The Growth Signal

Lifting weights challenges muscle fibers with mechanical tension. That tension is the main signal telling the muscle it needs to adapt by building more contractile protein.

Hard or unfamiliar training can also cause microscopic damage, which is why you may feel sore a day or two later. But soreness and damage are not the scoreboard. A well-designed workout can stimulate growth without leaving you wrecked.

This rebuilding is called muscle protein synthesis, and it doesn't fire instantly. It ramps up after you train and stays elevated for roughly a day or two while your body lays down new contractile proteins — the parts that actually pull when a muscle flexes.