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.