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Assume a plank position with your hands on a stability ball, about shoulder-width apart and palms aligned under your delts, legs together with your toes on the floor. Your body should be straight from head to heel. Hold this position, flexing your core to keep your hips from collapsing, for 30 seconds. Once you master the move, start working toward 60-second holds, increasing in five- to 10-second increments over time.