When you lift heavy weights, your muscles get more efficient at using less energy for the same effort, while lifting lighter weights doesn't change much. It's like upgrading your muscles to work...

When untrained young men lift heavy weights (80% of their max), their muscles and nerves become more efficient at lighter tasks, needing less effort over time. Lifting lighter weights (30% of max) doesn't give this same benefit.

Lifting heavier weights trains your brain and nerves better than lifting lighter weights, helping you get stronger even if your muscles don't grow bigger.

When you lift heavy weights, your muscles get faster at producing force quickly, but lifting lighter weights doesn't help much with this. It means your body responds better to heavier loads for getting stronger faster.

When you lift heavy weights, your muscles get stronger partly because your brain learns to control them better, not just because the muscles themselves grow bigger. Lighter weights don't have the same effect.

Lifting heavy weights until you can't lift anymore makes young male athletes much stronger than lifting lighter weights over 8 weeks, showing that heavier loads train muscles and nerves better.