Training one arm with heavy eccentric curls makes the other arm stronger too—even though it didn’t lift any weights—by about 10%, while the trained arm gets even stronger, by nearly 20%.

When you train one arm, the other arm’s muscles start firing more easily and more intensely during contractions—even without being exercised—making it stronger through better nerve signals, not bigger muscles.

Doing bicep curls one arm at a time makes your right arm stronger than doing them with both arms together, but doesn't make your biceps bigger than using both arms.

Training one arm at a time doesn’t make you stronger when lifting with both arms together — your total strength gain is the same as training both arms at once.

Whether you lift weights with one arm or both at the same time, your biceps grow about the same amount after 8 weeks of training.

Training one arm doesn’t just make it stronger—it also makes the muscles fire more smoothly and steadily, even in the other arm, leading to more controlled movements.