Effects of Resistance Profile Manipulation During Elbow Flexion on Regional Muscle Thickness of the Biceps Brachii and Brachialis in Males

Abstract

The mechanical tension generated during resistance training can be modulated by manipulating the resistance profile (RP), potentially influencing regional muscle hypertrophy. Although this theoretical mechanism has gained scientific interest, evidence regarding its effects on regional muscle thickness remains limited. The present study aimed to analyze the impact of two distinct resistance profiles, ascending (PRASC) and descending (PRDES), on regional muscle thickness of the biceps brachii (BB) and brachialis (B) during elbow flexion exercises. Eight untrained males (age: 19.5 ± 1.12 years; height: 1.72 ± 0.02 m; body mass: 66.26 ± 6.48 kg) participated in a six-week training intervention, performing unilateral elbow flexion twice weekly using a Scott bench. Each resistance profile was randomly assigned to one arm. PRASC was characterized by lower torque in the elongated phase and higher torque in the shortened phase, while PRDES imposed higher torque in the elongated phase. Muscle thickness was measured using B-mode ultrasonography at 50%, 60%, and 70% of the distance between the acromion and olecranon, pre- and 72–96 hours post-intervention. No significant changes were observed for PRASC (p > 0.05). However, significant increases in muscle thickness were found for PRDES at 50% (p=0.017, d=1.097) and 60% (p=0.028, d=0.980), with no change at 70% (p > 0.05). No significant differences were found between profiles. Although differences between conditions were not statistically significant overall, the results suggest that higher torque in lengthened positions (PRDES) may promote localized hypertrophy. Further studies with larger samples are needed to confirm these findings.