Increases in Motor Unit Action Potential Amplitudes Are Related to Muscle Hypertrophy Following Eight Weeks of High-intensity Exercise Training in Females

We examined the motor unit action potential amplitude versus recruitment threshold relationship (MUAP-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,  = 9) or control (CON,  = 18) condition and completed pre- (PRE) and post-testing (POST) during which maximal voluntary isometric leg extension strength (MVIT), (VL) muscle cross sectional area (mCSA), whole leg skeletal muscle mass (SMM), 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 and an absolute (ABS; Nm) and normalized (NORM; %MVIC) RT for each MU. Individual MUAP-RT slopes and intercepts were calculated for each subject. Changes in the pooled MUAP-RT relationships for each group were also examined. Finally, relationships among individual changes in slopes of MUAP-RT and individual changes in mCSA and SMM were examined. Training elicited increases in MVIT (+18%), mCSA (+12%), and mean and pooled slopes of MUAP-RT. The individual changes in slopes of both the MUAP-RT relationships were moderately to strongly ( = 0.48-0.68) related to changes in mCSA and SMM. Eight-weeks of high-intensity exercise elicited increases in MUAP-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-RT relationship were more subdued when MUAP was expressed relative to the absolute versus relative RT.