After a single session of weight training, human muscle cells show a temporary increase in proteins that promote muscle growth and energy production, but do not show a change in a protein that limits muscle growth.

After resistance exercise, human muscle activates different versions of the IGF-1 and PGC-1α genes depending on whether the movement is concentric or eccentric, suggesting that mechanical stress triggers distinct molecular signals.

Lifting heavy weights (70% of your max) makes your muscles turn up the volume on certain genes that help them grow and turn down a gene that holds them back, more than lifting light weights (16% of your max) does after a single workout.

When you lift weights until your muscles burn and feel fatigued, the buildup of waste products might help your muscles grow bigger, but the main reason they grow is still from the physical force you're using.

When you lift weights, the pulling force on your muscles is what makes them grow bigger—not hormones or feeling tired. This force turns on internal signals that tell your muscles to make more protein and get larger over time.

When you lift weights, your muscles sense the tension and turn on internal signals that help them grow bigger—and they might even become better at using leucine, a protein-building block, to help with that growth.