After a tough workout, your body releases a chemical called IL-6, and people who gain more muscle tend to have more of it—but it’s probably just a side effect, not the reason your muscles grow.

In guys who are new to weightlifting, a spike in a certain body chemical (IL-6) right after a workout is linked to bigger muscles after 16 weeks of training—but that spike alone doesn’t fully explain why some guys get bigger than others, because other internal changes in the muscle also matter.

Just because your muscles hurt or swell after lifting weights doesn’t mean that’s what’s making them grow—you can still get stronger and bigger even if you don’t feel sore or inflamed.

Lifting heavy weights just a few times doesn’t stress your body as much right after as doing lots of lighter reps.

Doing many reps triggers a bigger stress and inflammation response in the body right after exercise than doing fewer heavy reps.

When young men lift weights, if a specific protein in their muscles gets activated strongly exactly 5 hours after working out, they tend to build more muscle over the next 4 months—so this protein spike might tell us who’s going to grow the most from training.