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Reps: 30 seconds each side
Lie on your left side, resting your weight on your left forearm and placing your right hand on your uppermost thigh. With your right foot resting on top of your left, contract your abs and raise your hips so that your body forms a straight line from your ankles to your neck. Hold for 30 seconds, then swap sides.
It's working when your obliques are taut and your core and shoulders are working overtime to maintain balance.