Fluctuations in motor output during steady contractions are weakly related across contraction types and between hands

The presence of differences in motor unit activity across contraction types and between hands suggests that the magnitude of fluctuations in motor output is only weakly related when these conditions are compared. Twenty right-handed young (24.1 +/- 5.3 years) and old (72.5 +/- 4.9 years) adults performed three levels (相似文献   

Steadiness is reduced and motor unit discharge is more variable in old adults

The purpose of this study was to compare the steadiness and discharge rate of motor units during submaximal contractions performed by young and old adults. Subjects performed isometric and slow shortening and lengthening contractions with the first dorsal interosseous muscle. The steadiness of the isometric and slow anisometric contractions was less for the old subjects compared with young subjects, especially at the lower target forces and with the lightest loads. Furthermore, the steadiness of the lengthening contractions was less compared with the shortening contractions for the old subjects. Although the mean discharge rates of motor units were not different for the two groups of subjects, the variability of the discharge rates was greater for the old subjects during the isometric and anisometric contractions. We conclude that a more variable discharge by single motor units probably contributes to the reduced ability of old adults to perform steady muscle contractions.     

Time to task failure differs with load type when old adults perform a submaximal fatiguing contraction

Young adults exhibit a longer time to task failure when performing a submaximal isometric contraction by pushing against a force transducer (force task) than when supporting an equivalent inertial load (position task). The purpose of this study was to compare the time to failure for old adults when they performed a force task and a position task with the elbow flexor muscles. Eighteen old adults (72 +/- 4 years) performed the force and position tasks at 20% maximal voluntary contraction (MVC) force until task failure. The time to task failure was briefer for the position task (10.6 +/- 6.1 min) than the force task (22.8 +/- 9.1 min, P < 0.05). The rate of increase in electromyographic (EMG) bursting activity, ratings of perceived exertion, mean arterial pressure, heart rate, and fluctuations in motor output during the fatiguing contraction were greater for the position task. However, the increase in averaged EMG for the elbow flexor muscles was greater at termination of the force task. The difference in time to failure for the two tasks was due to a higher level of central neural activity during the position task and was similar to that observed for young adults. These findings indicate that the type of load supported influences the mechanisms and time to task failure for sustained contractions in old adults, and have implications for the design of tasks for rehabilitation and for tasks that minimize fatigue.     

The pain-induced decrease in low-threshold motor unit discharge rate is not associated with the amount of increase in spike-triggered average torque.

OBJECTIVE: Activation of nociceptive afferents decreases motor unit discharge rates in static contractions. There is also evidence that during experimental muscle pain the motor unit twitch force increases, which has been hypothesized to compensate for the decrease in discharge rate to maintain constant force. This study examined whether there is an association between the magnitude of change in motor unit discharge rate and the amount of increase in the spike-triggered average torque during experimental muscle pain. METHODS: Sixteen subjects performed three constant-torque isometric ankle dorsi-flexions at 10% of the maximal force (MVC) alternated with two contractions at constant discharge rate of a target motor unit, before and following injection of 0.5 ml of hypertonic (painful) or isotonic (control) saline into the tibialis anterior muscle. RESULTS: The discharge rate of the target unit at 10% MVC decreased following injection of hypertonic saline (P<0.05; mean+/-SD, before: 9.9+/-1.3 pulses per second, pps; after injection: 8.9+/-1.0 pps). The peak of the spike-triggered average torque increased with pain (P<0.05; before: 0.56+/-0.55 mNm; during pain: 0.95+/-1.02 mNm) but the increase was not correlated with the decrease in discharge rate (R=0.08). Propagation velocity and action potential peak-to-peak amplitude did not change with pain. CONCLUSIONS: The pain-induced modifications in the estimated motor unit twitch torque (1) were not caused by changes in muscle fiber action potential, and (2) were not associated with the decrease in discharge rate. SIGNIFICANCE: Maintenance of constant force during static painful contractions is not explained by a matching between changes in contractile and control motor unit properties.     

Practice and endpoint accuracy with the left and right hands of old adults: the right-hemisphere aging model

The purpose of the study was to quantify the aging-related differences in endpoint accuracy during isometric contractions of the left and right hands based on the prediction that declines in motor performance with aging may be greater for muscles controlled by the right hemisphere. Twelve young (6 men, 25 +/- 5 years) and 12 old (6 men, 76 +/- 6 years) adults performed a task that involved matching the peak of a force-time trajectory to a target. The old adults were less accurate than the young men and exhibited greater endpoint error with the left hand than the right hand on day 1, but not on days 2 and 3. Although electromyographic amplitude was similar between hands, old adults exhibited greater timing variability. These findings indicate that given sufficient practice there was no difference in endpoint accuracy between the left and right hands of old adults, which is not consistent with the prediction of an asymmetrical decline in motor performance by the right-hemisphere aging model. Conversely, an inability by an old adult to achieve similar accuracy with both hands during such tasks likely indicates an underlying motor impairment.     

Effect of muscle-fiber velocity recovery function on motor unit action potential properties in voluntary contractions

Measurements of muscle-fiber conduction velocity during voluntary contractions have been used in the diagnosis of neuromuscular diseases. However, the velocity of propagation of action potentials depends on the interspike interval of activation due to the velocity recovery function (VRF) of muscle fibers. The comparison of muscle-fiber conduction velocity estimates between individuals may thus be influenced by differences in motor unit discharge rate. This study investigates action potential properties of motor units of the sternocleidomastoid muscle during voluntary modulation of discharge rate with the purpose of assessing the effect of the VRF on motor unit properties in voluntary contractions. Nineteen healthy men trained to control a target motor unit with feedback of surface multichannel electromyographic (EMG) signals. The subjects performed three 30-s contractions of cervical flexion/rotation modulating the discharge rate of the target motor unit from 6.6 +/- 1.6 pps to 28.0 +/- 6.4 pps. Action potential conduction velocity was correlated to instantaneous discharge rate (R = 0.38 +/- 0.21). Action potential conduction velocity, peak-to-peak amplitude, and duration varied between minimum and maximum discharge rate (P < 0.01; percent change 12.3 +/- 5.0, -11.8 +/- 9.9, and -12.9 +/- 7.3). Thus, the properties of surface motor unit action potentials vary with modulation of discharge rate. This has implications for the use of conduction velocity values measured during voluntary contractions to differentiate patient populations from healthy individuals and for the development of normative data.     

Time-dependent structure in the discharge rate of human motor units.

OBJECTIVES: The aim of this study was to examine the influence of visual and motor processes on the deterministic and stochastic structure of force output and motor unit discharge variability. METHODS: Young adult subjects produced continuous, isometric force at 3, 6, 12, and 24% of their maximal voluntary contraction at low and high visual gain levels through abduction of the index finger. Force and fine-wire intramuscular electromyography were recorded. RESULTS: There was a linear increase in discharge irregularity with increases in the mean motor unit discharge rate (8-30 Hz). Recurrence analysis showed that the percentage of deterministic structure in discharge variability remained high, but decreased linearly with increased motor unit discharge rate. Surrogate analyses confirmed that the motor unit discharge variability was inconsistent with an uncorrelated and linearly correlated Gaussian noise process. Spectral analysis revealed that both the force output and the mean time-varying motor unit discharge time series had a dominant frequency of 0-2 Hz. Visual feedback gain did not affect the individual motor unit discharge patterns. CONCLUSIONS: The motor unit discharge rate has deterministic time-dependent structure. The motor unit discharge rate is modulated at multiple time scales likely by pre- and post-synaptic induced fluctuations from spinal level pathways impinging on the motor neuron.     

Muscle temperature has a different effect on force fluctuations in young and older women

ObjectiveTo investigate the effect of muscle temperature on force fluctuations during isometric contractions in young and older females.MethodsFifteen young and 11 older subjects performed 3 × 30-s long submaximal isometric ankle dorsi-flexions (5%, 10%, and 15% of the maximal force). Tibialis anterior muscle temperature was monitored with an intramuscular probe and manipulated to obtain a cold, control, and warm condition. The coefficient of variation (CofV) and the relative power in the frequency bands 0–3 Hz (low), 4–6 Hz (middle), and 8–12 Hz (high) of the force signal were computed to characterise steadiness. Intramuscular EMG signals were recorded from the tibialis anterior muscle to assess motor unit discharge pattern.ResultsCofV was higher in the older than in the young subjects (P < 0.001) in all conditions. In the older group only, CofV increased with cooling with respect to control temperature (P < 0.001), whereas in the young group only, relative power of force fluctuations at high frequency decreased with cooling. Motor unit discharge rate and inter-pulse interval variability were not different between groups and across temperatures.ConclusionsThe findings indicate a different effect of temperature on the ability to maintain constant force in young and older subjects.SignificanceThese results highlight the risk of further impairment to the motor control of older individuals with varying temperature.     

Dependence of the paired motor unit analysis on motor unit discharge characteristics in the human tibialis anterior muscle

The paired motor unit analysis provides in vivo estimates of the magnitude of persistent inward currents (PIC) in human motoneurons by quantifying changes in the firing rate (ΔF) of an earlier recruited (reference) motor unit at the time of recruitment and derecruitment of a later recruited (test) motor unit. This study assessed the variability of ΔF estimates, and quantified the dependence of ΔF on the discharge characteristics of the motor units selected for analysis. ΔF was calculated for 158 pairs of motor units recorded from nine healthy individuals during repeated submaximal contractions of the tibialis anterior muscle. The mean (SD) ΔF was 3.7 (2.5)pps (range -4.2 to 8.9 pps). The median absolute difference in ΔF for the same motor unit pair across trials was 1.8 pps, and the minimal detectable change in ΔF required to exceed measurement error was 4.8 pps. ΔF was positively related to the amount of discharge rate modulation in the reference motor unit (r2 = 0.335; P<0.001), and inversely related to the rate of increase in discharge rate (r2 = 0.125; P<0.001). A quadratic function provided the best fit for relations between ΔF and the time between recruitment of the reference and test motor units (r2 = 0.229, P<0.001), the duration of test motor unit activity (r2 = 0.110, P<0.001), and the recruitment threshold of the test motor unit (r2 = 0.237, P<0.001). Physiological and methodological contributions to the variability in ΔF estimates of PIC magnitude are discussed, and selection criteria to reduce these sources of variability are suggested for the paired motor unit analysis.     

Variation of amount of muscle discharges during ballistic isometric voluntary contraction in man

The effect of force velocity on the relation between voluntary force exertion and amount of muscle discharge was investigated. Surface e.m.g. from the adductor pollicis muscle and voluntary force curve were recorded simultaneously. Examined forces were selected from 500 to 3500 g at the peak force (about 30% of MVC) with time to peak of force curve between 60 ms and 500 ms. The amount of discharge during slow ramp contraction increased with the force increment. In ballistic contractions there was a wide variation in both the amount and rate of discharge. It is suggested that the modes of motor unit activities are different between force exertion with time to peak of less than 150 ms and force exertion with time to peak of over 150 ms.     

Age-related muscle fatigue after a low-force fatiguing contraction is explained by central fatigue

The contribution of central fatigue during and after low- and high-force isometric contractions sustained until failure with age is not established. We compared the time to failure and changes in voluntary activation measured using motor point stimulation of 15 young and 15 old adults for an isometric contraction sustained with the elbow flexor muscles at 20% and 80% of maximal voluntary contraction (MVC) force. Young adults had a briefer time to task failure than old adults for the 20% MVC fatiguing contraction, but a similar duration for the 80% task. Voluntary activation was reduced at the end of the 20% MVC task, but by greater magnitudes for old than young adults. The reduction in MVC torque after the low-force task was associated with the reduction in voluntary activation. After the 80% task, voluntary activation declined to similar levels for the young and old adults. Electromyographic activity levels (% MVC) of the biceps brachii and brachioradialis muscles during the fatiguing contraction were greater for the old than young for the 20% MVC task, but similar with age for the 80% MVC task. Our findings indicate that intensity and duration of contraction can be manipulated in young and old adults to induce varying magnitudes of fatigue within the central nervous system. Aging increases: (1) fatigue within the central nervous system immediately after a low-force fatiguing contraction, and (2) the potential for large neural adaptations during neuromuscular rehabilitation in old adults.     

Influence of contractile force on properties of motor unit action potentials: ADEMG analysis

We have used automatic decomposition electromyography (ADEMG) to measure the configurational and firing properties of 13,206 motor unit action potentials (MUAPs) in the brachial biceps, brachial triceps and anterior tibial muscles of 30 healthy adults (22 men, 8 women; mean age 48.6 +/- 16.9 years, range 20-76) at three levels of isometric contractile force: threshold, 10% of maximum voluntary contraction (MVC), and 30% MVC. In all muscles, the increment in contractile force from threshold to 10% MVC was associated with a significant (P less than 0.05, paired t-test) increase in mean MUAP firing rate and number of turns per MUAP. The increment from 10% to 30% MVC led to highly significant (P less than 0.005) increase in mean firing rate, number of turns, amplitude and rise rate. Each force increment was associated with an increase in the number of simultaneously-active MUAPs per recording site; and with a significant decrease in mean MUAP duration in all muscles, due to noise-dependency of the duration measurement. Quantitatively, the changes in MUAP properties with force were comparable to or exceeded the effects of age, gender differences, or intermuscular variability. Test-retest measurements 2 years apart in a subgroup of young adults showed good correspondence of mean MUAP properties with force standardization. These results demonstrate that contractile force is a major determinant of MUAP shape and behavior properties, and so must be precisely measured or controlled in clinical EMG studies.     

Motor unit physiology: some unresolved issues

The purpose of this review was to examine three issues that limit our understanding of motor unit physiology: (1) the range and distribution of the innervation ratios in a muscle; (2) the association between discharge rate and force; and (3) the variation in motor unit activity across contractions that differ in speed and type. We suggest that if more data were available on these issues, the understanding of neuromuscular function would be enhanced substantially, especially with regard to plasticity in the motor neuron pool, adequacy of the neural drive to muscle, and flexibility of activation patterns across various types of contractions. Current data are limited and these limitations influence our ability to interpret adaptations in muscle function in health and disease.     

Adaptations in maximal motor unit discharge rate to strength training in young and older adults

Six young (mean = 23 years) and 6 older (mean = 76 years) adults participated in isometric resistance training 5 days/week for 6 weeks. The task involved isometric fifth finger abduction. Maximal motor unit discharge rates (MUDRs) were obtained from the abductor digiti minimi of each hand at 0, 2, 14, and 42 days of training using a quadrifilar needle electrode and automatic spike recognition software. In agreement with previous findings, maximal MUDR at baseline was significantly lower in older adults (P < 0.001), averaging 51.5 (+/-17.13) HZ in young and 43.3 (+/-14.88) HZ in older adults. In response to resistance training, maximal voluntary force increased 25% in young and 33% in older subjects (P < 0.001). Maximal MUDR increased significantly (11% young, 23% older) on day 2 [F(3,36) = 2.58, P < 0.05], but in older subjects returned to baseline levels thereafter. These adaptations in abductor digiti minimi MUDR suggest a two-part response to strengthening fifth finger abduction: early disinhibition followed by altered MU activation.Copyright 2001 John Wiley & Sons, Inc.     

Low‐frequency component of rectified EMG is temporally correlated with force and instantaneous rate of force fluctuations during steady contractions

Introduction: The usefulness of surface EMG for assessing motor output variability during steady contraction is unclear. The purpose of this study was to examine the temporal correlation in signal characteristics between low‐frequency components of full‐wave rectified EMG (with or without high‐pass filtering before rectification) and force fluctuations or the instantaneous rate of force fluctuations (dF/dt) during steady contractions. Methods: Fourteen young adults produced steady force with the first dorsal interosseus muscle at various intensities. Cross‐correlation functions were calculated between the following signals: <5 Hz: force, dF/dt, and rectified EMG with or without high‐pass filtering at 300 Hz before rectification. Results: Rectified EMG correlated better with dF/dt compared with force itself, and high‐pass filtering of interference EMG before rectification improved the temporal correlation. Conclusions: The low‐frequency component of rectified EMG following high‐pass filtering may provide temporal information on the instantaneous rate of force fluctuations during steady contractions. Muscle Nerve 47: [?show $1534v]–[?show $1535v], 2013     

Age-related changes in fastest and slowest conducting axons of thenar motor units.

Single thenar motor unit F waves (FMUPs) were collected from 23 healthy volunteers (age range 21-91 years, mean 46 +/- 20 SD). In each subject, 10 distinct FMUPs were recorded, using surface stimulating and recording electrodes, and the conduction velocity (CV) of each motor unit was calculated. The distribution of CVs (overall range 42-66 m/s; individual FMUP CV dispersion range 6-27% of the maximal FMUP CV) was close to those previously reported whatever the technique used. With age, a progressive CV reduction was observed, and maximal FMUP CV was significantly correlated with age (r = -0.58, P < 0.01), whereas no statistically significant correlation was found between minimal FMUP CV and age (r = -0.27, ns). Individual FMUP CV dispersion presented a statistically significant decrease with age (r = -0.46, P < 0.05). Furthermore, thenar motor unit number (MUNE), estimated by the adapted multiple point stimulation method, decreased progressively with age and was statistically correlated with maximal FMUP CV (r = 0.59, P < 0.01), whereas there was no correlation with minimal FMUP CV (r = 0.34, ns). Thus, we propose that motor unit loss is progressive with age throughout life, affecting particularly the largest and fastest conducting motor units. Preferential involvement of these fibers could be responsible for the age-related changes in motor nerve CV.     

Properties of motor unit action potentials recorded with concentric and monopolar needle electrodes: ADEMG analysis

We compared the configurational and firing properties of 7270 motor unit action potentials (MUAPs) recorded with either concentric (CNE) or monopolar (MNE) needle electrodes from the brachial biceps and anterior tibial muscles of 10 healthy young adults (mean age 27 +/- 4.5 years) using automatic decomposition electromyography (ADEMG). In both muscles, mean MUAP amplitude, rise rate, and number of turns were significantly greater when recorded with MNE (paired t-test, P less than 0.001 in each case). Similar findings were observed at all three tested levels of isometric contractile force: threshold, 10% of maximum voluntary contraction (MVC), and 30% MVC. In contrast, there was no significant difference between electrode types on measurements of mean MUAP duration or firing rate (P greater than 0.05 in each case). These findings indicate that it is acceptable to generalize normative data on MUAP duration and firing rate from one electrode type to another, but that measures of MUAP amplitude and complexity require independent normative databases.     

Muscle-fiber conduction velocity estimated from surface EMG signals during explosive dynamic contractions

Muscle-fiber conduction velocity (CV) was estimated from surface electromyographic (EMG) signals during isometric contractions and during short (150-200 ms), explosive, dynamic exercises. Surface EMG signals were recorded with four linear adhesive arrays from the vastus lateralis and medialis muscles of 12 healthy subjects. Isometric contractions were at linearly increasing force from 0% to 100% of the maximum. The dynamic contractions consisted of explosive efforts of the lower limb on a sledge ergometer. For the explosive contractions, muscle-fiber CV was estimated in seven time-windows located along the ascending time interval of the force. There was a significant correlation between CV values during the isometric ramp and explosive contractions (R = 0.75). Moreover, CV estimates increased significantly from (mean +/- SD) 4.32 +/- 0.46 m/s to 4.97 +/- 0.45 m/s during the increasing-force explosive task. It was concluded that CV can be estimated reliably during dynamic tasks involving fast limb movements and that, in these contractions, it may provide important information on motor-unit control properties.     

Two-dimensional spatial distribution of surface mechanomyographical response to single motor unit activity

In order to better understand the mechanisms of generation of mechanomyography (MMG) signals, the two-dimensional distribution of surface MMG produced by the activity of single motor units was analyzed by a novel two-dimensional recording method. Motor unit action potentials were identified from intramuscular electromyographic (EMG) signals and used to trigger the averaging of MMG signals detected over the tibialis anterior muscle of 11 volunteers with a grid of 5x3 accelerometers (20-mm inter-accelerometer distance). The intramuscular wires were inserted between the first and second accelerometer in the middle column of the grid, proximal to the innervation zone. The subjects performed three contractions with visual feedback of the intramuscular EMG signals. In each contraction, a new motor unit was recruited at the minimum stable discharge rate (mean+/-S.D., N = 11 subjects, 7.3+/-2.3 pulse/s), resulting in torque of 2.4+/-2.8% of the maximal voluntary contraction (MVC), 4.6+/-2.7% MVC, and 6.3+/-3.1% MVC (all different, P < 0.01). For 23 out of 33 detected motor units, it was possible to extract the motor unit surface acceleration map (MUAM). A negative MUAM peak (-2.7+/-2.2 mm/s2) was detected laterally and a positive MUAM peak (4.1+/-2.4 mm/s2) medially (P < 0.001). The time-to-peak was shorter in the medial part of the muscle (2.9+/-0.4 ms) than in the other locations (3.4+/-0.5 ms, P < 0.001). The double integrated signals (muscle displacement) indicated negative deflection in the lateral part and inflation close to the tibia bone. The maps of acceleration showed spatial dependency in single motor unit MMG activities. The technique provides a new insight into motor unit contractile properties.