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.

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.

Just because your testosterone, growth hormone, and IGF-1 levels spike after a workout doesn't mean those spikes are what make your muscles grow.

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.

Even if your body releases a lot more of the 'muscle-building' hormones after a workout, it doesn’t mean you’ll grow more muscle than someone whose hormone levels stay low—studies show both groups end up with the same muscle size.

Even if your body releases a lot of muscle-building hormones after a workout, that doesn’t mean you’ll grow more muscle than someone whose hormone levels don’t spike as much—both people end up getting just as strong and muscular.