Child-Adult Differences in Discrete Motor-Unit Activation: Insights from sEMG Decomposition

Abstract

The overall purpose of this thesis was to examine age-related differences in the discrete motor-unit (MU) activation of two muscles, differing by size and recruitment strategy (vastus lateralis (VL) and flexor carpi radialis (FCR)) as well as during contractions where torque was developed at low and high rates. These research objectives were assessed within one research project and reported in three studies. Study 1 (Chapter 3) examined differences in muscle performance and discrete MU activation of the VL (large muscle, broad RT range) between boys and men during moderate-intensity isometric contractions. Study 2 (Chapter 4) examined differences in muscle performance and discrete MU activation of the FCR (small muscle, narrow RT range) between boys and men during moderate-intensity isometric contractions. Lastly, study 3 (Chapter 5) examined differences in discrete MU activation between boys and men during moderate-intensity contractions performed at low and high contraction rates in both the VL and FCR. The collective findings of this work indicate that during moderate-intensity isometric contractions, boys activate smaller MUs that have lower MU firing rates (MUFR) compared with men. Although this general trend was observed in both muscles as well as during slow and fast contractions, the specific age-related differences in discrete MU activation patterns varied between muscles. In the VL, men recruited relatively larger high-threshold MUs compared with boys even when accounting for differences in muscle size. Moreover, lower MUFR in boys was present in low- and high-threshold MUs. On the other hand, in the FCR there were no differences in the relative size of high-threshold MUs between groups, but boys demonstrated a narrower recruitment range which may explain why lower MUFR in boys were especially pronounced among high-threshold MUs. Overall, lower MUFRs in boys compared with men can partly explain age-related differences in size-normalized strength. While our findings provide some support for the notion of lower type-II MU activation in children, other emergent differences in MU activation suggest that there are other explanatory factors contributing to differences in muscle performance between children and adults.