It looks easy, but this one move works your legs, glutes, lower and upper back, shoulders, and hip flexors.
How to do it: Start with feet shoulder-width apart and a 45-degree bend in your knees. Extend one leg out to the side, keeping toes forward and knees bent the entire time. Bring your leg back in and repeat to the other side. Pump your arms as if you were skating and continue alternating legs until you complete 20 reps.