"This move really challenges the core without hurting the spine, and also works the hips, glutes and upper body, giving you a lot of bang for your buck," says Matthews.
1. Start in a high plank position, with palms on the ground underneath the shoulders (not slightly out to the side, as in a push-up position), arms straight, toes tucked under, quads and abs engaged and head aligned with your spine. Bend your left knee and bring your left foot up toward the outside of your left hand. (Don't worry if you can only get it halfway up there.)
2. Step the left foot back to meet the right foot. You should be in plank position again.
3. Repeat the move with your right foot.