When people train one leg at a time, that leg gets much stronger than the other leg, even if the other leg didn’t do any training.

Doing leg exercises with both legs at the same time for 12 weeks makes you stronger at lifting with both legs together, while doing them one leg at a time makes you stronger at lifting with each leg separately.

Whether you train both legs together or one at a time, your thigh muscles grow about the same amount after 12 weeks of heavy lifting.

For exercises that move just one joint—like bicep curls—lifting heavier weights leads to better strength gains than lifting lighter weights, even if both are done until exhaustion.

If you lift heavy weights (85% of your max) instead of light ones (30% of your max) for nine weeks, you’ll get stronger on exercises like squats and deadlifts—even if your muscles don’t grow any bigger than if you’d lifted light weights.

Working out one arm can make the other arm stronger and more steady, even if you didn't touch it—your brain is learning to control it better.