You’re proud of your plank. You admire your squat form. You know your bicep curls look good. But could you be doing these common exercises all wrong? It’s totally possible, says physical therapist David Reavy.

According to Reavy, 99% of the people he sees in his physical therapy clinic are doing these types of movements incorrectly. Oops. One reason people perform standard exercises wrong has to do with their musculature. For example, super strong quads can compromise your lunge form, driving your front knee too far forward.

Do you usually have a friend hold down your feet while you do sit-ups at the gym? That’s no good either. It can kill the activation in your abdominals, meaning you work less and miss out on results.

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So how do you correct your fitness faux-pas? The best way is to use props, like resistance bands, and really focus on proper form. Doing moves the right way will engage your muscles more efficiently, so you’ll build strength faster. Plus, you’ll also lower your risk of injury; if you’re not careful, the wrong move could put you out of the gym altogether.

In this video, Reavy shows Health’s senior fitness editor the exercises many people tend to mess up—including lunges, bicep curls, squats, and planks—and how to correct the most common mistakes. Follow along to figure out what moves you might be doing wrong, and learn how fixing them can help you get fitter faster.