I've been warming up with high knees [1] for years: during sideline runs on my high school field hockey field, in sweaty dorm room HIIT workouts like Insanity, before runs or strength training sessions today. It's simple but effective, intense but not too hard on your body. And ever since those Insanity days, listening to Shaun T yelling to "engage your abs [2]!" with every step and drive, I've assumed high knees are great move for your abs, too.

Are High Knees a Good Ab Workout?

As it turns out, high knees might not actually be the core workout they're cracked up to be. "Doing high knees will work your abs, but probably not to the degree you are hoping for," said fitness coach Nick Leyden, MS, CSCS. High knees, he explained, are really meant to ramp up your heart rate and burn calories and fat. Your abs come into play, especially your lower abs and obliques. But, Nick explained, your hip flexors typically take over the exercise, becoming "the predominant and primary mover of our lower leg as we bring our knees up and down."

So should you still be sweating it out with high knees? Sure; Nick recommended using the move in HIIT workouts, as a high-intensity "finisher" set, or even in between strength training sets to rev up your heart rate. High knees are also helpful in a running warmup, he said, because the exercise wakes up your body and helps to develop a proper leg driving motion.

What Should I Do Instead of High Knees?

Want a move that's more core-effective than high knees and easier on your joints to boot? Nick gave us three.

• Slider mountain climbers: Get in a high plank position with your feet on sliders (towels or paper plates work too). Brace your core and keep your back flat while sliding your knees towards your chest one by one. Compared to high knees, Nick said, this move "stimulates and targets the lower abs more effectively and takes the demand off your hip flexors." Here's how to do sliding mountain climbers [3].
• Hanging toes to bar or hanging alternating knee raises: Hang from a pull-up bar with your arms straight and use your core to lift your toes to the bar. (See the move here [4].) If this is too tough, alternate driving your knees up to your chest instead. With this move, "try to keep yourself from swaying back and forth," Nick said.
• Farmer's march: Similar to a farmer's carry [5], hold a heavy dumbbell in each hand and march forwards, lifting each knee up to your chest. You'll be working to stabilise your core and driving your knee up as high as you can will also work your lower abs and external obliques, Nick told POPSUGAR.

High knees are great for getting your heart rate up and warming up for a run or workout, but ultimately, "high knees provide very little stimulus to your abs and a lot of impact on your joints," Nick said. Try one of his recommendations for a more effective core workout or peruse our 30 favourite ab exercises [6] for more ideas.