When female athletes don't eat enough to cover both their workouts and basic body needs, their muscles stop building protein as well, which could make them weaker or slower to recover.

Even if female athletes eat enough protein, not eating enough total calories can still stop their muscles from getting stronger or recovering properly during training.

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.

Taking a lot of creatine for 5 days doesn’t seem to boost muscle repair or growth right after a workout in young, healthy guys—even though it does increase creatine levels in their muscles. So, creatine might help muscles grow in other ways, not by speeding up protein building right after exercise.

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.

Even if your muscles are low on stored sugar (glycogen), your body still builds muscle just as well after lifting weights, as long as you eat enough protein.