Mountain climbers are a better ab workout than crunches — if you do them right. Here's how to nail your technique.

• Mountain climbers are an effective exercise for building core strength without a gym.
• Common mistakes, like caving the lower back or raising the hips, can make the exercise less effective.

Mountain climbers are a popular bodyweight exercise that can ramp up your heart rate and help build full-body strength without a gym.

Done correctly, mountain climbers and other plank variations can be more effective ab exercises than crunches, Miriam Fried, NYC-based personal trainer and founder of MF Strong, told Insider.

"What people don't realize is that you can't spot reduce fat, you can't do a ton of crunches and blast fat away," she said.

"Instead, shift from the mindset of 'I need to work for a flat belly', to thinking of the core as an important foundation for everything, and crunches aren't functional."

Mountain climbers can help you improve your core stability, which helps prevent injury and boost effectiveness in other kinds of exercise.

However, common mistakes, like unstable hips or misaligned arms, can make the exercise less effective. To get the most out of your mountain climbers, focus on a strong plank and slow, careful movement, and work your way up to longer or faster exercise.

Mountain climbers require a sturdy plank position

To do mountain climbers, you need to start in a plank position, with your hands on the floor under your shoulders, legs and back straight, and toes planted on the ground behind you.

The plank position is effective for building full body strength and stability by itself, and can be plenty challenging alone.

If you're still working on your plank, practice holding a plank for longer with scaled versions of the exercise such as elevating your hands on a box or chair, or dropping your knees to the floor.

Common mistakes include lifting the hips too high or sagging in the core

Once you've got the plank down, mountain climbers involve bringing one knee up toward your elbow and then returning it to the plank position behind you, alternating sides.

The movement can sometimes prompt people to get out of position with their plank, Fried said, losing some of the stability and effectiveness of the movement.

"What I often see is people's hips start rising up, or their lower back start caving in," she said.

Both can be signs that you're not engaging your core, using your abs muscle to resist gravity and the rotation of the movement. To fix it, squeeze your abs and glutes and hold your hips lower than your shoulders without dropping them toward the ground. Maintain a straight, diagonal line running down from your head to your heels.

Keep your hips square and avoid shifting them up and down or side to side to tap into your obliques (the ab muscles along the sides of your core).

Another common mistake is positioning your hands in front of your shoulders, instead of directly under them. As you get tired, you might be tempted to let your shoulders slump back behind your hands to relieve the weight of your body, but it can mess up your hip position. Instead, keep your wrists, elbows, and shoulders stacked the entire time.

If you're struggling to maintain good form during mountain climbers, Fried recommends moving more intentionally. Frantically driving the knees up or rushing the exercise can distract from holding a stable foundation, which is key to getting the most out of the movement.

"Slow it down so you can focus on maintaining the plank position," she said.

It can also be helpful to focus on shorter sets of the exercise, performing 20 to 30 seconds of mountain climbers with good form and working up to longer sets.