When guys who’ve never lifted weights before train their legs for 15 weeks, the more their muscles grow, the stronger they get—so muscle growth is mostly why they get stronger.

When guys who’ve never lifted weights before start training their legs for 15 weeks, the more their thigh muscles grow, the stronger they get—so muscle growth is probably why they’re getting stronger, not just better at lifting.

When untrained men get stronger in a static leg press, the main reason is that their thigh muscle near the hip gets bigger and its fibers angle more, which helps push harder.

If older people who didn’t get stronger from doing just one set of leg exercises start doing four sets instead, they’ll likely see bigger thigh muscles after 10 weeks of training twice a week.

Whether you take short or long breaks between leg exercises, as long as you do the same total number of reps, your inner and outer thigh muscles grow about the same.

If you're new to lifting and using light weights, you need to push your muscles until they can't do another rep to grow them bigger—otherwise, even doing more reps won't help.