Jeremy Frisch, owner and director of Achieve Performance Training in Clinton, Massachusetts, trains everyone from college football players to middle schoolers. (Want to train like a professional athlete? Then check out Warrior Cardio to build power, speed, and agility.)

As you'd expect, the college athletes are much, much stronger than the kids. But there's one exercise that the kids can do no problem, but the athletes struggle with: The crab foot reach. Watch the video below to see how it's done:

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"When I try to do it, it's so tough," Frisch admits. "But my 5-year-old son can do it easy. He’s so mobile and he has a free range of movement."

Most adults struggle with hip mobility and glute strength. Blame sitting all day, which leaves your hips tight and your glutes weaker than a frat party keg beer. The crab foot reach targets both, which is why it's such a challenge -- and why it's so important to do, since those weak points can limit your deadlift and squat.

If you can't do the crab foot reach as shown in the video, start by sitting on the ground with your knees bent, your feet flat on the floor. Place your hands on the ground behind you. Grab your left foot with your right hand. While still holding your foot, raise your butt off the floor.

Think this exercise is challenging? Here's one that will annihilate your core: The Crunch That Works Every Muscle.