Doing a tough leg workout with lots of sets makes your leg muscles start building new proteins much faster—2 to 3 times more than usual—right after the workout.

When you do strength training, your body gets better at building the proteins that make muscles strong, while cutting back on making other less important proteins. It's like your body learns to focus on what really matters for muscle power.

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.

Lifting weights at a challenging level makes your muscles build more protein, and this boost happens just as much in arms or legs that are used to exercising as in ones that aren't.

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.

People respond to weight training in wildly different ways—some grow muscles a lot, others hardly at all—and those differences between people are about 40 times bigger than the tiny differences you’d see if you trained one leg harder than the other in the same person.