Energy Systems

Author: Rod Ferris CPT, CPAFLA
Have you ever wondered about where your body gets its energy from? Your body chooses to use ATP (the body's stored form of energy) from a certain system depending on how long you have been training, what is available, and which route is the most efficient.

The nonoxidative energy system is for short burst of exercise or explosive strength. In this energy system your body produces adenosine triphosphate (ATP or energy) by binding phosphate to adenosine diphosphate with energy from phosphocreatine. This energy system only lasts for around 14 seconds so for activities that extend beyond this period would have to revert to a different system.

The glycolytic system is used for continuous repetitions near maximum intensity. In this nonoxidative state, glycogen is used to produce ATP. Glucose is stored in your liver and muscles in the form of glycogen and through various glycolytic pathways is converted to energy when needed.

The final energy system is the oxidative system which is used in endurance and activities lasting longer than 15 minutes. This is the most complex system which uses fat as its primary fuel source.

Notes: Carbohydrates provide 4.1 kcal of energy per gram. Fat provides 9 kcal/g while carbohydrates are the most accessible.