Author(s): KOBAYASHI, Y., MAEO, S., ICHINOSE, H., NISHIZAWA, N., SHINZAKI, S., KANEHISA, H., ISAKA, T., Institution: RITSUMEIKAN UNIVERSITY, Country: JAPAN, Abstract-ID: 1866

INTRODUCTION:
Biarticular muscles exhibit greater hypertrophy when trained at longer muscle lengths. Previous studies comparing exercises performed at different joint positions, such as seated (hip-flexed) versus prone (hip-extended) leg curl for the hamstrings and standing (knee-extended) versus seated (knee-flexed) calf raise for the triceps surae, have reported greater hypertrophy when these biarticular muscles are trained at longer lengths (Maeo et al., 2021; Kinoshita et al., 2023). In these comparisons, joint position was manipulated to alter muscle length in biarticular muscles. Similarly, incline curls performed in a shoulder-extended position (50° of shoulder extension, placing the biceps brachii at a relatively longer length) elicit greater biceps brachii hypertrophy than preacher curls performed in a shoulder-flexed position (Kobayashi et al., 2024). However, it remains unclear whether training at even longer muscle lengths within the same exercise enhances hypertrophy. Therefore, this study examined whether performing cable incline curls at a greater shoulder extension angle (90° vs 50°) provides additional hypertrophic benefits to the elbow flexors, specifically the biarticular biceps brachii.
METHODS:
Nine untrained healthy young adults participated in this study. Using a cable machine, participants performed incline curls with one arm at 50° of shoulder extension and the contralateral arm at 90° of shoulder extension at 70% of the position-specific one-repetition maximum. Each arm completed 5 sets of 10 repetitions per session (2 s concentric and 2 s eccentric phases), twice weekly for 12 weeks. After each session, participants reported ratings of perceived exertion (RPE) for each condition using the Borg scale (i.e., 6–20). Before and after the intervention, T1-weighted axial 3-T magnetic resonance images were obtained to assess muscle volumes of the biceps brachii, brachialis, brachioradialis, and the whole elbow flexor muscle group.
RESULTS:
RPE did not differ between the two conditions across all training sessions (18.2 ± 1.1 vs 18.1 ± 1.1, P = 0.885). Muscle volume significantly increased in all three muscles and in the whole elbow flexors under both conditions. Whole elbow flexor volume increased similarly in the 50° and 90° shoulder extension conditions (+11.7% vs +11.8%, P = 0.991). No differences were observed between conditions for hypertrophy of the biceps brachii (+15.9% vs +16.3%, P = 0.831), brachialis (+9.8% vs +8.8%, P = 0.569), or brachioradialis (+9.1% vs +10.0%, P = 0.393).
CONCLUSION:
The present study indicates that performing cable incline curls at 90° of shoulder extension does not confer additional hypertrophic benefits to the biceps brachii and elbow flexors compared with the 50° condition.

REFERENCES:
Maeo et al. (2021) Med Sci Sports Exerc, 53(4), 825-37.
Kinoshita et al. (2023) Front Physiol, 14, 1272106.
Kobayashi et al. (2024) ECSS Glasgow.