You’ve heard the drumbeat so many times that you read right over the words: “Use good form, squeeze the muscle at the point of peak contraction, use a full range of motion.” Yeah, you already do all those things…or do you?

Arguably, at least 90% of the people I see at the gym make one crucial error on biceps curling movements even though they think they’re working their arms the best way possible. And what they don’t know they’re doing can be counterproductive to their goals of trying to build big biceps. Here’s why.

During many curling exercises, you start with your elbows by your sides, contracting your biceps to raise the weight. But if you’re going all the way up to the point where the bar is beneath your chin or at shoulder height, your front delts have assisted in the move, pulling your elbows forward.

Limit your cheating

“People tend to bring their elbows forward for two simple reasons,” says Ernesto Osorio, C.S.C.S., a personal trainer with One2One Fitness in Houston. “The first is trying to achieve a greater peak contraction of the biceps. The second reason is that of pride and ego—you can typically use more weight when you’re not isolating the target muscle, a nice way of saying you’re cheating. And while it’s okay to occasionally use cheating techniques to spark growth, doing so on every rep can hamper your progress.

“Bringing your elbows forward can increase the workload, but this brings in the front delts and increases tension on the biceps tendon,” he adds. “Ultimately, this could lead to impingement and cause undue wear and tear.” And Osorio points out that when you need to use your shoulders to assist in curling heavier weight, you tend to lean backward, which could cause low-back discomfort or pain.

When you allow your elbows to travel too far forward, the weight is directly over the elbows, meaning the bi’s don’t have to work as hard against gravity. You may think it looks good, but the overall effect on just the biceps is less than if your elbows were firmly planted by your sides. And for those looking for an incredible burn and pump, try this strict form for an entire set. You’ll be surprised how full your biceps get when you don’t let your shoulders become too heavily involved.

Osorio suggests that if you’re one of the 90% of those who want bigger guns but don’t pin your elbows at your sides as you curl, start by backing off on the weight you use. “Remember, the biceps is only a small muscle group with two heads, so the key is to work smart and efficient with various angles,” he says. Then, on occasion, allow your shoulders to assist in moving more weight than you could otherwise when using superstrict form.

And while it’s often easy to overlook training advice that seems redundant, always turning a single-joint move, such as a biceps curl, into a multi-joint exercise could be doing your physique more harm than good.