Side Plank with Rotation

Set up in a side plank, forearm on the floor under your shoulder and your body in a straight line from head to feet, hips lifted. Reach your top arm underneath your body, threading it through the gap between your torso and the floor, rotating your chest down and in. Then reverse the rotation, opening your chest back up and reaching that same arm toward the ceiling, following it with your eyes as your torso twists open. Your hips should stay lifted and level through the whole motion; if they drop every time you rotate, the plank itself needs work before the rotation does.