Butt muscles & heavy breathing

Bigger muscles stimulate breathing. Although the body requires oxygen for metabolism, low oxygen levels do not stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide levels.

Much of the lactic acid formed as a result of anaerobic metabolism dissociates into carbon dioxide and water. Carbon dioxide stimulates respiration. Therefore, anaerobic exercise is NOT aerobic just because you're breathing hard.

The Gluteus Maximus is the largest muscle in the human body (with the exception of the uterus, which becomes larger in pregnant women). Therefore, the glutes are potentially responsible for creating the largest amount of lactic acid (or, in this case, ass-id) during exercise.

Furthermore, the bigger the butt, the bigger the burn!