Core and upper body. Start in a push-up position, shoulders above your wrists (drop to your knees if needed). Squeeze your glutes, tighten your core and keep your body as straight as possible as you hold a plank. For advanced, add a mountain climber by pulling one knee at a time up to your chest. Rotate to a left-side plank, extend your top arm and hold. If you can, raise and lower the top leg. Return to the centre, rotate to a right-side plank and hold.
Hold each plank for 30-60 seconds.