When people do strength exercises like knee extensions, how hard they think they're working doesn't always match how hard their body is actually working. Even trained athletes often think they can't do more reps when they actually could.

People who lift weights for over a year often guess wrong about how many more reps they can do before getting tired—they usually think they can do about 2 fewer than they actually can.

If new guys do leg exercises with slow reps (6 seconds) instead of faster ones (2 seconds), they might get stronger in a specific knee position—even if they do the same number of sets and their muscles grow the same amount.

If untrained guys do leg exercises to failure using medium-heavy weights, going slow or going a bit faster on each rep gives about the same strength gains.

When doing intense leg exercises with 5-minute breaks between sets, people can do more total work than with only 2-minute breaks, because their muscles recover better.

Whether you lift heavy weights for a few reps or lighter weights for more reps until you can't do any more, both ways make your thigh muscles grow about the same amount in people who already work out.