The contractile properties of human motor units during voluntary isometric contractions

Abstract

SUMMARY1. The electrical activity of single motor units has been recorded from the first dorsal interosseus muscle of normal human subjects during voluntary, isometric contractions, together with the force generated by the muscle.2. By averaging the force correlated with the impulses from a single motor unit, the contraction time and twitch tension generated by that motor unit could be measured. When the rate of discharge was limited, either voluntarily or by automatic selection of intervals for analysis, the time for the tension to decline to half its maximum value (half-relaxation time) could also be measured for some motor units.3. Under our experimental conditions the trains of impulses from different motor units in most subjects were generated quite independently as tested by (a) measuring the correlation between activity 'in single units and that in the whole muscle as recorded by the surface electromyogram (e.m.g.), (b) measuring the cross-correlations between pairs of single units and (c) comparing the tension generated by stimulating single motor units with the average tension correlated in time with voluntary activity of single units in the same location.4. In one normal subject evidence of synchronization between separate motor units was obtained. Cross-correlation studies suggested that the cause of the synchronization was the presence of substantial common excitation received by the various motor units in the muscle.5. The frequency response for the contractions of single motor units was well fitted by that for a linear, second-order system with nearly critical damping. However, when stimulation of a few motor units was superimposed on a voluntary contraction, underdamped (oscillatory) responses were seen which were probably of reflex origin.

Control scheme governing concurrently active human motor units during voluntary contractions

“…3) point strongly to the fact that a muscle is incapable of generating purely isotonic contractions under isometric conditions. Small, low-frequency fluctuations in isometric force output mentioned by Milner-Brown, Stein & Yemm, (1973a) can now be seen as the manifestation of an underlying common drive to all concurrently active motor units. the purpose for this slowly fluctuating synaptic drive is not obvious, although an attractive idea emerges from work on the frequency response ofmotor-unit tension development (Stein, French, Mannard & Yemm, 1972).…”

Relation between location of a motor unit in the human biceps brachii and its critical firing levels for different tasks

“…Therefore, reliable twitch shapes could be determined only for MUs with large twitch amplitudes. By means of a microprocessor-based averager, twitches preceded or followed by another twitch of the same MU within 100 ms were excluded from averaging, thereby reducing contamination of the twitch shape and amplitude (26). medial recording site are recruited first, and reversal of the tasks (Fig.…”

Relationship between hamstring activation rate and heel contact velocity: Factors influencing age-related slip-induced falls

“…Also, many studies [7,9,[31][32][33][34][35] proposed that a possible mechanism in association with the decreased muscle activation rate among older adults was changes in proportions in muscle fiber types. Previously, an implication of the higher potential risk of slip-induced falls among older adults was presumed upon faster heel contact velocity found among older adults [14,22,25,36,37] while they were exhibiting slower walking velocity than younger adults.…”