Small muscle mass exercise enhances muscular adaptations? Effects of unilateral and bilateral biceps curl on maximum strength and muscle size changes.

PURPOSE: We compared the effects of bilateral and unilateral biceps curl training on strength and muscle size changes.

METHODS

Untrained young women were randomly allocated to perform bilateral (B-BC, n = 28) or unilateral (U-BC, n = 26) biceps curl training. Participants performed 2 sets of 8-12 repetitions maximum (RM) per session, 3 d·wk-1, for 8 weeks. We measured the muscle thickness of the elbow flexors at 60% (EF60) and 70% (EF70) of arm length. We used the sum of the two-muscle thickness as a proxy for EF as a whole (Whole-EF). Strength was assessed using 1RM tests in the bilateral (1RM BIL) and unilateral (1RM UNI) biceps curl exercise.

RESULTS

There was no between-group differences on muscle size changes for EF60 [∆ -0.01 (-0.08, 0.05) cm], EF70 [∆ 0.03 (-0.04, 0.11) cm], and Whole-EF [∆ 0.02 (-0.10, 0.15) cm]. The 1RM BIL changes were not different between groups [∆ -0.28 (-1.23, 0.66) kg]. U-BC exhibited greater changes in 1RM UNI right arm [∆ 0.75 (0.22, 1.27) kg], but no between-group difference was observed in 1RM UNI left arm [∆ 0.43 (-0.24, 0.93) kg].

CONCLUSION

Our findings suggest that exercise selectionࣧ-whether bilateral or unilateralࣧ-influences maximum strength changes. On muscle size, we observed no evidence of differential muscle growth. Whether exercises that involve greater absolute difference in muscle recruited affect the magnitude of muscle size changes remains an open question and warrants further investigation.