Metabolism
on-line - the virtual tutorial room
copyright © 2008 - 2015 David A Bender
Selection of fuels for muscle contraction
Key points from this exercise:
In exercise, creatine phosphate acts as a reservoir of "high energy" phosphate to maintain the intracellular concentration of ATP until metabolic activity increases. It also acts to shuttle "high energy" phosphate from ATP formed in mitochondria to the sites where ATP is required for muscle contraction.
The selection of fuels for muscle work depends on:
the intensity of the exercise
the duration of the exercise
whether in the fed or fasting state
At rest, muscle is relatively poorly perfused and metabolises mainly its own glycogen reserves, largely anaerobically, producing lactate.
In short duration, high intensity, exercise, which uses glycolytic white muscle fibres, the main fuels are muscle glycogen and plasma glucose. As much as possible is metabolised aerobically, but the rate of uptake of oxygen is insufficient and there is much anaerobic glycolysis to form lactate.
In moderate exercise the main fuel initially is glucose muscle glycogen. As the exercise continues, and muscle glycogen begins to be depleted, so there is an increasing uptake of plasma glucose and fatty acids. The oxidation of fatty acids increases as glucose and glycogen reserves are depleted.
In strenuous exercise, the oxidation of fatty acids is insufficient to meet the need for ATP formation, and an increasing proportion of glucose is oxidised as the intensity of the exercise increases.
In the fasting state the main fuel for muscle is fatty acids. In the fed state, when much glucose is available and insulin secretion is high, muscle takes up and uses mainly glucose.