Motor unit discharge behavior in older adults during maximal-effort contractions

1995 ◽  
Vol 79 (6) ◽  
pp. 1908-1913 ◽  
Author(s):  
G. Kamen ◽  
S. V. Sison ◽  
C. C. Du ◽  
C. Patten
Keyword(s):  
Older Adults ◽  
Motor Unit ◽  
Discharge Rate ◽  
Force Production ◽  
Motor Units ◽  
Older Individuals ◽  
Unit Discharge ◽  
Maximal Force ◽  
Motor Unit Discharge ◽  
Common Observation

A reduction in maximal force production is a common observation in older individuals. In an effort to determine whether aging is accompanied by reductions in central motoneuron drive limiting motor performance, motor unit discharge records were obtained from seven young (21–33 yr) and seven older (> 67 yr) adults. Informed consent was obtained from all subjects. The task required the subject to perform a maximal abduction of the second digit under isometric conditions. Motor unit potentials in the first dorsal interosseous were monitored by using a selective four-wire needle electrode and identified off-line with the aid of a Dantec electromyograph. The maximal discharge rate in the older adults (31.1 impulses/s) was significantly smaller (P < 0.05) than that in the younger subjects (50.9 impulses/s). These findings suggest that reductions in maximal force capability in older adults are partially due to an impaired ability to fully drive the surviving motor units.

Fluctuations in Isometric Force Are Associated With Motor Unit Discharge Rate Variability in Older Adults

2004 ◽  
Vol 36 (Supplement) ◽  
pp. S123 ◽  
Author(s):  
Jennifer L. Stephenson ◽  
Katrina S. Maluf ◽  
Brian L. Tracy ◽  
Sandra K. Hunter ◽  
Roger M. Enoka
Keyword(s):  
Older Adults ◽  
Motor Unit ◽  
Isometric Force ◽  
Discharge Rate ◽  
Unit Discharge ◽  
Motor Unit Discharge

Motor-unit behavior in humans during fatiguing arm movements

1996 ◽  
Vol 75 (4) ◽  
pp. 1629-1636 ◽  
Author(s):  
K. J. Miller ◽  
S. J. Garland ◽  
T. Ivanova ◽  
T. Ohtsuki
Keyword(s):  
Motor Unit ◽  
Force Production ◽  
Motor Units ◽  
Isometric Contractions ◽  
Triceps Brachii ◽  
Unit Discharge ◽  
Recruitment Threshold ◽  
Motor Unit Discharge ◽  
Discharge Behavior ◽  
The Mean

1. The activity of 40 triceps brachii motor units was recorded from the dominant arms of 9 healthy adult volunteers (age 27.8 +/- 4.4 yr, mean +/- SD) during a fatigue task that included both isometric and anisometric contractions. The fatigue task lasted 8.3 min and consisted of 50 extension and 50 flexion movements of the elbow. Each movement (40 degrees in 0.8s) was separated by an isometric contraction. A constant load resisting extension of 17.7 +/- 3.0% of maximal voluntary contractions (MVC) was applied throughout the task. This paradigm enabled the direct contrast of motor-unit discharge behavior during the different types of fatiguing contractions. 2. Motor-unit behavior was examined to determine the relative contribution of two mechanisms for optimizing force production under fatiguing conditions: recruitment of motor units and modulation of motor-unit discharge following recruitment. Threshold torques for motor-unit recruitment thresholds were determined by ramp-and-hold isometric contractions. Motor-unit discharge was evaluated during the fatigue task by contrasting the number of motor-unit potentials (spikes) per contraction for concentric eccentric, and isometric contractions. 3. The fatigue task resulted in a 30 +/- 12% decline in the mean MVC of elbow extension. Recruitment of nine new motor units (23%) was evident during the fatiguing extension movements, often within five to seven movements (i.e., within 25-35 s). Each newly recruited motor unit had the largest recruitment threshold torque in that experiment. 4. Analysis of the motor units that were active from the beginning of the fatigue task revealed that the mean number of motor-unit spikes per contraction increased, or remained constant as fatigue ensued, yet for the majority of motor units it increased or remained constant. None of the newly recruited motor units demonstrated decreased number of mean spikes per contraction after recruitment. Further, concurrently active motor units displayed different discharge behavior in two-thirds of the subjects. It is proposed that if the neural drive to the muscle is distributed uniformly upon the motoneuron pool, peripheral feedback from the exercising muscle may modulate specific motoneuron discharge levels during fatigue.

scholarly journals Motor-Unit Synchronization Increases EMG Amplitude and Decreases Force Steadiness of Simulated Contractions

2000 ◽  
Vol 83 (1) ◽  
pp. 441-452 ◽  
Author(s):  
Wanxiang Yao ◽  
Rew J. Fuglevand ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Action Potentials ◽  
Isometric Force ◽  
Discharge Rate ◽  
Motor Units ◽  
Average Force ◽  
Unit Discharge ◽  
Motor Unit Synchronization ◽  
Motor Unit Discharge ◽  
High Level

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 ∼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.

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

2001 ◽  
Vol 24 (4) ◽  
pp. 542-550 ◽  
Author(s):  
Carolynn Patten ◽  
Gary Kamen ◽  
Daniel M. Rowland
Keyword(s):  
Older Adults ◽  
Strength Training ◽  
Motor Unit ◽  
Discharge Rate ◽  
Unit Discharge ◽  
Motor Unit Discharge

Why humans are stronger but not faster after isometric strength training: specific neural, not muscular, motor unit adaptations

2021 ◽  
Author(s):  
A. Del Vecchio ◽  
A. Casolo ◽  
J. Dideriksen ◽  
P. Aagaard ◽  
F. Felici ◽  
...  
Keyword(s):  
Strength Training ◽  
Motor Unit ◽  
Discharge Rate ◽  
Force Production ◽  
Population Data ◽  
Motor Units ◽  
Isometric Strength ◽  
Maximal Force ◽  
Contraction Speed ◽  
Rapid Generation

AbstractWhile maximal force increases following short-term isometric strength training, the rate of force development (RFD) may remain relatively unaffected. The underlying neural and muscular mechanisms during rapid contractions after strength training are largely unknown. Since strength training increases the neural drive to muscles, it may be hypothesized that there are distinct neural or muscular adaptations determining the change in RFD independently of an increase in maximal force. Therefore, we examined motor unit population data during the rapid generation of force before and after four weeks of strength training. We observed that strength training did not change the RFD because it did not influence the number of motor units recruited per second or their initial discharge rate during rapid contractions. While strength training did not change motoneuron behaviour in the force increase phase of rapid contractions, it increased the discharge rate of motoneurons (by ∼4 spikes/s) when reaching the plateau phase (∼150 ms) of the rapid contractions, determining an increase in maximal force production. Computer simulations with a motor unit model that included neural and muscular properties, closely matched the experimental observations and demonstrated that the lack of change in RFD following training is primarily mediated by an unchanged maximal recruitment speed of motoneurons. These results demonstrate that maximal force and contraction speed are determined by different adaptations in motoneuron behaviour following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.

scholarly journals Lack of increased rate of force development after strength training is explained by specific neural, not muscular, motor unit adaptations

2021 ◽  
Author(s):  
Alessandro Del Vecchio ◽  
Andrea Casolo ◽  
Jakob Lund Dideriksen ◽  
Per Aagaard ◽  
Francesco Felici ◽  
...  
Keyword(s):  
Strength Training ◽  
Motor Unit ◽  
Discharge Rate ◽  
Force Production ◽  
Population Data ◽  
Motor Units ◽  
Rate Of Force Development ◽  
Force Development ◽  
Maximal Force ◽  
Contraction Speed

While maximal force increases following short-term isometric strength training, the rate of force development (RFD) may remain relatively unaffected. The underlying neural and muscular mechanisms during rapid contractions after strength training are largely unknown. Since strength training increases the neural drive to muscles, it may be hypothesized that there are distinct neural or muscular adaptations determining the change in RFD independently of an increase in maximal force. Therefore, we examined motor unit population data acquired from surface electromyography during the rapid generation of force before and after four weeks of strength training. We observed that strength training did not change the RFD because it did not influence the number of motor units recruited per second or their initial discharge rate during rapid contractions. While strength training did not change motoneuron behaviour in the force increase phase of rapid contractions, it increased the discharge rate of motoneurons (by ~4 spikes/s) when reaching the plateau phase (~150 ms) of the rapid contractions, determining an increase in maximal force production. Computer simulations with a motor unit model that included neural and muscular properties, closely matched the experimental observations and demonstrated that the lack of change in RFD following training is primarily mediated by an unchanged maximal recruitment speed of motoneurons. These results demonstrate that maximal force and contraction speed are determined by different adaptations in motoneuron behaviour following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.

Fluctuations in Isometric Force Are Associated With Motor Unit Discharge Rate Variability in Older Adults

2004 ◽  
Vol 36 (Supplement) ◽  
pp. S123
Author(s):  
Jennifer L. Stephenson ◽  
Katrina S. Maluf ◽  
Brian L. Tracy ◽  
Sandra K. Hunter ◽  
Roger M. Enoka
Keyword(s):  
Older Adults ◽  
Motor Unit ◽  
Isometric Force ◽  
Discharge Rate ◽  
Unit Discharge ◽  
Motor Unit Discharge

scholarly journals Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle

2005 ◽  
Vol 99 (5) ◽  
pp. 1835-1842 ◽  
Author(s):  
Minoru Shinohara ◽  
Chet T. Moritz ◽  
Michael A. Pascoe ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Stretch Reflex ◽  
Discharge Rate ◽  
Motor Units ◽  
Muscle Vibration ◽  
Constant Force ◽  
Unit Discharge ◽  
Force Fluctuations ◽  
Motor Unit Discharge ◽  
Hand Muscle

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 ± 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials ( P < 0.01) and by 38% during the Recovery trials ( P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials ( P < 0.05) and by 28% during the Recovery trials ( P < 0.01). The discharge rate of motor units increased from 10.3 ± 2.7 pulses/s (pps) before vibration to 12.2 ± 3.1 pps ( P < 0.01) during the After trials and to 11.9 ± 2.6 pps during the Recovery trials ( P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.

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