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...

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.

When people do a special warm-up exercise, regular active folks get more tired, but top runners don't get any more tired—showing that your past exercise habits change how your muscles react.

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

When people do a special workout called pre-exhaustion, some muscles work harder right away, but the main muscle doesn't always get more active. Also, these quick changes don't tell us if the workout will build more muscle over time.

Doing intense drop-set workouts until your legs can't do another rep causes clear signs of muscle tiredness in beginners, based on electrical signals from the muscles — making it a solid way to push muscles hard during a workout.