Begin at the end of your mat in plank position with your hands underneath your shoulders, body in one straight line.
Simultaneously cross your right hand over your left, as you step your left foot to the left. Then simultaneously step your left hand and right foot to the left, returning to plank position. Your hands move together as your feet step apart.
Repeat this for three steps to the left to complete one rep. Then reverse directions. Be sure not to let your pelvis lower or lift as you move side to side.
This full-body exercise is a great way to tone your deltoids (aka your shoulders) while firing up your core.