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

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 (FRDEREC) relative to FR at recruitment (FRREC) vs. FRREC relationship for each subject. Correlations were performed between the y‐intercepts and slopes with type I %MHC. Results The majority of MUs had greater FRDEREC than FRREC. 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 FRDEREC than FRREC, however, individuals with higher type I %MHC had a greater propensity to have MUs with FRREC > FRDEREC 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.