The cable machine has been the subject of more hilarious gym fail videos on YouTube than we can count. But the truth is, the cable machine is one of the most misunderstood pieces of equipment at the gym, and even people who believe they’re using it correctly may be making form mistakes that could make workouts less effective, or worse, lead to injury. 

Cable machine crunches are one exercise that Planet Fitness trainer Maria Almonte sees done wrong all the time. In this video, she shows you the right way (and the wrong way) to perform this ultra-effective ab burner. 

First, kneel underneath a machine with rope cables. Grab the rope handles and bring them to your forehead. Contract your abs and bend over until your chest meets your thighs. That’s one rep.

Here’s what not to do: Don’t hold the rope near your chest. Grip the rope at your forehead, and keep it there throughout the exercise. You also need to be sure you don’t arch your back. And finally, don’t pull your elbows all the way to the ground. They should not go past your knees during the crunch.