When rats do strength exercises for two weeks, their muscles don't get bigger or heavier, even though their muscle-building processes speed up. This might mean the muscles are just reorganizing themselves instead of growing in the short term.

When young men lift weights to build muscle, their muscle-building process speeds up at first but slows down after 10 weeks, even if they keep increasing the difficulty.

In rats, lifting weights for two weeks made their muscles build more protein even though the usual muscle-building signals in their bodies went down. This shows muscles can grow without needing those normal signals.

For men who exercise regularly, doing both weight training and intense cardio together doesn't slow down muscle building compared to just weight training alone, even after 10 weeks of working out.

After 10 weeks of working out, men who are already somewhat active don't build muscle as efficiently as before, and their bodies don't respond as strongly to exercise—this happens whether they're doing cardio or weight training.

Your muscles make more protein at first when you start lifting weights, but after a few weeks, that boost fades—even if you keep lifting harder.