Whether you lift weights with one arm or both at the same time, your biceps grow about the same amount after 8 weeks of training.

Scientific Claim

In untrained young women, 8 weeks of bilateral or unilateral biceps curl training results in no significant difference in muscle thickness gains of the elbow flexors at 60% or 70% of arm length, indicating similar hypertrophic responses regardless of exercise configuration.

Original Statement

“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 RCT design with precise ultrasound measurements and narrow confidence intervals around zero supports a definitive conclusion of no difference in muscle growth between conditions.

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

Both ways of doing bicep curls—using both arms at once or one arm at a time—led to the same amount of muscle growth in the arms after 8 weeks, so neither way is better for building bigger biceps.

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

Similar Assertions

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.

91%

Training one arm at a time doesn’t make you stronger when lifting with both arms together — your total strength gain is the same as training both arms at once.

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Whether you lift your arm weights slowly or with a little swing, your biceps and triceps both get noticeably bigger after eight weeks of training—your triceps grow even more than your biceps.

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Training your right arm alone makes your right arm stronger, but doesn’t help your left arm get stronger — the benefit stays on the side you trained.

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Doing bicep curls with one arm makes that arm stronger for one-arm curls, but doesn’t make you stronger for two-arm curls or the other arm.

78%