Stand tall with your back straight, abs engaged, shoulders relaxed, legs together, and arms at your sides. Bend forward to touch the floor (bend knees if necessary), shift your weight into your hands, then jump or walk your legs behind you to start in a plank position (hands under shoulders, abs pulled in tight, body in a straight line).
Breathe deeply, keeping your back straight and abs engaged as "march" by bringing one knee toward your chest, return it to the floor, and then bring the opposite knee toward your chest. Repeat, continuously switching sides.
Master the Plank exercise before attempting this one. Plank Jumping Jacks are an advanced exercise that require core strength, shoulder stability and muscular endurance. Practice caution with this exercise if you have problems in those areas. Keep your belly pulled in tight at all times and make sure your hips do not fall toward the floor or lift toward the ceiling as you move your legs. You can march in this position (pictured) for a lighter intensity or move more quickly into a jog or run to increase your intensity.