Skeletal muscle contraction enables our joints to move and allows us to engage in physical activity and exercise. This diagram provides a view of energy metabolism within a skeletal muscle.
Hydrolysis of adenosine triphosphate (ATP) provides chemical energy required for the mechanical work of skeletal muscle contraction. Because the supply of ATP within muscles is small, metabolic pathways that lead to resynthesis of ATP must be activated to keep pace with demands for continued muscle contraction. These pathways are either anaerobic (without oxygen) or aerobic (oxygen required). ATP is synthesized from adenosine diphosphate (ADP) with energy provided by phosphagens, anaerobic glycolysis, or oxidative metabolism.
Creatine phosphate (CP) is stored in larger amounts than ATP and can rapidly resynthesis ATP anaerobically. Breakdown of CP within skeletal muscle occurs quickly during high intensity exercise.
Glycolysis is a form of anaerobic metabolism. Glucose, obtained mainly from intramuscular glycogen stores, is metabolized to lactic acid under conditions of short duration, high intensity exercise when oxygen supply is limited. Glycolysis provides ATP more rapidly than oxidative metabolism.
As the duration of exercise continues, oxygen availability to contracting muscles increases and aerobic metabolism provides most of the energy for skeletal muscle contraction. Carbohydrate and fat are the major fuel substrates for oxidative metabolism, while protein serves as aminor alternative energy source. Significantly more energy (36 ATP) is obtained from aerobic metabolism of a glucose molecule than from anaerobic metabolism (2 ATP).
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