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Motor unit recruitment and derecruitment induced by brief increase in contraction amplitude of the human trapezius muscle
Author(s) -
Westad C.,
Westgaard R. H.,
Luca C. J.
Publication year - 2003
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2003.00645.x
Subject(s) - motor unit , motor unit recruitment , electromyography , contraction (grammar) , muscle contraction , muscle fatigue , electrophysiology , physical medicine and rehabilitation , medicine , psychology , anatomy , neuroscience
The activity pattern of low‐threshold human trapezius motor units was examined in response to brief, voluntary increases in contraction amplitude (‘EMG pulse’) superimposed on a constant contraction at 4–7% of the surface electromyographic (EMG) response at maximal voluntary contraction (4–7% EMG max ). EMG pulses at 15–20% EMG max were superimposed every minute on contractions of 5, 10, or 30 min duration. A quadrifilar fine‐wire electrode recorded single motor unit activity and a surface electrode recorded simultaneously the surface EMG signal. Low‐threshold motor units recruited at the start of the contraction were observed to stop firing while motor units of higher recruitment threshold stayed active. Derecruitment of a motor unit coincided with the end of an EMG pulse. The lowest‐threshold motor units showed only brief silent periods. Some motor units with recruitment threshold up to 5% EMG max higher than the constant contraction level were recruited during an EMG pulse and kept firing throughout the contraction. Following an EMG pulse, there was a marked reduction in motor unit firing rates upon return of the surface EMG signal to the constant contraction level, outlasting the EMG pulse by 4 s on average. The reduction in firing rates may serve as a trigger to induce derecruitment. We speculate that the silent periods following derecruitment may be due to deactivation of non‐inactivating inward current (‘plateau potentials’). The firing behaviour of trapezius motor units in these experiments may thus illustrate a mechanism and a control strategy to reduce fatigue of motor units with sustained activity patterns.