Increases in motor unit action potential amplitudes are related to muscle hypertrophy following eight weeks of high‐intensity exercise training in females

Abstract

We examined the motor unit action potential amplitude versus recruitment threshold relationship (MUAP_AMP-RT) as an indicator of MU-specific hypertrophy following high-intensity exercise training in females. Participants were assigned to either a high-intensity exercise (EX, <i>n</i> = 9) or control (CON, <i>n</i> = 18) condition and completed pre- (PRE) and post-testing (POST) during which maximal voluntary isometric leg extension strength (MVIT), <i>vastus lateralis</i> (VL) muscle cross sectional area (mCSA), whole leg skeletal muscle mass (SMM_RL), and high-density surface EMG (HD-sEMG) signals were recorded from the VL during an isometric ramp contraction at 70% MVIT. The HD-sEMG signals were decomposed and yielded a MUAP_AMP and an absolute (ABS; Nm) and normalized (NORM; %MVIC) RT for each MU. Individual MUAP_AMP-RT slopes and intercepts were calculated for each subject. Changes in the pooled MUAP_AMP-RT relationships for each group were also examined. Finally, relationships among individual changes in slopes of MUAP_AMP-RT and individual changes in mCSA and SMM_RL were examined. Training elicited increases in MVIT (+18%), mCSA (+12%), and mean and pooled slopes of MUAP_AMP-RT_NORM. The individual changes in slopes of both the MUAP_AMP-RT relationships were moderately to strongly (<i>r</i> = 0.48-0.68) related to changes in mCSA and SMM_RL. Eight-weeks of high-intensity exercise elicited increases in MUAP_AMP-RT slope in females. Further, the observed change in slope was related to both VL mCSA and SMM of the tested leg. However, changes in slope for the MUAP_AMP-RT relationship were more subdued when MUAP_AMP was expressed relative to the absolute versus relative RT.