharge characteristics of the same motor unit in biceps brachii, fluctuations in motor output (force or acceleration), mean arterial pressure, heart rate, and rating of perceived exertion. Despite similar increases in the amplitude of the averaged EMG (% MVC) for the elbow flexor muscles during both tasks (P = 0.60), the rates of increase in mean arterial pressure (P < 0.001), rating of perceived exertion (P = 0.023), and fluctuations in motor output (P = 0.003) were greater during the position task compared with the force task. Consistent with these differences, mean discharge rate declined at a greater rate during the position task (P = 0.03), and the coefficient of variation for discharge rate increased only during the position task (P = 0.02). Furthermore, more motor units were recruited during the position task compared with the force task (P = 0.01). These findings indicate that despite a comparable net muscle torque, the rate of increase in the motor output from the spinal cord was greater during the position task.     

Motor-unit coherence during isometric contractions is greater in a hand muscle of older adults

The purpose of this study was to quantify the strength of motor-unit coherence from the first dorsal interosseus muscle in young and old adults using data obtained in a previous study, where no differences in motor-unit synchronization between the two groups were observed. The strength of motor-unit coherence was quantified from 47 motor-unit pairs in 11 young adults (age 24.1 +/- 4.1 yrs) and from 48 motor-unit pairs in 14 old adults (age 70.4 +/- 5.9 yrs). The strength of motor-unit coherence was greater in old adults, particularly at low frequencies of 5-9 Hz (85% greater in old adults at 5 Hz). In addition, the older adults expressed an extra oscillation at approximately 12-13 Hz that was not present in the young subjects. These data demonstrate that common oscillatory inputs to motor neurons (motor-unit coherence) are enhanced in older adults despite no age-related difference in the strength of shared inputs (synchronization). Furthermore, the data emphasize that measures of motor-unit synchronization and coherence highlight different features of the same common input, and a coherence analysis may be a more sensitive tool to characterize shared input to motor neurons.     

Multiple features of motor-unit activity influence force fluctuations during isometric contractions

To identify the mechanisms responsible for the fluctuations in force that occur during voluntary contractions, experimental measurements were compared with simulated forces in the time and frequency domains at contraction intensities that ranged from 2 to 98% of the maximum voluntary contraction (MVC). The abduction force exerted by the index finger due to an isometric contraction of the first dorsal interosseus muscle was measured in 10 young adults. Force was simulated with computer models of motor-unit recruitment and rate coding for a population of 120 motor units. The models varied recruitment and rate-coding properties of the motor units and the activation pattern of the motor-unit population. The main finding was that the experimental observations of a minimum in the coefficient of variation (CV) for force (1.7%) at approximately 30% MVC and a plateau at higher forces could not be replicated by any of the models. The model that increased the level of short-term synchrony with excitatory drive provided the closest fit to the experimentally observed relation between the CV for force and the mean force. In addition, the results for the synchronization model extended previous modeling efforts to show that the effect of synchronization is independent from that of discharge-rate variability. Most of the power in the force power spectra for the models was contained in the frequency bins below 5 Hz. Only a model that included a low-frequency oscillation in excitation, however, could approximate the experimental finding of peak power at a frequency below 2 Hz: 38% of total power at 0.99 Hz and 43% at 1.37 Hz, respectively. In contrast to the experimental power spectra, all model spectra included a second peak at a higher frequency. The secondary peak was less prominent in the synchronization model because of greater variability in discharge rate. These results indicate that the variation in force fluctuations across the entire operating range of the muscle cannot be explained by a single mechanism that influences the output of the motor-unit population.     

Motor unit activation patterns during concentric wrist flexion in humans with different muscle fibre composition

Muscle activity was recorded from the flexor carpi radialis muscle during static and dynamic-concentric wrist flexion in six subjects, who had exhibited large differences in histochemically identified muscle fibre composition. Motor unit recruitment patterns were identified by sampling 310 motor units and counting firing rates in pulses per second (pps). During concentric wrist flexion at 30% of maximal exercise intensity the mean firing rate was 27 (SD 13) pps. This was around twice the value of 12 (SD 5) pps recorded during sustained static contraction at 30% of maximal voluntary contraction, despite a larger absolute force level during the static contraction. A similar pattern of higher firing rates during dynamic exercise was seen when concentric wrist flexion at 60% of maximal exercise intensity [30 (SD 14) pps] was compared with sustained static contraction at 60% of maximal voluntary contraction [19 (SD 8) pps]. The increase in dynamic exercise intensity was accomplished by recruitment of additional motor units rather than by increasing the firing rate as during static contractions. No difference in mean firing rates was found among subjects with different muscle fibre composition, who had previously exhibited marked differences in metabolic response during corresponding dynamic contractions. It was concluded that during submaximal dynamic contractions motor unit firing rate cannot be deduced from observations during static contractions and that muscle fibre composition may play a minor role.     

Frequency modulation of motor unit discharge has task-dependent effects on fluctuations in motor output

The rate of change in the fluctuations in motor output differs during the performance of fatiguing contractions that involve different types of loads. The purpose of this study was to examine the contribution of frequency modulation of motor unit discharge to the fluctuations in the motor output during sustained contractions with the force and position tasks. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task). The force and position tasks were performed in random order at a target force equal to 3.6 +/- 2.1% (mean +/- SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of an isolated motor unit from the biceps brachii. Each subject maintained the two tasks for an identical duration (161 +/-93 s) at a mean target force of 22.4 +/-13.6% MVC. As expected, the rate of increase in the fluctuations in motor output (force task: SD for detrended force; position task: SD for vertical acceleration) was greater for the position task than the force task (P < 0.001). The amplitude of the coefficient of variation (CV) and the power spectra for motor unit discharge were similar between tasks (P > 0.1) and did not change with time (P > 0.1), and could not explain the different rates of increase in motor output fluctuations for the two tasks. Nonetheless, frequency modulation of motor unit discharge differed during the two tasks and predicted (P < 0.001) both the CV for discharge rate (force task: 1-3, 12-13, and 14-15 Hz; position task: 0-1, and 1-2 Hz) and the fluctuations in motor output (force task: 5-6, 9-10, 12-13, and 14-15 Hz; position task: 6-7, 14-15, 17-19, 20-21, and 23-24 Hz). Frequency modulation of motor unit discharge rate differed for the force and position tasks and influenced the ability to sustain steady contractions.     

Motor-unit synchronization increases EMG amplitude and decreases force steadiness of simulated contractions

The purpose of the study was to determine the effect of motor-unit synchronization on the surface electromyogram (EMG) and isometric force using a computer model of muscle contraction. The EMG and force were simulated by generating muscle fiber action potentials, defining motor-unit mechanical characteristics and territories, estimating motor-unit action potentials, specifying motor-unit discharge times, and imposing various levels of motor-unit synchronization. The output (EMG and force) was simulated at 11 levels of excitation, ranging from 5 to 100% of maximum. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; however, the number of motor units recruited and the average discharge rate of each unit was constant across synchronization conditions for a given level of excitation. Two levels of synchronization were imposed on the discharge times: a moderate and a high level, which approximated the experimentally observed range of motor-unit synchronization. The moderate level of synchrony caused the average EMG to increase by approximately 65%, whereas the high level caused a 130% increase in the EMG with respect to the no-synchrony condition. Neither synchrony condition influenced the magnitude of the average force. However, motor-unit synchronization did increase the amplitude of the fluctuations in the simulated force, especially at intermediate levels of excitation. In conclusion, motor-unit synchronization increased the amplitude of the average rectified EMG and decreased the steadiness of the force exerted by the muscle in simulated contractions.     

Low-frequency oscillations of the neural drive to the muscle are increased with experimental muscle pain

We investigated the influence of nociceptive stimulation on the accuracy of task execution and motor unit spike trains during low-force isometric contractions. Muscle pain was induced by infusion of hypertonic saline into the abductor digiti minimi muscle of 11 healthy men. Intramuscular EMG signals were recorded from the same muscle during four isometric contractions of 60-s duration at 10% of the maximal force [maximal voluntary contraction (MVC)] performed before injection (baseline), after injection of isotonic (control) or hypertonic saline (pain), and 15 min after pain was no longer reported. Each contraction was preceded by three 3-s ramp contractions from 0% to 10% MVC. The low-frequency oscillations of motor unit spike trains were analyzed by the first principal component of the low-pass filtered spike trains [first common component (FCC)], which represents the effective neural drive to the muscle. Pain decreased the accuracy of task performance [coefficient of variation (CoV) for force: baseline, 2.8 ± 1.8%, pain, 3.9 ± 1.8%; P < 0.05] and reduced motor unit discharge rates [11.6 ± 2.3 pulses per second (pps) vs. 10.7 ± 1.7 pps; P < 0.05]. Motor unit recruitment thresholds (2.2 ± 1.2% MVC vs. 2.4 ± 1.6% MVC), interspike interval variability (18.4 ± 4.9% vs. 19.1 ± 5.4%), strength of motor unit short-term synchronization [common input strength (CIS) 1.02 ± 0.44 vs. 0.83 ± 0.22], and strength of common drive (0.47 ± 0.08 vs. 0.47 ± 0.06) did not change across conditions. The FCC signal was correlated with force (R = 0.45 ± 0.06), and the CoV for FCC increased in the painful condition (5.69 ± 1.29% vs. 7.83 ± 2.61%; P < 0.05). These results indicate that nociceptive stimulation increased the low-frequency variability in synaptic input to motoneurons.     

Effects of aging on motor-unit control properties

It was hypothesized that the age-related alterations in the morphological properties of a motor unit would be accompanied by modifications in the control aspects of the motor unit, as either an adaptive or compensatory mechanism to preserve smooth force production. In specific, the objective of the study was to investigate the age-related alterations in the concurrent firing behavior of multiple motor units in the first dorsal interosseous (FDI) muscle in isometric contractions at 20 and 50% of the subject's voluntary contraction level. Analysis of the data collected from 10 young (24-37 yr of age) and 10 elderly (65-88 yr of age) subjects led to three novel observations regarding the firing behavior of aged motor units. 1) Among elderly subjects, there is a decrease in the common fluctuations that are observed among the firing rates of motor units in the young. 2) The relationship observed between the firing rate and recruitment threshold of young subjects is disturbed in the elderly. Although in young subjects, at any point in a given submaximal contraction, earlier recruited motor units have higher firing rates than later-recruited units; in aged subjects this dependency of firing rate on recruitment rank is compromised. 3) The progressive decrease observed in the firing rates of concurrently active motor units in constant-force contractions in the young is not seen in the aged. In addition to these original findings, this study provided support for earlier reports of 1) decreased average firing rates probably reflecting the slowing of the muscle, 2) a shift in recruitment thresholds toward lower force levels in line with the shift toward type I fibers, and 3) multiphasic action potential shapes indicative of the reinnervation process that takes place during aging. Taken as a whole, these findings indicate significant age-related modifications in the control properties of human motor units.     

Changes in force, surface and motor unit EMG during post-exercise development of low frequency fatigue in vastus lateralis muscle

We investigated the effects of low frequency fatigue (LFF) on post-exercise changes in rectified surface EMG (rsEMG) and single motor unit EMG (smuEMG) in vastus lateralis muscle (n=9). On two experimental days the knee extensors were fatigued with a 60-s-isometric contraction (exercise) at 50% maximal force capacity (MFC). On the first day post-exercise (15 s, 3, 9, 15, 21 and 27 min) rsEMG and electrically-induced (surface stimulation) forces were investigated. SmuEMG was obtained on day two. During short ramp and hold (5 s) contractions at 50% MFC, motor unit discharges of the same units were followed over time. Post-exercise MFC and tetanic force (100 Hz stimulation) recovered to about 90% of the pre-exercise values, but recovery with 20 Hz stimulation was less complete: the 20–100 Hz force ratio (mean ± SD) decreased from 0.65±0.06 (pre-exercise) to 0.56±0.04 at 27 min post-exercise (P<0.05), indicative of LFF. At 50% MFC, pre-exercise rsEMG (% pre-exercise maximum) and motor unit discharge rate were 51.1±12.7% and 14.1±3.7 (pulses per second; pps) respectively, 15 s post-exercise the respective values were 61.4±15.4% (P<0.05) and 13.2±5.6 pps (P>0.05). Thereafter, rsEMG (at 50% MFC) remained stable but motor unit discharge rate significantly increased to 17.7±3.9 pps 27 min post-exercise. The recruitment threshold decreased (P<0.05) from 27.7±6.6% MFC before exercise to 25.2±6.7% 27 min post-exercise. The increase in discharge rate was significantly greater than could be expected from the decrease in recruitment threshold. Thus, post-exercise LFF was compensated by increased motor unit discharge rates which could only partly be accounted for by the small decrease in motor unit recruitment threshold.     

Motor-unit synchronization alters spike-triggered average force in simulated contractions

The purpose of the study was to quantify the effect of motor-unit synchronization on the spike-triggered average forces of a population of motor units. Muscle force was simulated by defining mechanical and activation characteristics of the motor units, specifying motor neuron discharge times, and imposing various levels of motor-unit synchronization. The model comprised 120 motor units. Simulations were performed for motor units 5-120 to compare the spike-triggered average responses in the presence and absence of motor-unit synchronization with the motor-unit twitch characteristics defined in the model. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; this kept the number of action potentials constant across the three levels of synchrony for each motor unit. Because there was some overlap of motor-unit twitches even at minimal discharge rates, the simulations indicated that spike-triggered averaging underestimates the twitch force of all motor units and the contraction time of motor units with contraction times longer than 49 ms. Although motor-unit synchronization increased the estimated twitch force and decreased the estimated contraction time of all motor units, spike-triggered average force changed systematically with the level of synchrony in motor units 59-120 (upper 90% of the range of twitch forces). However, the reduction in contraction time was similar for moderate and high synchrony. In conclusion, spike-triggered averaging appears to provide a biased estimate of the distribution of twitch properties for a population of motor units because twitch fusion causes an underestimation of twitch force for slow units and motor-unit synchronization causes an overestimation of force for fast motor units.     

Force control is impaired in the ankle plantarflexors of elderly adults

This study determined the amplitude of force fluctuations for the ankle dorsiflexor (DF) and plantarflexor (PF) muscles of young and elderly adults. Maximal voluntary contraction (MVC) force and isometric DF and PF steadiness (2.5–80% MVC) was assessed in 11 young (23 ± 3 years, 5 women, 6 men) and 10 elderly (73 ± 6 years, 5 women, 5 men) adults. The coefficient of variation (CV) and power spectrum of the force was measured from the steadiness trials. MVC force was lower for elderly adults for PF (38% lower, P = 0.002) but not DF (20% lower, P = 0.14). For PF, the CV of force was greater for elderly than young adults at 2.5% (2.64 vs. 1.71%) and 5% MVC (1.78 vs. 1.24%), similar at 10, 50, and 80% MVC, and greater for young than elderly at the 30% MVC target force. For DF, the CV of force was similar for young and elderly at all target forces (P > 0.05). The CV of force was 49% lower for the PF compared with DF muscles across all target forces (P < 0.0001). This difference was significantly greater at the 2.5 (58%), 5 (58%), and 10% MVC (44%) target forces compared with higher target forces. The power spectra of the force fluctuations for both muscles were consistently dominated by frequencies below 2 Hz. For elderly adults, the neuromuscular factors that underlie both muscle strength and force fluctuations during low-force contractions are impaired in the ankle plantarflexors but not the dorsiflexors.     

Change in muscle fascicle length influences the recruitment and discharge rate of motor units during isometric contractions

This study examines the effect of fascicle length change on motor-unit recruitment and discharge rate in the human tibialis anterior (TA) during isometric contractions of various intensities. The torque produced during dorsiflexion and the surface and intramuscular electromyograms (EMGs) from the TA were recorded in eight subjects. The behavior of the same motor unit (n = 59) was compared at two ankle joint angles (+10 and -10 degrees around the ankle neutral position). Muscle fascicle length of the TA was measured noninvasively using ultrasonography recordings. When the ankle angle was moved from 10 degrees plantarflexion to 10 degrees dorsiflexion, the torque produced during maximal voluntary contraction (MVC) was significantly reduced [35.2 +/- 3.3 vs. 44.3 +/- 4.2 (SD) Nm; P < 0.001] and the average surface EMG increased (0.47 +/- 0.08 vs. 0.43 +/- 0.06 mV; P < 0.05). At reduced ankle joint angle, muscle fascicle length declined by 12.7% (P < 0.01) at rest and by 18.9% (P < 0.001) during MVC. Motor units were activated at a lower recruitment threshold for short compared with long muscle fascicle length, either when expressed in absolute values (2.1 +/- 2.5 vs. 3.6 +/- 3.7 Nm; P < 0.001) or relative to their respective MVC (5.2 +/- 6.1 vs. 8.8 +/- 9.0%). Higher discharge rate and additional motor-unit recruitment were observed at a given absolute or relative torque when muscle fascicles were shortened. However, the data indicate that increased rate coding was mainly present at low torque level (<10% MVC), when the muscle-tendon complex was compliant, whereas recruitment of additional motor units played a dominant role at higher torque level and decreased compliance (10-35% MVC). Taken together, the results suggest that the central command is modulated by the afferent proprioceptive information during submaximal contractions performed at different muscle fascicle lengths.     

Discharge of biceps brachii motor units is modulated by load compliance and forearm posture

The purpose of this study was to compare the discharge characteristics of motor units in the biceps brachii during brief isometric contractions of the elbow flexors as subjects matched either a target force or a target joint angle with the forearm placed in one of two postures. One task required force control to exert a constant force against a rigid restraint (force task), whereas the other task involved position control to maintain a constant elbow angle while supporting an inertial load (position task). The left arm of right-handed subjects was rotated forward so that the upper arm was horizontal and the forearm was vertical. When the elbow flexor muscles were contracted, the wrist exerted a force in a horizontal direction. Subcutaneous electrodes were used to record the discharge of 20 motor units in neutral and supinated forearm postures during both force and position control. Motor unit recruitment thresholds ranged from 1.3 to 37.9% of maximal voluntary contraction force. Discharge rate was similar at the start of the force and position tasks in both the neutral posture (13.1 ± 0.6 and 12.6 ± 0.6 pps, P = 0.54) and the supinated posture (14.7 ± 1.6 and 14.0 ± 0.9 pps, P = 0.4) and declined during both tasks in the two forearm postures (P < 0.001). Nonetheless, the decrease in discharge rate (P < 0.001), increase in the coefficient of variation for interspike interval (P = 0.04), and increases in the standard deviation of acceleration (P = 0.02) were greatest for the position task in the supinated posture. These findings indicate that the influence of load compliance on the adjustments in motor unit activity during brief isometric contractions with the elbow flexors was modulated by changes in forearm posture.     

The amplitude of force variability is correlated in the knee extensor and elbow flexor muscles

The purpose of this study was to determine motor output variability for different muscles in the same individuals. Ten young (21.7 ± 3.4 years) and ten elderly (72.1 ± 3.9 years) men underwent assessment of maximal isometric (MVC) and dynamic (1-RM) strength, and performed constant-force (2–50% MVC), constant-load (5–50% 1-RM load), and unloaded postural contractions as steadily as possible with the first dorsal interosseus (FDI), elbow flexors (EF), and knee extensors (KE). The coefficient of variation (CV) of force for isometric contractions and the standard deviation (SD) of acceleration for concentric, eccentric, and postural contractions were calculated. The 1-RM load, the CV of force for four of five isometric target forces, and the SD of acceleration during postural contractions were correlated between the EF and KE muscles. MVC force, 1-RM load, and SD of acceleration during postural contractions were not correlated between the FDI/EF or FDI/KE. The CV of force was correlated between the FDI/EF and FDI/KE for two of five isometric target forces. The SD of acceleration during concentric and eccentric contractions was not correlated between muscles. The normalized fluctuations during isometric contractions were greater for the FDI compared with the EF and KE. Elderly adults displayed greater fluctuations only for the FDI during low-force isometric and postural contractions. The dominant frequency of fluctuations was similar for the EF and KE muscles. The correlated fluctuations for the EF and KE muscles, within subjects, suggests that the two motor neuron pools transform the various neural inputs similarly.     

Adaptations in motor unit discharge activity with force control training in young and older human adults

Six young (aged 18–22 years) and six older (aged 66–76 years) healthy humans participated in a visually guided isometric force modulation training program designed to improve accurate control of force during ankle dorsiflexion. Isometric force and the discharge activity of motor units (MU) supplying the tibialis anterior muscle were sampled concurrently at the beginning of the study, following 2 weeks of force modulation training and again after a 4 week retention period which followed immediately. The initial maximal voluntary force (MVC) and MU discharge rates were similar between young and older adults at 10–60% MVC while MU discharge rates during maximal effort were significantly reduced in older adults. Following the 2 weeks of force modulation training, both young and older adults demonstrated significant improvements in force accuracy (44% young, 48% older) and significantly reduced MU discharge rates at 30%, 40%, and 60% MVC. Young adults also demonstrated increased MVC force (11%), while older adults demonstrated significantly increased (30%) maximal MU discharge rate. Thus, following 2 weeks of force modulation training, young and older individuals demonstrated similar MU discharge rates at all force levels. The MU discharge rate adaptations were retained after the 4 week retention period. In young adults, improved force accuracy and increased MVC force were accompanied by significantly reduced MU recruitment thresholds. In the older subjects, improved force accuracy was accompanied by an increase in the difference between the recruitment-derecruitment force threshold and significantly reduced antagonist co-contraction. Age-related alterations in force regulation and MU discharge activity cannot be explained solely on the basis of contractile changes in senescent muscle. Rather, reliance on compensatory neuromuscular changes including antagonist muscle co-contraction is suggested. Accepted: 27 June 2000     

Doublet discharges in motoneurons of young and older adults

The purpose of this study was to investigate the occurrence of motor unit doublet discharges in young and older individuals at different rates of increasing force. Participants included eight young (21.9 +/- 3.56 yr) and eight older (74.1 +/- 8.79 yr) individuals, with equal numbers of males and females in each group. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a four-wire needle electrode. Subjects performed three ramp contractions from zero to 50% maximal voluntary contraction (MVC) force at each of three rates: 10, 30, and 50% MVC/s. Overall, the occurrence of doublets was significantly higher in the young than in the older individuals. However, neither group showed differences in the occurrence of doublets across the three rates of force production. Doublet firings were observed in 45.6 (young) and 35.1% (old) of motor units at 10% MVC/s; 48.6 (young) and 22.5% (old) of motor units at 30% MVC/s; and 48.4 (young) and 31.4% (old) at 50% MVC/s. The maximal firing rate was significantly higher and the force at which the motor units were recruited was significantly lower for those units that fired doublets than those that did not. The force at which doublets occurred ranged from 3.42 to 50% MVC in the young subjects and from 0 (force onset) to 50% MVC in the older subjects. The results of this study suggest that the occurrence of doublets is dependent on both motor unit firing rate and force level. The lower incidence of doublets in older individuals may be attributable to changes in the intrinsic properties of the motoneurons with aging, which appear to play a role in doublet discharges.