Single-leg work builds no more muscle than both legs together

Lifters and coaches argue endlessly about exercise selection, and one persistent claim is that unilateral exercises, meaning one limb at a time, are superior to bilateral exercises where both limbs work together. The argument matters because it shapes how people build programmes, from split squats versus back squats to single-arm versus two-arm pressing. Despite the strength of opinion on both sides, an evidence-based answer had not been established, particularly for muscle hypertrophy. This review set out to settle whether the choice actually changes how much muscle and strength people gain.

The authors systematically searched PubMed, Scopus and Web of Science for trials directly comparing unilateral and bilateral resistance training. From 703 studies retrieved, nine met the inclusion criteria and entered the meta-analyses. Risk of bias was judged using the Cochrane Risk of Bias 2 tool. The analyses used robust variance estimation with small-sample corrections, and the outcomes were muscle hypertrophy and gains in both bilateral and unilateral strength.

There was no significant difference in muscle hypertrophy between unilateral and bilateral training. Strength, however, followed the principle of specificity: bilateral training produced a superior increase in bilateral strength, while unilateral training produced a superior increase in unilateral strength. Both strength effects were statistically significant. The included studies overall presented a moderate risk of bias, which tempers confidence in the precise sizes of these effects.

This result pushes back against the idea that unilateral exercise is a superior muscle-building choice, an argument that had circulated ahead of the evidence. It also reinforces one of the oldest ideas in training science: strength gains are specific to how you train. The literature remains limited, with only nine qualifying trials, so questions stay open about whether the picture differs across muscle groups, training experience levels or longer interventions. For now, the burden of proof sits with anyone claiming a hypertrophy advantage for either approach.

If your goal is muscle size, this is a reason to think the unilateral versus bilateral debate matters far less than turning up and training hard, since neither approach showed an advantage. If your goal is a specific expression of strength, the evidence points to training that expression directly: two-limb lifts for two-limb strength, single-limb work for single-limb strength. Exercise selection can then come down to preference, equipment and comfort rather than a supposed hypertrophy secret.