Motor unit firing rates and synchronisation affect the fractal dimension of simulated surface electromyogram during isometric/isotonic contraction of vastus lateralis muscle

Motor unit firing rates in human muscle can be determined from recordings made with small-diameter microelectrodes inserted directly into the muscle during voluntary contraction. Frequently, these counts are pooled to give an average motor unit firing rate under a given set of conditions. Since the fibers of one motor unit are dispersed among the cells of several others, it is conceivable that discharge rates can be measured in more than one cell from the same unit. If this occurred frequently, the distribution of firing rates could be influenced by those from units counted more than once. Based on literature values, we made the following assumptions: vastus lateralis contains approximately 300 motor units, with an average innervation ratio of 1500. Muscle cell diameter is about 50 to 100 μm and cells are randomly distributed over a motor unit territory of 10 μm diameter. The recording range of a microelectrode is about 600 μm. Given the distribution of cells normally found in whole human muscle, the probability of recording from two or more cells of the same motor unit at 50% MVC follows a Poisson distribution with a mean of 0.44. This model suggests that although there is a low probability of some duplication in this technique, the extent to which it influences the distribution of average motor unit firing rates is minimal over the entire range of forces produced by vastus lateralis. Key words: probability, motor unit, single unit recording, human muscle, rate coding

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The compensatory interaction between motor unit firing behavior and muscle force during fatigue

Journal of Neurophysiology ◽

10.1152/jn.00347.2016 ◽

2016 ◽

Vol 116 (4) ◽

pp. 1579-1585 ◽

Cited By ~ 43

Author(s):

Paola Contessa ◽

Carlo J. De Luca ◽

Joshua C. Kline

Keyword(s):

Motor Unit ◽

Muscle Force ◽

Vastus Lateralis ◽

Motor Units ◽

Vastus Lateralis Muscle ◽

Clear Understanding ◽

Contraction Force ◽

Firing Behavior ◽

Motoneuron Pool ◽

Motor Unit Firing

Throughout the literature, different observations of motor unit firing behavior during muscle fatigue have been reported and explained with varieties of conjectures. The disagreement amongst previous studies has resulted, in part, from the limited number of available motor units and from the misleading practice of grouping motor unit data across different subjects, contractions, and force levels. To establish a more clear understanding of motor unit control during fatigue, we investigated the firing behavior of motor units from the vastus lateralis muscle of individual subjects during a fatigue protocol of repeated voluntary constant force isometric contractions. Surface electromyographic decomposition technology provided the firings of 1,890 motor unit firing trains. These data revealed that to sustain the contraction force as the muscle fatigued, the following occurred: 1) motor unit firing rates increased; 2) new motor units were recruited; and 3) motor unit recruitment thresholds decreased. Although the degree of these adaptations was subject specific, the behavior was consistent in all subjects. When we compared our empirical observations with those obtained from simulation, we found that the fatigue-induced changes in motor unit firing behavior can be explained by increasing excitation to the motoneuron pool that compensates for the fatigue-induced decrease in muscle force twitch reported in empirical studies. Yet, the fundamental motor unit control scheme remains invariant throughout the development of fatigue. These findings indicate that the central nervous system regulates motor unit firing behavior by adjusting the operating point of the excitation to the motoneuron pool to sustain the contraction force as the muscle fatigues.

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Non-invasive method to detect motor unit contractile properties and conduction velocity in human vastus lateralis muscle

Medical & Biological Engineering & Computing ◽

10.1007/bf02522514 ◽

1995 ◽

Vol 33 (4) ◽

pp. 558-562 ◽

Cited By ~ 1

Author(s):

H. Nishizono ◽

T. Fujimoto ◽

H. Kurata ◽

H. Shibayama

Keyword(s):

Motor Unit ◽

Conduction Velocity ◽

Vastus Lateralis ◽

Contractile Properties ◽

Vastus Lateralis Muscle ◽

Non Invasive ◽

Lateralis Muscle ◽

Invasive Method

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Non-Propagating Components of Surface Electromyogram Reflect Motor Unit Firing Rates

IEEE Access ◽

10.1109/access.2019.2931609 ◽

2019 ◽

Vol 7 ◽

pp. 106155-106161 ◽

Cited By ~ 1

Author(s):

Luca Mesin

Keyword(s):

Motor Unit ◽

Surface Electromyogram ◽

Firing Rates ◽

Motor Unit Firing

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Recruitment Order of Motor Units in Human Vastus Lateralis Muscle Is Maintained During Fatiguing Contractions

Journal of Neurophysiology ◽

10.1152/jn.00179.2003 ◽

2003 ◽

Vol 90 (5) ◽

pp. 2919-2927 ◽

Cited By ~ 105

Author(s):

Alexander Adam ◽

Carlo J. De Luca

Keyword(s):

Motor Unit ◽

Time Course ◽

Vastus Lateralis ◽

Motor Units ◽

Voluntary Contraction ◽

Vastus Lateralis Muscle ◽

Force Output ◽

Similar Time ◽

Lateralis Muscle ◽

Recruitment Order

Motor-unit firing patterns were studied in the vastus lateralis muscle of five healthy young men [21.4 ± 0.9 (SD) yr] during a series of isometric knee extensions performed to exhaustion. Each contraction was held at a constant torque level, set to 20% of the maximal voluntary contraction at the beginning of the experiment. Electromyographic signals, recorded via a quadrifilar fine wire electrode, were processed with the precision decomposition technique to identify the firing times of individual motor units. In repeat experiments, whole-muscle mechanical properties were measured during the fatigue protocol using electrical stimulation. The main findings were a monotonic decrease in the recruitment threshold of all motor units and the progressive recruitment of new units, all without a change of the recruitment order. Motor units from the same subject showed a similar time course of threshold decline, but this decline varied among subjects (mean threshold decrease ranged from 23 to 73%). The mean threshold decline was linearly correlated ( R2 ≥ 0.96) with a decline in the elicited peak tetanic torque. In summary, the maintenance of recruitment order during fatigue strongly supports the notion that the observed common recruitment adaptations were a direct consequence of an increased excitatory drive to the motor unit pool. It is suggested that the increased central drive was necessary to compensate for the loss in force output from motor units whose muscle fibers were actively contracting. We therefore conclude that the control scheme of motor-unit recruitment remains invariant during fatigue at least in relatively large muscles performing submaximal isometric contractions.

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