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.

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.

Doing a pullover exercise before a rowing exercise doesn't make your back muscles work harder than if you just did the rowing exercise by itself, at least for people who aren't trained.