The same metabolic systems are present in muscle as in all other parts of the body. However, special quantitative measures of the activities of three metabolic systems are exceedingly important in understanding the limits of physical activities like sports. These systems are (1) the phosphocreatinine-creatinine (phosphagen) system, (2) the glycogen-lactic acid (anaerobic) system, and (3) the aerobic system (2000).


The source of energy used to cause muscle contraction is adenosine triphosphate (ATP), which has the following basic formula:


Adenosine-PO3 ~ PO3 ~ PO3 ~


The bonds attaching the last two phosphate radicals to the molecule, designated by the symbol ~, are high energy phosphate bonds. Each of these bonds store 7300 calories of energy per mole of ATP under standard conditions and even slightly more than this under the physical conditions in the body ( 2001).


The amount of ATP present in the muscles, even in a well-trained athlete, is sufficient to sustain maximal muscle power for only about 3 seconds, maybe enough for one half of a 50-meter dash. Therefore, except for a few seconds at a time, it is essential that new ATP be formed continuously, even during the performance of short athletic events (2000).


The figure below shows the overall metabolic system, demonstrating the breakdown or splitting of ATP first to adenosine diphosphate (ADP) and then to adenosine monophosphate (AMP), with the release of energy to the muscles for contraction.




  1. Phosphocreatinine è Creatinine + PO3      è




  1. Glycogen                 è Lactic Acid             è        ì         éê      î


ADP         Energy for muscle


iii. Glucose                  î                                             î         éê      ì         contraction


Fatty Acids             è + O2 à CO2 + H2O           è                    AMP


Amino Acids           ì                     +




Source: 2000




The aerobic system is the oxidation of foodstuffs in the mitochondria to provide energy (2001). As shown in the table, glucose, fatty acids/fats, and amino acids from the foodstuffs – after some immediate processing – combine with oxygen to release tremendous amounts of energy that are used to convert AMP to ADP into ATP.


The phosphagen system is the one used by the muscle for power surges of a few seconds, and the aerobic system is required for prolonged athletic activity. In between is the glycogen-lactic acid system, which is especially important for giving extra power during intermediate races as the 200- to 800-meter runs (2000).


In anaerobic glycolysis, the process converts glycogen or glucose to lactic acid and regenerate energy stores of ATP. However, if lactic acid is allowed to build up, muscle contraction is compromised. This buildup of lactic acid is thought to be a component of muscle fatigue (1997).


The fuels or substrates used during exercise are primarily carbohydrates and fats (1997). Carbohydrates (CHO) are largely used by muscles during exercise and other sports activities. In addition to the large usage of CHO by the muscles, large amounts of fat in the form of fatty acids and acetoacetic acid are also used. To a much less extent proteins in the form of amino acids are also used. In fact, even under the best conditions, in those sports events that last longer than 4 to 5 hours, the glycogen stores of the muscle become almost totally depleted and are of little further use for energizing muscle contraction. Instead, the muscle now depends on energy from other sources, mainly from fats ( 2000).


The table below shows the energy systems used in various sports:






Phosphagen System


100-meter dash, jumping, weight lifting, diving, football dashes


Phosphagen & Glycogen-lactic acid systems


200-meter dash, basketball, baseball home run, ice hockey dashes


Glycogen-lactic acid system mainly (Anaerobic)


400-meter dash, 100-meter swim, tennis, soccer


Glycogen-lactic acid and aerobic systems


800-meter dash, 200-meter swim, 1500-meter skating, boxing, 2000-meter rowing, 1500-meter run, 1-mile run, 400-meter swim


Aerobic system


10000-meter skating, cross-country skiing, marathon run (26.2 miles, 42.2 km), jogging


Source: 2000