When men lift weights with dumbbells or barbells instead of using machines, their bodies produce more of the hormone testosterone right after the workout—so free weights might give a bigger hormonal boost.

During a tough leg workout, testosterone levels spike temporarily, but they go back to normal by the next day and don’t stay high.

Even after a super hard workout, the body’s baseline testosterone level doesn’t stay high the next day — it goes back to normal quickly.

Taking much more testosterone than your body naturally makes can make your muscles bigger and stronger—even if you don’t lift weights or do any exercise.

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 some guys get much bigger from weight training and others don’t, it’s not because the big responders have more of these male hormones inside their muscles — both groups have about the same levels, so something else must be causing the difference.