The influence of sex, body composition and training status on neuromuscular measures of the bicep brachii

Abstract

The purpose of the study was to determine how sex, body composition, and chronic resistance training influence corticospinal excitability (CSE) of the biceps brachii. Fifty participants including 15 male resistance trained (male RT), 15 non-resistance trained (male non-RT), 10 female RT, and 10 female non-RT participated in this study. Anthropometry, body composition, and biceps brachii subcutaneous fat and muscle thicknesses were recorded. Participants were then asked to performed maximum voluntary contractions (MVC) of the dominant elbow flexors (3×5s). Following the MVCs, peak twitch force (via motor point stimulation at rest) and maximal compound action potential (Mmax, via Erb's point stimulation) (during 10% MVC) were assessed. Next, participants received transcranial magnetic stimulation (TMS) to determine resting motor threshold (RMT) and active motor threshold (AMT) of the biceps brachii. After 3 min rest, participants performed the interpolated twitch technique (ITT) to determine voluntary activation (VA). Five minutes following the ITT participants performed 14, 5s contractions at 10% MVC. During the 10% MVC participants received TMS or an Erb's point stimulation at 2.5 seconds. A main effect of training demonstrated that RT participants exhibited significantly greater muscle thickness (28%), motor evoked potential (MEP) amplitude (81%), and Mmax amplitude (79%), alongside shorter half relaxation time (½RT) (19%) and MEP latency (9%) compared to non-RT participants (all p<0.05). RT participants also demonstrated lower body fat percentage (38%), fat mass (45%), and subcutaneous fat thickness (48%). A significant main effect of sex demonstrated that males exhibited greater muscle thickness (64%) but lower body fat percentage (24%) compared to females. Regression analyses revealed that training status was the strongest positive predictor of both MEP and Mmax amplitudes, while higher fat indices were negatively associated. Notably, muscle thickness increased MEP only in trained individuals, with effects differing by sex. This study highlights the sex-specific neuromuscular and corticospinal adaptations, with body composition playing a key role in modulating excitability. These findings highlight the importance of considering central and peripheral factors, and interpreting normalized excitability measures cautiously, when assessing training effects.