Your posture and core will thank you for breaking into this quick and easy workout. Lie on your stomach with your arms reaching overhead, pulling your abs away from the floor to protect and lengthen your lower back. Lift your legs, arms, and head off the mat. Keeping your knees straight, lift your entire right leg and left arm a few inches away from the floor. As you lower your right leg and left arm, lift your left leg and right arm away from the mat to complete one rep. Continue alternating sides in a slow controlled manner, keeping your torso stable, for a total of 20 reps.