Doing bicep curls one arm at a time makes your right arm stronger than doing them with both arms together, but doesn't make your biceps bigger than using both arms.

Scientific Claim

In untrained young women, 8 weeks of unilateral biceps curl training increases 1-repetition maximum strength in the trained right arm by 0.75 kg more than bilateral biceps curl training, but produces no greater increase in muscle thickness of the elbow flexors.

Original Statement

“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]. 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].”

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

Evidence Quality Assessment

Claim Status

appropriately stated

Study Design Support

Design supports claim

Appropriate Language Strength

definitive

Can make definitive causal claims

Assessment Explanation

The study is a Level 1b RCT with randomization, control group, and direct measurement of outcomes. The effect size is precise and statistically significant for the right arm, justifying definitive language within the studied population and intervention.

Evidence from Studies

Supporting (1)

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

Randomized Controlled Trial

Human

2025 Dec 8

The study found that doing one-arm curls made the right arm 0.75 kg stronger than doing two-arm curls, but both types of training made the arm muscles grow the same amount.

Contradicting (0)

No contradicting evidence found

Source Study

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

Score: 61

DOI: 10.1007/s00421-025-06033-4

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