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

The purpose of the study was to determine how sex, anthropometry, and chronic resistance training affect neuromuscular measures of the biceps brachii. Fifteen male resistance-trained (RT), 15 male nonresistance-trained (non-RT), 10 females RT, and 10 females non-RT individuals (<i>n</i> = 50) participated in this study. First, participants' anthropometry measurements were recorded. Second, participants performed isometric maximum voluntary contractions (MVCs) of the dominant elbow flexors and the interpolated twitch technique. Then, participants received transcranial magnetic stimulation (TMS) to determine resting and active motor threshold (during 10% MVC) to elicit a motor evoked potential (MEP) and Erb's point stimulation to determine maximal compound action potential (M_max) of the biceps brachii. Finally, participants performed 14, 5-s isometric contractions at 10% MVC and received TMS or an Erb's point stimulation. Males had greater muscle thickness (64%) but lower body fat percentage (24%) compared with females. RT participants exhibited significantly greater muscle thickness (28%), MEP (81%), and M_max amplitudes (79%), and shorter half relaxation time (19%) and MEP latency (9%) compared with non-RT participants (all <i>P</i> < 0.05). The MEP/M_max ratio did not differ between any of the groups. RT was the strongest positive predictor of both MEP and M_max amplitudes, whereas higher fat indices were negatively associated. Muscle thickness increased MEP only in trained individuals, with the association being stronger in males than in females. This study highlights training and sex-specific differences when assessing various central and peripheral neuromuscular measures that are due, in part, to morphological and physiological variances.<b>NEW & NOTEWORTHY</b> Here we show that training and sex alter central and peripheral neuromuscular output measures due to morphological and physiological differences and adaptations. These differences should be taken into consideration when designing studies to determine mechanisms underlying changes in neuromuscular output, especially when considering corticospinal excitability and whether or not it is normalized between and within participants.