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.

Even if you've been lifting weights for a year, you might still not be good at guessing how many reps you have left in the tank, because people in a study kept thinking they could do more than they actually could.

When people do strength exercises like knee extensions, how hard they think they're working doesn't always match how hard their body is actually working. Even trained athletes often think they can't do more reps when they actually could.

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.

If untrained guys do leg exercises to failure using medium-heavy weights, going slow or going a bit faster on each rep gives about the same strength gains.

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.