Even when young men get stronger and their muscles grow bigger from lifting weights with one arm, their body’s hormone levels don’t go up or down — they stay the same.

Even if your body has more of these muscle-building hormones like testosterone and IGF-1, it doesn’t mean you’ll grow bigger muscles from lifting weights over 12 weeks.

After working out with weights, the levels of certain hormones in the blood don’t predict whether a guy will gain more or less muscle over 16 weeks — so those hormones probably aren’t the main reason some people grow bigger muscles than others.

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.

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 your testosterone, growth hormone, and IGF-1 levels spike after a workout, that doesn’t mean you’ll grow more muscle over time—those hormone spikes don’t seem to explain why some guys get bigger than others after 16 weeks of lifting weights.