We’ve all been there before one way or another: You’re in class and the instructor calls for a series of push-ups and you just cannot muster the strength to complete them. Instead of doing mediocre push-ups you take them to your knees and complete the set.

While it’s good – and recommended – to make modifications when you need them, there are a couple of things we’re doing wrong when we take them – and the push-up is the most difficult exercise to modify.

Common exercise modification mistakes

We talked with Molly Key, general manager of Enrgi Fitness, to learn about what common mistakes we make when using modifications – like when we take push-ups down to our knees – and how we can fix them.

Not taking them when you need to

According to Key, the number one mistake she sees with modifications is that people just aren’t taking them when they need to. She suggests this may be because there is a little bit of a stigma surrounding the idea of modifications and that some people think it means you’re not working out as hard as you could or as the others around you. But this is totally not the case – you have to take care of your body – and one way to do so includes doing work outs that fit your physically ability.

“If you need a modification and you don’t do it, you could end up seriously injuring yourself,” says Key. “If your body doesn’t want you doing jump squats because your doctor says you can’t do any high impact activity, don’t do it. Just do an air squat!”

Relying on modifications

On the flip side of not taking modifications, another mistake Key sees is that some people get so comfortable with them they never take their moves to the next level or challenge themselves.

Modifications are great for injuries and if you’re trying a new workout, but if your body allows you to – and you’re comfortable with it – try the traditional move first and then if you need to, move on to the modifications.

Making the exercise easier

“When taking modifications, you have to make sure you’re not making the exercise any easier for yourself,” says Key.

You want to make sure you’re getting the same results you would if you were doing the exercise without modifications.

For example, if you can’t run and you need to walk on the treadmill instead, “walk at a steep incline so you get the same cardiovascular benefits as running,” says Key.

Not working the same muscles

If you’re doing modifications, make sure you’re working the same muscles the original exercise was supposed to work – not an entirely different set of muscles.

“When we do pull-ups in class, sometimes, people who can’t do pull-ups often try to do pushups instead,” says Key. But, this is a major mistake. “Although you work a few of the same muscles, the mechanics of both moves are totally different. There’s a reason that one is called a pull and the other is called a push.”

If you can’t do a pull-up – and if there are no assistant bands around—instead of doing push-ups, Key suggest grabbing a resistant band, putting it over the pull-up bar, sitting on the ground and slowly pull your arms down so you are mimicking the pull-up motion with your arms and back while keeping your body seated.

Forgetting they go both ways

It’s easy to forgot modifications can go both ways. If you’re finding an exercise is too easy, ask your instructor for a modification to make it harder. “If you are at the point where you can do box jumps for days, change the height of the box or throw a little weight on,” says Key. “The goal of modifications is to tailor the workout to what your body is able to do to help you get stronger.”

Not properly modifying a push-up

Push-ups are really hard, so there’s absolutely no shame (ever!) in modifying your push-up technique. However, if you take them from your knees, you’ll want to be extra careful. So, follow Key’s advice the next time you modify your push up:

“To get into the proper position, start in a plank, and then drop to your knees. From here, lift your ankles up to a 45-degree angle – make sure to keep your hips right where they are. Many people tend to lift their hips when taking pushups on their knees, which reduces the weight your chest and arms – the muscles you’re trying to work – hold.  Another issue is people tend to rest on their knee joint. You actually want the fleshy part right above your knee to be the part hitting the ground.”

Moving too fast

Another mistake Key often sees, especially if an original exercise called for weights, is moving too fast. For example, if you’re instructed to do a weighted squat, but are unable to do those, your modification will be an air squat. “Without the weight, though, people tend to go a little faster and don’t complete full reps, says Key. “Make sure you’re still getting nice and low and actively squeezing your glutes when you stand up. Sometimes taking an exercise slowly is more difficult that rushing through it!”