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...

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 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 untrained men lift heavy weights, their muscles work harder and show more activity than when they lift lighter weights, no matter how many times they lift until they can't anymore.

Doing single-muscle exercises first makes you tired, so you can build muscle with lighter weights on bigger exercises.