Training one arm at a time with biceps curls leads to the same increase in strength in both arms as training both arms together, in women who have not previously trained.

We analyzed one assertion on this question and found that training one arm at a time with biceps curls leads to the same increase in strength in both arms as training both arms together in untrained young women [1]. This single piece of evidence suggests that unilateral training — working one arm at a time — may produce similar strength gains in the trained and untrained arm as bilateral training — working both arms together. The evidence we’ve reviewed so far does not show any studies contradicting this finding. However, we only have one assertion to work with, and it does not include details like sample size, training duration, or how strength was measured. Because of this, we cannot say whether this result holds across different conditions or populations. In everyday terms, this means that if you’re new to lifting and only do biceps curls with one arm, you might still get stronger in the other arm — even if you never directly train it. This could be due to how the nervous system adapts when one side is worked, but we don’t have enough data to explain why. We don’t yet know if this applies to men, older adults, or people with training experience. More studies are needed to see if this pattern holds under different conditions. For now, if you’re an untrained young woman and prefer training one arm at a time — perhaps due to injury, equipment limits, or personal preference — the evidence we’ve seen so far doesn’t suggest you’re missing out on strength gains in the other arm.