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There are planks, then there are planks. This multi-tasking version works your core, chest, glutes, outer thighs and shoulders all at once, and three sets of 10 on each side will toast even the most hardcore planker!
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Setup: Kneel in front of a Swiss ball and place your hands about shoulder-width apart on the floor. Extend your legs one at a time behind you to rest on top of the ball. Lift your hips so they are in line with your heels and head.
Move: Slowly take one leg off the ball and lower it down and to the side to briefly touch the floor. Bring it back to the start and repeat on the other side. Continue, alternating sides, for reps.
Tip: Press down into the ball with your remaining leg and squeeze your glutes to stabilize yourself while your other leg is moving.