Motor unit fring rates during slow and fast contractions in boys and men

Abstract

Background Motor unit (MU) activation during maximal contractions is lower in children compared with adults. Among adults, discrete MU activation difers, depending on the rate of contraction. We investigated the efect of contraction rate on discrete MU activation in boys and men. Methods Following a habituation session, 14 boys and 20 men completed two experimental sessions for knee extension and wrist fexion, in random order. Maximal voluntary isometric torque (MVIC) was determined before completing trapezoidal isometric contractions (70%MVIC) at low (10%MVIC/s) and high (35%MVIC/s) contraction rates. Surface electromyography was captured from the vastus lateralis (VL) and fexor carpi radialis (FCR) and decomposed into individual MU action potential (MUAP) trains. Results In both groups and muscles, the initial MU fring rate (MUFR) was greater (p<0.05) at high compared with low contraction rates. The increase in initial MUFR at the fast contraction in the VL was greater in men than boys (p<0.05). Mean MUFR was signifcantly lower during fast contractions only in the FCR (p<0.05). In both groups and muscles, the rate of decay of MUFR with increasing MUAP amplitude was less steep (p<0.05) during fast compared with slow contractions. Conclusion In both groups and muscles, initial MUFRs, as well as MUFRs of large MUs were higher during fast compared with slow contractions. However, in the VL, the increase in initial MUFR was greater in men compared with boys. This suggests that in large muscles, men may rely more on increasing MUFR to generate torque at faster rates compared with boys.