During exercise, muscle cells immediately use ATP as the energy source for contractions.
The only immediate source of energy for muscular contraction is ATP. Within muscle fibers, ATP is replenished by creatine phosphate (anaerobic respiration), glycolysis (anaerobic respiration), and cellular respiration (aerobic respiration). As the duration and intensity increase, carbohydrates (glucose/glycogen) become the primary source of fuel. If exercise is sustained for an extended period of time, fatty acids will be used as a fuel source when glycogen stores are nearly depleted.
ATP is a form of instant energy. The body cannot obtain energy directly from glucose. Glucose is not a direct source of energy. Instead, the body uses the energy released from the breakdown of glucose to drive ADP phosphorylation.
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