When people are lifting weights and getting really tired, they're pretty good at guessing how many more reps they can do if they're almost done—only off by about one rep. But if they could do a few more, their guess is less accurate, off by more than two reps.

People who lift weights for over a year often guess wrong about how many more reps they can do before getting tired—they usually think they can do about 2 fewer than they actually can.

Whether you're new to lifting weights or have been doing it for years, you're just as good at guessing how many more reps you can do before your muscles give out.

When people do more reps in a row during weight training, they become worse at guessing how many more reps they could do. This means longer sets make it harder to judge your own effort accurately.