Why do your muscles ache and tire during hard exercise, and sometimes cramp?
During intense effort the muscles need more oxygen than the blood can supply, so they switch to anaerobic respiration to keep making ATP.
During intense effort the muscles need more oxygen than the blood can supply, so they switch to anaerobic respiration to keep making ATP. This produces lactic acid, which builds up in the muscle and causes the burning, fatigue and sometimes cramps you feel. When you rest and oxygen returns, the lactic acid is cleared and the muscles recover. It's an oxygen shortage, not damage.
Key point
Hard exercise forces anaerobic respiration, building up lactic acid that causes fatigue and cramps.
Common mistake
thinking muscle burn during exercise means the muscle is being injured.
Memory tip
too little O₂ → anaerobic respiration → lactic acid → ache and cramp.
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