When young men lift weights for 12 weeks, a small rise in their stress hormone (cortisol) right after working out seems to be linked to a little bit more muscle growth—even though we usually think stress hormones break down muscle.

When young guys burn more calories than they eat and work out hard, their stress hormone (cortisol) changes—and that’s linked to shifts in muscle and fat. But it doesn’t explain much about why people’s bodies change differently.

When young men do intense leg workouts, their body releases more lactate and cortisol—stress and energy chemicals—but that doesn’t mean they’ll grow bigger muscles or get stronger faster than if those chemicals stayed low.

If young guys lift weights for 12 weeks, their muscle strength gains in the leg press don’t seem to be linked to how much their hormones spike right after each workout.

When young men lift weights for 12 weeks, a small spike in a hormone called GH right after working out is slightly linked to their muscle fibers getting bigger — even though we don’t think GH directly makes muscles grow.

If you're a guy who lifts weights, doing more reps with lighter weights (which spikes your hormones more) doesn't build more muscle or strength over 8 weeks than lifting heavier weights with fewer reps—even though the hormone changes look different.