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Load type influences motor unit recruitment in biceps brachii during a sustained contraction
Journal article   Open access   Peer reviewed

Load type influences motor unit recruitment in biceps brachii during a sustained contraction

Stéphane Baudry, Thorsten Rudroff, Lauren A Pierpoint and Roger M Enoka
Journal of neurophysiology, Vol.102(3), pp.1725-1735
09/2009
DOI: 10.1152/jn.00382.2009
PMCID: PMC2746780
PMID: 19625539
url
https://doi.org/10.1152/jn.00382.2009View
Published (Version of record) Open Access

Abstract

Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task (P=0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% (P=0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (-16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.
 Torque Humans Male Muscle, Skeletal - physiology Reaction Time - physiology Recruitment, Neurophysiological - physiology Weight-Bearing - physiology Action Potentials - physiology Young Adult Time Factors Muscle Contraction - physiology Adolescent Adult Elbow - physiology Female Electromyography - methods Task Performance and Analysis

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