Martin Bilodeau

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.

EMG frequency content changes with increasing force and during fatigue in the quadriceps femoris muscle of men and women

The purpose of this study was to determine the effect of gender on changes in electromyographic (EMG) signal characteristics of the quadriceps muscles with increasing force and with fatigue. A total of fourteen healthy adults (seven men, seven women) participated in the study. Subjects had to perform isometric ramp contractions in knee extension with the force gradually increasing from 0 to 100% of the maximal voluntary contraction (MVC) in a 6-s period. Subjects then performed a fatigue task, consisting of a sustained maximum isometric knee extension contraction held until force decreased below 50% of the pre-fatigue MVC. Subjects also performed a single ramp contraction immediately after the fatigue task. The Root Mean Square (RMS) amplitude, mean power frequency (MPF) and median frequency (MF) of EMG signals obtained from the vastus lateralis, vastus medialis and rectus femoris were calculated at nine different force levels from the ramp contractions (10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC), as well as every 5 s during the fatigue task. The main results were a more pronounced increase in EMG RMS amplitude for the three muscles and in MPF for the VL muscle with force in men compared with women. No significant effect of gender was found with regards to fatigue. These observations most likely reflect a moderately greater type II fiber content and/or area in the VL muscle of men compared to that of women.

Normality and stationarity of EMG signals of elbow flexor muscles during ramp and step isometric contractions

The purpose of this study was to test the stationarity and normality of electromyographic (EMG) signals obtained while exerting isometric contractions: (a) where a steady force level is maintained (step contractions); and (b) where the force level is increased linearly over time (ramp contractions). Ramp elbow flexions were performed from 0 to 100% of the maximum voluntary contraction (MVC) in a 5-s period. For the step contractions, four force levels (20, 40, 60 and 80% MVC) were maintained for a period of 3 s each. EMG signals of the biceps brachii (BB) and brachioradialis (BR) muscles of 16 subjects were recorded with surface electrodes and digitized at a sampling frequency of 2000 Hz. Tests of normality (Shapiro-Wilk test) and stationarity (reverse arrangement test) were performed locally on short finite time records (512-ms windows). Results show that, in general, EMG signals present a non-Gaussian amplitude distribution and are stationary. Furthermore, the amplitude distribution characteristics and the stationarity of the signal were not dependent on the muscle investigated, nor on the type of contraction or force level tested. The finding of local stationarity for both tasks is important, because it suggests that performing standard spectral analysis is applicable for both step and ramp contractions. It also allows a direct comparison between results obtained under both conditions.

STRENGTH TRAINING CAN IMPROVE STEADINESS IN PERSONS WITH ESSENTIAL TREMOR

We evaluated the effect of a strength‐training program on the ability of persons with essential tremor to exert steady forces with the index finger. Thirteen subjects with a diagnosis of essential tremor were assigned to three different groups: one group trained with heavy loads, one with light loads, and one did not perform any training. Subjects attempted to generate steady contractions during both postural and constant‐force tasks. Steadiness was quantified by the root mean square amplitude of acceleration during postural tasks and the standard deviation and coefficient of variation of force during the constant‐force tasks. Subjects who performed the training program with heavy loads experienced an increase in steadiness around the target force during the constant‐force tasks. Subjects in the other two groups did not exhibit any changes. These findings suggest that strength training can decrease the magnitude of tremor. However, we did not observe any associated improvements in functional abilities.

Task-dependent effect of limb immobilization on the fatigability of the elbow flexor muscles in humans

Because short‐term limb immobilization produces selective adaptations in the neuromuscular system that probably interact with the task‐dependent expression of muscle fatigue, the purpose of this study was to determine the effects of limb immobilization on the ability of human subjects to sustain isometric contractions at low and moderate submaximal forces. Four weeks of elbow joint immobilization caused a substantial decrease in the daily activity of biceps brachii during immobilization, a significant reduction in the cross‐sectional area and volume of the elbow flexor muscles as measured by magnetic resonance imaging, and a decline in the maximum voluntary contraction (MVC) activation and force of the elbow flexor muscles. Immobilization had a task‐dependent effect on muscle fatigue with a substantially increased endurance time (reduced fatigability) at a low force (20% MVC) and no statistical effect at a moderate force (65% MVC). Despite atrophy of the elbow flexor muscles due to the immobilization, the twitch force elicited in biceps brachii by electrical stimulation was greater after immobilization. The selective improvement of fatigue resistance for the low‐force contraction and the absence of a change in the time course of the twitch suggests that the immobilization‐induced adaptations included an improved efficacy of some excitation‐contraction processes and underscored the major role of these mechanisms in determining the endurance time for low‐force, long‐duration contractions.

Neuromuscular fatigue of elbow flexor muscles of dominant and non-dominant arms in healthy humans

The purpose of this study was to assess differences in fatigue-related changes in variables related to structures within the neuromuscular system, between the dominant and non-dominant elbow flexor muscles of right-handed individuals. Two experimental sessions were performed on the right arm and one on the left arm. For each session, maximum voluntary torque, level of voluntary activation, M-wave amplitude, twitch/train or twitch/doublet torque ratio and EMG median frequency were obtained before and up to 20 min after a sustained maximum isometric fatigue task. Our main results were: 1) reproducible fatigue-induced changes in all variables of interest between the two sessions performed with the right arm, 2) significantly greater failure in voluntary activation and neuromuscular propagation with sustained activity for the non-dominant compared with dominant side, and 3) no effect of dominance on MVC torque, endurance time, and fatigue-induced changes in EMG median frequency and elicited torques. These results suggest that the preferential use of elbow flexor muscles with the dominant arm leads to more fatigue resistance in certain structures/mechanisms of the neuromuscular system, but not in others.