The bicycle sit-up combines an isometric hold with a concentric contraction, stabilizing our body while maintaining our shoulders and thighs off the ground. The combination of both movements makes this a valuable and effective ab exercise that will give us a stronger midline. To do it, lie on your back, feet and thighs off the ground. Bring your hands to your ears and crunch your body so your left knee and right elbow meet each other above your bellybutton. Switch sides (right knee meets left elbow), alternating as quickly as you can.