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Energy Supply for Muscle Contraction

ATP, adenosine triphosphate (there are three phosphates in ATP), is not stored to a great degree in

cells. Once muscle contraction starts the regeneration of ATP must occur rapidly. There are three

primary sources of ATP which, in order of their utilization, are creatine phosphate (CP), anaerobic

glycolysis, and oxidative phosphorylation.

Energy from ATP derives from cleaving of the terminal

phosphate of the ATP molecule. The resulting molecule is

called ADP, adenosine diphosphate. Creatine phosphate

converts ADP back to ATP by donating its phosphate in the

presence of an enzyme which is called either creatine kinase

(CK) or creatine phosphokinase (CPK). The reaction of CP

with ADP to form ATP is very rapid but short lived, since

the cell does not store high amounts of CP. However during

short, high intensity contractions, CP serves as the major

source of energy. This form of energy generation is oftencalled alactic anaerobic because it neither produces lactate nor requires oxygen. It is of paramount

importance in sports requiring bursts of speed or power such as sprints of 10 seconds or less in

duration.

As soon as muscle contraction starts, the process of

anaerobic glycolysis also begins. Anaerobic

glycolysis does not contribute as large an amount of

energy as CP in the short term, but its contribution is

likely to last from 30 to 60 seconds. Duringglycolysis, locally stored muscle glycogen and

possibly some blood born glucose, supply the

substrate for energy generation. No oxygen is

required so the process is called anaerobic. Lactic

acid (lactate is the salt) is formed as the end product

of pure anaerobic glycolysis. Sufficient lactic acid

formation can lower the pH of the cell to the extent

that metabolism is turned off in the cell. The major substrate for anaerobic glycolysis is glycogen, so

prior hard exercise without adequaterepletion of glycogen is going to limit further high intensity,

short term work by muscles.

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The final, and virtually limitless supply of

energy, comes from the process of

oxidative phosphorylation. Maximum

energy production rates from oxidative

phosphorylation are not as high as from

glycolysis. Aerobic events like the

marathon are run at a considerably slowerpace than a 440 because of this fact. The

substrates for oxidative metabolism are

primarily glucose and fat (free fatty acids,

not cholesterol), although protein can also

act as an energy source through

intermediate conversions to glucose,

glucose precursors or free fatty acids.

Because fat can be metabolized

aerobically, most well nourished humans have a near limitless supply of energy for low intensity

exercise. Limitation of low intensity exercise is rarely due to substrate depletion, although depletion

of muscle glycogen may also result in fatigue during aerobic events. The reasons for this are beyondthe scope of this description.