entrations (1:407). Nonetheless, it is generally accepted that about 800 milligrams of caffeine will elicit a urine level of 12 micrograms per milliliter in a 70-kilogram athlete. That is approximately equal to between six and eight cups of coffee (9:215-219).
At the cellular level, numerous mechanisms of action have been described to account for caffeine's physiologic effects. The main three theories--in chronological order of their discovery--include the following: (1) the intracellular mobilization of calcium from the sarcoplasmic reticulum of skeletal muscle; (2) the inhibition of phosphodiesterases in various tissues including muscles and adipocytes; and (3) the antagonism of central nervous system adenosine receptors (8:139-155).
The mobilization of intracellular calcium by caffeine was first demonstrated in skeletal muscle. At concentrations of between 1 and 2 millimolar, caffeine lowers the muscle's excitability threshold and prolongs the duration of the active period of muscle contraction. These effects result primarily from the enhancement of
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