Effects of resistance profile manipulation during elbow flexion on regional muscle thickness of the biceps brachii and brachialis in males

Abstract

The level of muscle tension generated during a movement can be influenced by the applied resistance profile (RP). Muscle tension produces mechanical stress on muscle fibers, which, depending on its magnitude and duration, is considered one of the primary stimuli for muscle hypertrophy. In resistance training, modifying the resistance profile allows for targeted manipulation of mechanical demand across different joint angles, potentially influencing regional hypertrophy. Despite this theoretical relevance, the effects of resistance profile manipulation on regional muscle thickness remain poorly understood. Therefore, the present study aimed to investigate whether increased muscle tension at specific ranges of motion leads to greater regional hypertrophy. Specifically, the objective was to analyze the effects of two distinct resistance profiles, Ascending (ASCRP) and Descending (DESRP), during elbow flexion exercises on the muscle thickness (MT) of the biceps brachii (BB) and brachialis (B) muscles. Eight untrained male participants (age: 19.5 ± 1.12 years; height: 1.72 ± .02 m; body mass: 66.26 ± 6.48 kg; body mass index: 22.43 ± 2.11 kg/m²) underwent a six-week training intervention, with two sessions per week. Participants performed unilateral elbow flexion exercises using a Scott bench, with the resistance profile applied randomly to each arm. In one arm, the ascending profile (ASCRP) was used, characterized by lower torque in the elongated phase and higher torque in the shortened phase. In the contralateral arm, the descending profile (DESRP) was applied, involving higher torque in the elongated phase and lower torque in the shortened phase. Muscle thickness was assessed by B-mode ultrasonography at three points (50, 60, and 70% of the distance from the acromion to the olecranon), both pre-intervention (PRE) and 72 to 96 hours post-intervention (POST). No significant differences were observed between PRE and POST for ASCRP in any measured region (p > .05). However, significant increases in MT were found in DESRP at 50% (p = .017, d = -1.097) and 60% (p = .028, d = -.980), with no changes at 70% (p > .05). No significant inter-profile differences were observed (p > .05). In conclusion, while overall differences between resistance profiles were not statistically significant, DESRP elicited regional hypertrophy in the more proximal areas. These findings suggest that resistance profile manipulation may influence the regional adaptation of elbow flexors, although further research is required to confirm these trends in larger and more diverse populations.