The quads, calves and abs are amongst the strongest and largest muscle groups in the body. This makes them critical to posture, overall strength and, of course, athletic performance. Cardio Crunch doesn't just train them, it also loads your cardiovascular system and your lungs, expanding your VO2 range so that you can then also become capable of working out longer, without getting tired. Add EC (you know you really want to) and workout through the burn.
Extra Credit: 1 minute rest between sets.