If you train one leg at a time, that leg gets much stronger than the other when tested alone — but if you train both legs together, both legs get stronger at the same rate, with no one leg getting ahead.

Training one leg at a time makes it harder for both legs to work together at full power, while training both legs together makes them work better together — the way you train changes how your legs coordinate.

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.

Training one arm or leg at a time helps older women build muscle just as well as training both at once—but it doesn’t help them use both sides together any better.

Doing leg extensions one leg at a time for 12 weeks makes each leg stronger and more active when tested alone, better than doing both legs together — even though both methods improve overall leg strength similarly.

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.