The first supports high intensity
short duration (few seconds) activities because the amount
of ATP and CP reserves in muscle fibers is small. Anaerobic
glycolysis produces ATP quickly to sustain muscle
actions for a couple of minutes but the end products (H?,
lactate) impair muscle function and are associated with
muscle fatigue. Finally, the energy for exercise performed
at intensities that can be sustained for longer duration
(minutes to hours) is supplied by oxidative phosphorylation
within the mitochondrial network. A network of capillaries
transports oxygen to the active muscle fibers. The extent of
this network correlates with the metabolic demand on the
muscle fiber. The architectural relationship between these
two structures was described very early in the twentieth
century by August Krogh and recently discussed [31]. It is
important to note that the utilization of these metabolic
pathways is not an ‘‘all or none’’ phenomenon.
Pathways overlap and can be activated at different points in
time during a single session of exercise depending on the
intensity of the effort.