This energy system fuels all of your activity known as Aerobic, Cardiorespiratory or Cardiovascular exercise.

Cardio training is typically long in duration, beyond 2 to 3 minutes of continuous activity, and usually targets creating, improving or maintaining endurance, leanness and heart muscle health.

After about 4 to 5 minutes of, for instance, jogging, oxygen consumption has stabilized and remains fairly constant throughout the duration of athlete's sustained run, unless intensity varies. Once this steady rate of oxygen consumption is reached exercise can continue for protracted periods of time, as in running a 5K race, competing in an Ironman Triathalon, or walking around the block. It's of course the least powerful of our energy systems, and utilizes oxygen to metabolize sugar and fat as fuel. (If exertion is long enough and intense enough protein will also be metabolized to fuel activity.)