Effects Of Resistance Training On Maximal Motor Unit Firing Rates In Young And Older Males

Abstract

It is unknown if resistance training (RT) has differential effects on maximum firing rates (MAXFR) of motor units (MUs) across the recruitment threshold (RECT), and whether or not age has an influence. PURPOSE: To examine the effects of short-term RT on MAXFR of MUs in young and older males. METHODS: Thirteen young and seventeen older males were randomly assigned to either a training (young group [YTG]: n = 8, age = 21 ± 1.6 yrs; old group [OTG]: n = 10, age = 64.1 ± 7.4 yrs) or control (young group [YCG]: n = 5, age = 22 ± 3.1 yrs; old group [OCG]: n = 7, age = 64 ± 9.3 yrs) group. RT involved knee extensions for 4 sets of 10 repetitions for 2 weeks. Before (PRE) and after (POST) RT subjects performed 2 maximum isometric ramp contractions (MVCs) of the knee extensors while 4 surface electromyography (sEMG) signals were recorded from the vastus lateralis. The raw sEMG signals were then decomposed into their constituent MU action potential trains. RECT, defined as the relative force level (%MVC) when the MU began firing, and MAXFR were calculated for each MU. The highest 500 ms average for torque was considered peak torque (PT). Linear regression was used on the pooled and individual data for the groups separately, to examine the relationship between RECT and MAXFR. Two-way (group [young vs. old] × time [PRE vs. POST]) repeated measures analyses of variance were used to compare individual slope coefficients (SLPc) and PT across time. RESULTS: A total of 1,403 (PRE = 713; POST = 690) MUs were detected. SLPc (p = 0.136) and PT (p = 0.781) remained unaltered in the control groups at POST. No significant group × time interactions were observed for SLPc (p = 0.678) or PT (p = 0.100), but a main effect for time was demonstrated for SLPc (-44.1%; p = 0.001) and PT (+12.5%; p = 0.001). CONCLUSION: These findings, in addition to a qualitative, visual inspection of the pooled regression lines (figure below), indicate RT induced a slightly more negative relationship between RECT and MAXFR and this effect was not influenced by age.Figure