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This exercise is also suitable for the strengthening of the deep back muscles thus it is specifically suitable for people doing desk jobs. By strengthening your back muscles you can correct your incorrect posture. Lie down in a prone position onto the ball, support yourself with your hands on the ground. Your head should be the extension of your torso, legs should be straight. Slowly lift upwards both of your legs, hold them for a few seconds then let them down. Repeat the exercise until you get exhausted.