Unilateral vs Bilateral

What It Is

The choice between training one limb at a time or both at once.

Why It Matters

Whether you train one limb at a time (unilateral exercises like lunges, split squats, or single-arm presses) or both together (bilateral exercises like the squat or bench press) appears to matter less than how hard you train. The evidence is limited (mostly small and short studies), and what signal exists points to specificity: you tend to get strongest in the way you actually train. The more consistent finding is that two-limb training builds two-limb strength best, while the mirror effect for single-limb strength shows up in some studies but not others.

For muscle growth, no clear difference has been found between the two, though the data are too sparse and imprecise to call them genuinely equivalent. The specificity that does appear is thought to be mostly a matter of coordination (your nervous system learning a particular movement) rather than a change in the muscle itself. Correcting a side-to-side imbalance may not require single-limb work either: in one study of older women, bilateral exercises alone still narrowed the gap, as the weaker leg gained a little more.

In practice, build your program around the lifts you can load and progress most comfortably, since no muscle-growth difference has been shown between them. Reach for unilateral work when you specifically want single-limb strength or need to train around a sore joint. Rotating both over time trains strength across more of the movement patterns you rely on to stay strong and independent with age.

• Liao, K.F. et al. (2022). Effects of unilateral vs. bilateral resistance training interventions on measures of strength, jump, linear and change of direction speed: a systematic review and meta-analysis. Biology of Sport. doi.org/10.5114/biolsport.2022.107024
• Nunes, J.P. et al. (2022). Differential Responsiveness for Strength Gain Between Limbs After Resistance Training in Older Women: Impact on Interlimb Asymmetry Reduction. Journal of Strength and Conditioning Research. doi.org/10.1519/JSC.0000000000004240
• Kassiano, W. et al. (2025). Comparison of Muscle Growth and Dynamic Strength Adaptations Induced by Unilateral and Bilateral Resistance Training: A Systematic Review and Meta-analysis. Sports Medicine. doi.org/10.1007/s40279-024-02169-z