Armstrong, N., & Welsman, J. (1997). Children in sport and exercise: Bioenergetics and anaerobic exercise. British Journal of Physical Education, 28(1), 3-6.

Glycogen: the storage form of carbohydrates (e.g., glucose) in the body. Glycogen is stored in skeletal muscle and the liver. Its breakdown (glycogenolysis) to pyruvic acid results in the production of ATP.

Glycogenolysis: the breakdown of glycogen to supply glucose for glycolysis.

The Energy of Activity

ATP-CP (alactacid) energy system. The energy for muscular contractions, muscles being the mechanical engines of movement, is produced by the splitting of high energy phosphates, primarily adenosine tri-phosphate (ATP). The amount of ATP stored in muscles is limited and will support only the briefest of bursts of movement (about one second's worth). ATP needs to be regenerated quickly if exercise is to continue. Extra ATP is derived from splitting another high-energy phosphate, creatine phosphate (CP). CP stores are also limited but sustain high-intensity exercise for another four to five seconds. The ATP-CP energy system, the alactacid system, can sustain from 5 to 7 seconds of intense activity.

The lactacid (glycolytic) system. Before ATP-CP stores are depleted, further ATP must be re-synthesized and this occurs during the breakdown (catabolism) of glycogen (the process of glycogenolysis) or glucose (the process of glycolysis) to pyruvic acid and eventually lactic acid. Glucose is obtained from the diet and is stored in the muscles and liver as glycogen. Liver glycogen is primarily used to maintain glucose concentration in the blood, which is vital for optimal functioning of muscles and the nervous system including the brain. Although glycogen is the primary source, blood glucose is a secondary source of energy for muscular contraction. ATP re-synthesis through either the splitting of CP or glycogenolysis does not require oxygen and is therefore, anaerobic.

The aerobic system. Prolonged activity depends upon the delivery of oxygen to the working muscles via the aerobic energy system. This system is the most efficient for ATP production because of its ability to use fatty acids as well as carbohydrates. It has a vastly greater capacity to supply energy (ATP production) than either anaerobic system. However, the aerobic system is slow to adapt to the demands of exercise. In the early stages of work, the demands for ATP normally exceed the ability of the slowly-responding aerobic system, pyruvic acid production exceeds the capacity of aerobic system to oxidize it, and so energy has to be derived also from anaerobic sources. If pyruvic acid builds up it forms into lactic acid that accumulates in the muscles eventually bringing muscular contraction to a halt.

The presence of these three types of energy system in an individual is genetically determined. The types of muscle fibers that exist in a person is relatively stable from six years on and so energy producing capacities are established very early in young children.

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