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(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, n = 9) or control (CON, n = 18) condition and completed pre- (PRE) and post-testing (POST) during which maximal voluntary isometric leg extension strength (MVIT), vastus lateralis (VL) muscle cross sectional area (mCSA), whole leg skeletal muscle mass (SMMRL), 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 SMMRL were examined. Training elicited increases in MVIT (+18%), mCSA (+12%), and mean and pooled slopes of MUAP(AMP)-RTNORM. The individual changes in slopes of both the MUAP(AMP)-RT relationships were moderately to strongly (r = 0.48-0.68) related to changes in mCSA and SMMRL. 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.