Pioneer Valley Youth Sports and Fitness Institute

the source for sound athletic advice

Where Do We Get All That Energy?

Sport-specific training workouts and athletic events might be understood better by examining the degree to which they use various anaerobic and aerobic processes in supplying energy.  Most changes that occur in the human body during exercise are related to increase in energy metabolism largely within the contractile muscle. Skeletal muscle contractions are powered by the energy released through the breakdown of adenosine triphoshate (ATP). The amount of ATP directly available in muscle at any time is small and, therefore, it must be re-synthesized continuously if exercise continues for more than a few seconds.  Anaerobic activities are those that use energy in the form of ATP with no oxidative processes occurring in order for the muscles to use that energy.  Aerobic activities are those that use oxidative processes as the major source of producing ATP. Our muscle fibers have the ability to produce ATP in three ways; the following will discuss these three pathways.

The first pathway, often referred as the creatine phosphate (CP) system allows us to perform activities for short duration. As mentioned above, we need ATP to produce a muscle contraction and ATP needs to be made; creatine phosphate, which is available in limited amounts can only help produce a small amount of ATP. This CP system does not need oxygen (O2) to produce the ATP, therefore this energy system and the muscle contractions that are produced with this system during exercise and sport is considered anaerobic.  The CP system provides ATP for events that last for 0 to 6 seconds in duration, see table 1.  

The second pathway, glycolysis, of which there are two types, fast glycolysis and slow glycolysis. Glycolysis is the breakdown of carbohydrates, either that which is stored in muscle as glycogen or that which is stored in blood as glucose to produce ATP.  The glycolitic system supplements the energy supply from the CP system for higher intensity muscular activity.  Fast glycolysis occurs during periods of reduced oxygen availability in the muscle cells and through a series of reactions results in the formation of pyruvate which is converted to lactic acid and ATP at a fast rate. Muscular fatigue experienced during exercise is often associated with high tissue concentrations of lactic acid.  Fast glycolysis occurs in events that are generally between 30 seconds to 2 minutes, see table 1  . Slow glycolysis occurs when a series of reactions results in the formation of pyruvate and O2 is present in sufficient quantities in the mitochondria, the power house of the cells. At the point ATP is produced pyruvate is not converted to lactic acid but is sent to the mitichondria where O2 is present and aids in the production of ATP which is the third pathway. Slow glycolysis will take the place of fast glycolysis during events that are 2 minutes to three minutes in duration, see table 1. Because glycolysis itself does not depend on O2, the terms anaerobic and aerobic are not practicle for this system.

The third pathway, the oxidative system, uses primarily carbohydrates and fats.  When exercise continues past the approximate 3 minute mark, slow glycolysis is then taken over by the oxidative system. As previously mentioned when O2 is present, the end product of glycolysis, pyruvate, is not converted to lactic acid but is sent to the mitochondria where it enters what is known as the krebs cycle. The krebs cycle is where a series of reactions take place in order to produce molecules that enter what is known as the electron transport chain where energy is provided for ATP production. From 1 molecule of glucose approximately 38 ATP’s can be produced. The oxidative system occurs when exercise last for 3 minutes and greater in duration, see table one. The oxidative system is considered solely aerobic.     

The CP system provides ATP primarily for short-term, high intensity activities (e.g., weight training and sprinting) and is active at the start of all exercise regardless of intensity. Glycolysis is the breakdown of carbohydrates. If a high rate of energy is needed such as during resistance training, fast glycolysis is primarily used, If the energy demand is not as high and oxygen is present in sufficient quantities in a cell, slow glycolysis is activated. When exercise is performed for a prolonged period of time, greater than 3 minutes, and O2 is present in sufficient quantities the oxidative system is activated to produce multiple amounts of ATP.

Len Haggerty is a Sports Performance coach operating STRIDES Performance & Wellness, Inc. in Northampton MA. He provides an experience for youths, athletes, and adults of all ages and abilities to discover their athletic potential. Len can be reached at len@strideshpi.net to answer any questions regards to sports performance and conditioning.