3A Jump Squat
Stand with your arms at your sides and your feet shoulder-width apart. Keeping your lower back in its natural arch, bend your hips and knees until your thighs are about parallel to the floor. Explode upward, jumping as high as you can. Land with soft knees and immediately begin the next rep. Perform reps for 20 seconds straight with a light load, and then rest 10 seconds. Then go on to the bicycle crunch.