When active young men do 50 knee extension exercises to exhaustion, their muscles get tired faster afterward—about 22% more tired—showing this workout really wears out their muscles.

For top runners, doing 50 knee exercises in a special machine doesn't make their leg muscles get tired in a way that changes how quickly they fatigue, so it doesn't really affect their muscle tiredness after the workout.

When resting 5 minutes between hard leg pushes, your muscles stay more 'turned on' than when you only rest 2 minutes.

When untrained guys do leg extensions with a medium weight until they can't do any more, moving faster (2 seconds per rep) makes their quads work harder for most of the movement — but moving slower (6 seconds per rep) makes them work harder right at the end. So, how fast you lift changes which part of the move your muscles feel most.

After doing hard leg pushes with 5-minute breaks, people’s muscles stay stronger and more activated afterward than after 2-minute breaks, meaning they don’t get as tired.

Doing a lot of reps with lighter weights right after each other makes your legs feel way more tired right after than doing fewer reps with heavier weights.