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The change in motor unit firing rates at de‐recruitment relative to recruitment is correlated with type I myosin heavy chain isoform content of the vastus lateralis in vivo
Author(s) -
Herda T. J.,
Miller J. D.,
Trevino M. A.,
Mosier E. M.,
Gallagher P. M.,
Fry A. C.,
Vardiman J. P.
Publication year - 2016
Publication title -
acta physiologica
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.591
H-Index - 116
eISSN - 1748-1716
pISSN - 1748-1708
DOI - 10.1111/apha.12624
Subject(s) - myosin , isometric exercise , motor unit , vastus lateralis muscle , gene isoform , in vivo , chemistry , medicine , major histocompatibility complex , motor unit recruitment , pi , biology , endocrinology , anatomy , skeletal muscle , biophysics , electromyography , biochemistry , genetics , neuroscience , gene
Aim To investigate the change in motor unit ( MU ) firing rates ( FR ) at de‐recruitment relative to recruitment and the relation to % type I myosin heavy chain isoform content (type I % MHC ) of the vastus lateralis ( VL ) in vivo . Methods Ten subjects performed a 22‐s submaximal isometric trapezoid muscle action that included a linearly increasing, steady force at 50% maximal voluntary contraction, and linearly decreasing segments. Surface electromyographic signals were collected from the VL and were decomposed into constituent MU action potentials trains. A tissue sample from the VL was taken to calculate type I % MHC . The y‐intercepts and slopes were calculated for the changes (Δ) in FR at de‐recruitment ( FR DEREC ) relative to FR at recruitment ( FR REC ) vs. FR REC relationship for each subject. Correlations were performed between the y‐intercepts and slopes with type I % MHC . Results The majority of MU s had greater FR DEREC than FR REC . The y‐intercepts ( r = −0.600, P = 0.067) were not significantly correlated, but the slopes ( r = −0.793, P = 0.006) were significantly correlated with type I % MHC . Conclusion The majority of the motoneuron pool had greater FR DEREC than FR REC , however, individuals with higher type I % MHC had a greater propensity to have MU s with FR REC > FR DEREC as indicated by the slope values. Overall, the contractile properties of the muscle ( MHC ) could partially explain the differences in MU firing rates at de‐recruitment relative to recruitment. Thus, suggesting the fatigability of the muscle influences the alterations in MU firing rates from recruitment to de‐recruitment.