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.

Whether you train one leg or both legs at once for 12 weeks, your legs get just as big and strong overall — the way you do the exercise doesn’t change how much muscle you build or how strong you become in general.

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.

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.

Even though people who trained one leg at a time ended up lifting heavier weights than those who trained both legs together, both groups ended up with the same muscle size and overall strength — so lifting heavier with one leg doesn’t automatically make you stronger overall.

When older women train by pushing or pulling with both arms or legs at the same time, they get better at using both sides together, making their strength more efficient.