Surprise: The triceps actually make up the bulk of your upper arm (no matter how many biceps curls you do).

“Triceps are the big, primary movers for upper-body ‘push’ motion, meaning extension of the arms,” says personal trainer Chris Clough, based in Baltimore and Washington, DC. “The most common mistake beginners make is ignoring their triceps.”

The second most common mistake? “Overworking your triceps—that is, not recognizing the work triceps do while you’re bench pressing.”

The solution? Do this beginner-focused triceps workout once or twice a week, but make sure you don’t do it the day after you focus on a chest-heavy workout.

Aim for four sets of 12–15 reps, with just 15 to 30 seconds of rest between sets. (Increasing muscle mass requires volume.) Use a load heavy enough so it’ll be challenging to complete your reps, but not so heavy that you can’t maintain good form.

For the sake of clarity: “Upper arm” refers to the part that’s closer to your shoulder.