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.

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.

Some people who get really strong from weight training might have special tiny molecules in their muscles that help boost a protein involved in muscle growth, but we’re not sure how or if this really works yet.

When muscles are strongly activated during physical exercise, the body produces more insulin-like growth factor 1 (IGF-1), a protein involved in tissue growth and repair.

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.

When you lift weights, your muscles temporarily stop building protein, but right after you finish, they kick into high gear to repair and grow—this happens because key signaling molecules in your muscles get activated during recovery.