There are three primary energy systems in the human body: the ATP-PC system, the glycolytic system, and the aerobic system. Each system produces energy to fuel different types and durations of physical activity.

  1. ATP-PC system (Phosphagen system): This system uses stored creatine phosphate (CP) to quickly produce ATP (adenosine triphosphate), the primary source of energy for muscle contraction. This system is used during high-intensity, short-duration activities, such as weightlifting or sprinting. The ATP-PC system can provide energy for up to 10-15 seconds of activity before needing to be replenished.

  2. Glycolytic System: This system is used for moderate to high-intensity activities lasting longer than 15 seconds but less than two minutes, such as a 400-meter run or a set of 15-20 repetitions of weightlifting. It relies on the breakdown of glycogen (stored carbohydrates) to produce ATP. This system can provide energy for up to two minutes of intense activity.

  3. Aerobic system: This system uses oxygen to break down carbohydrates, fats, and proteins to produce ATP. It is used during low-intensity, long-duration activities, such as Ironman or Multisport. The aerobic system produces ATP slowly but can sustain energy production for hours. The byproduct of this system is carbon dioxide and water.


All three energy systems are always active to some extent, but the contribution of each system depends on the type and intensity of the activity. The body will typically use the most efficient energy system for the given task to conserve energy and minimize fatigue. During exercise, the body's demand for energy increases, and it will switch between the different energy systems to meet this demand.

In summary, the human body's energy systems work together to produce energy to fuel physical activity, with each system being used to varying degrees depending on the intensity and duration of the activity.