When people who don't exercise much do a special warm-up called pre-exhaustion before rowing exercises, their shoulder muscle gets tired faster than if they didn't do the warm-up.

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

Doing a special warm-up before lifting weights lets you finish your workout faster while still building just as much muscle.

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 you tire out a muscle with one exercise before doing a bigger workout, you can't do as many reps in the big workout, so you end up doing less overall exercise than if you just did the big workout alone.

When people who don't exercise regularly get tired out before doing certain workouts, they can't do as many repeats afterward—about 25% fewer.