When teenage rugby players train one leg at a time on the leg press machine, they get much stronger in that single leg than when they train both legs together at the same time.

Doing leg press exercises twice a week for five weeks makes teenage rugby players much stronger when pushing with both legs at once, whether they use one leg or both legs at a time.

Training one leg at a time makes that leg much stronger than training both legs together — even though both methods make you equally strong when using both legs at once.

Even though the players got stronger, their ability to jump higher or more powerfully didn’t improve after five weeks of leg press training — meaning strength on the machine doesn’t automatically make you jump better.

Even though the players got stronger from doing leg presses, they didn’t run any faster after five weeks — meaning just getting stronger on a machine doesn’t automatically make you sprint better.

If you only work out one leg with resistance training, that leg gets stronger, but the other leg doesn’t get stronger at all—even if you don’t train it.