When men who lift weights start training, doing more reps with lighter weights causes a bigger short-term boost in certain hormones compared to lifting heavy weights for fewer reps. But this hormone boost gets smaller the longer they keep training.

When young men lift weights, their hormone levels spike right after, but those spikes don’t tell us who’s going to get stronger or bigger — some people grow a lot even with small hormone changes, so something else must be behind why people respond differently.

Even if you lift heavy or light weights for 12 weeks, your body’s short-term hormone spike after each workout doesn’t change—and that means those hormone surges probably aren’t why your muscles get bigger or stronger over time.

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.

When young men lift weights regularly for 15 weeks, their body’s immediate hormone spike after each workout stays about the same—no matter how many times they do it. It’s like their body keeps reacting the same way every time, without getting used to it.

Even if your hormones spike a lot right after your first few workouts, that doesn’t mean you’ll grow more muscle or get stronger later on.