The American Council on Exercise defines high-intensity interval training (HIIT) as a system of organizing cardio-respiratory training with repeated bouts of short duration and high-intensity exercise intervals intermingled with periods of lower intensity intervals of active recovery. When done right, you will leave feeling like you got a great workout — your muscles may feel fatigued and achey, but being in pain is another thing entirely.

HIIT workouts involve a lot of jumping and Kim Kelly, personal trainer and HIIT instructor, says that "can pose a problem specifically when people push up off the ground and when they land." And knees and joints take the brunt of the impact. “It's absolutely okay to push yourself in any class, especially a HIIT type of class, but you should know the difference between ‘good pain’ and ‘bad pain,’” she adds. The good pain is when you feel the burn — as in your muscles are working. Bad, or dangerous, pain is when you hear something snap, pop, crack or feel a sharp pain.

Knowing what not to do can help prevent improper form and lower your risk of injury. Here are a few common mistakes to be aware of — and how to fix them.

(As always, remember to always consult with a physician before starting a workout routine and if you feel any pain.)

Mistake #1: Skipping a Proper Warmup

When pressed for time, an instructor may skip or condense a warmup to get right to the meat of the class. But this is critical mistake: “A warmup is how your body prepares for what’s about to happen, so neglecting the necessary preparation your joints and muscles need in order to deliver power is only setting you up for possible pain in the near future,” says Nathalia Ferreira, STRONG by Zumba master instructor.

While we typically associate a warmup with prepping muscles, joints and tendons for movement, it also serves to prepare our brains, says Jeffrey Duarte, a performance specialist at NY Sports Science Lab. “Our brain’s number one priority is to make sure our body is safe. If we complete a warmup, our brain will recognize the movements and allow our body temperature to gradually increase to help our heart pump blood more efficiently to working muscles,” he says. “The warmup allows your muscles time to fill up with blood which will help decrease stiffness," Duarte explains.

Fix #1: Warm up while in class

Pay attention to how the class starts; If you jump right in to hardcore moves, take it slower and do some dynamic stretching.

If you need to slow down and walk yourself through a warmup, try this: Stand in a lunge position with your right foot forward, and stretch the left hip flexor for 2 seconds; then straighten the right leg and lean forward to stretch the hamstring for 2 seconds; switch from bent leg to straight leg a few times. Repeat this with the left leg forward. For your upper body, do some arm circles and rotate at your shoulder and wrists to loosen everything up. Then march in place and reach your arms up towards the ceiling 20 times.

Fix #2: Do your own warmup before class

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If you know that your HIIT class dives right in without dynamic stretching or a light cardio warmup, take matters into your own hands. Get to the class 5 minutes early, set up your place in the room and then head over to the treadmill or elliptical. Go at a moderate pace for 3 minutes and swing your arms forward and back to get some upper body mobility in while warming up your lower body.

Mistake #2: Landing Incorrectly

If you’re jumping, running in place or doing plyometric exercises, a common mistake is to keep your knees straight or even locked instead of softly bending your knees. This can wreak havoc not only on your knees, but also on your hip and ankle joints. Jumping up and then landing down puts a lot of pressure and strain on your joints, so it’s important to pay attention to your form especially if you’re moving through exercises quickly. “We naturally have these shock absorbers in our knees called menisci; their primary role is absorbing forces that we impose on our knees whether it’s walking, running or jumping,” Duarte explains. However, our muscles are meant to absorb some of that shock. He explains that if you land with your knees straight then all of that force will be placed on these menisci and none of that force will be controlled or mitigated through the muscles, which are significantly stronger than a piece of cartilage.

Fix #1: Land softly

Focus on keeping the knees slightly bent when performing movements like burpees and jump squats. “Landing softly with a soft bend in the knees will allow for the muscles located in your legs to activate and control the forces being placed [on the knees and joints],” Duarte says.

Landing softly with a soft bend in the knees will allow for the muscles located in your legs to activate and control the forces being placed [on the knees and joints].

Two ways to train yourself to land softly: Think about jumping up and down on a hard wood floor with someone sleeping below you — your goal is to try not to wake them up. Another tactic is to listen to the sound you make when you land: if you hear a loud “thud," chances are you’re placing too much force on the knee joint and not recruiting enough of your leg core muscles to soften the blow. Try to land without making a sound.

Fix #2: Modify your jumps

If you have a hard time eliminating that “thud”, try cutting your jump height in half. If you do jumping lunges, make the jumps lower to the ground so that you can keep your knees bent and control the movement more. If jumping is not an option, turn the jump into a reach on your toes. Still bending your knees as if you’re going to jump, simply stand up and press up onto your tip toes while reaching your arms up. That is your modified jump. For even more of a modification, bend your knees as if you’re going to jump, and then just stand up tall (without coming onto your tip toes) and reach your arms up overhead.

Mistake #3: Speed Breaks Proper Form

In HIIT workouts, one of the goals is to move quickly. Combine speed with fatigue and you may start to sacrifice proper form, which makes the likelihood of injury greater, says Monique Crous, founder of HOT HITT.

Plus, when your body is used to performing a certain exercise, you start to pay less attention to the mechanics of the movement. This leads to mistakes like squatting with your knees going too far past your toes, which can cause twinges and strains around the knee joint, and forgetting to engage your abdominals during the squat, which can put stress on your low back. Similarly, if your front knee goes too far forward in a lunge, you’re putting more stress on the knee joint and not getting the full benefit of the exercise. Instead of the quads and glutes doing the work, the knee joint over-performs.

Fix #1: Slow down

When the music is cranked up and your fellow classmates are going full speed ahead, it can be hard to slow down and focus on your form. However, at the start of each new circuit or exercise, commit to doing the first few reps slower than usual. Get the form right, and then turn up the speed. "Focus on moving from the core. If you are moving more mindfully, even if you have to slow down, you will avoid injury and see results much faster,” says Crous. Continue to remind yourself to pull your naval in towards your spine in all exercises throughout the class.

Fix #2: Check yourself out

Instructors can’t keep an eye on everyone’s form at all times, so watch yourself in the mirror to keep an eye on your own. “Sometimes people get in a group environment and feel the need to 'match' their neighbors’ abilities or push themselves too far,” says Kelly. This is problematic because everyone’s abilities and fitness levels are different. Plus, your neighbor may not be using correct form either, so copying them isn’t always the best bet.

If you’re really worried about intensity of a HIIT class, Crous recommends taking a low impact HIIT class, as opposed to a high impact one. This way you’ll avoid the chance of injury while you get used to the movements.

Remember to speak up if something feels too intense or if you’re unsure of how to modify exercises to suit your needs. “You can always ask your instructor for low impact modifications to high impact exercises,” Crous says.

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