What percentage of ATP used by muscle activity comes from aerobic respiration?

• A. 50%
• B. 75%
• C. 95%
• D. 100%

Answer: (C)

Energy for muscle contraction comes from multiple systems that kick in at different times. Right at the start, the immediate phosphagen system (phosphocreatine) supplies ATP for a few seconds, and anaerobic glycolysis provides additional ATP during short, intense efforts. But as activity continues and oxygen delivery meets demand, mitochondria switch to oxidative phosphorylation, using glucose and fatty acids to make ATP with high efficiency. In sustained, steady muscle activity, the vast majority of ATP—about 95%—is produced aerobically, with a small remainder coming from anaerobic pathways to cover brief bursts or the ramp-up phase. So, roughly 95% from aerobic respiration best fits how energy is supplied during longer exercise. The other percentages would imply a greater, or complete, reliance on aerobic sources than actually occurs, given the involvement of quick, anaerobic systems early on.