Answer:
Intense pain in the muscle is caused due to production of lactic acid (as a result of anaerobic respiration)
Step-by-step explanation:
When an athlete exercises intensely, the muscles carry out lactic acid fermentation in the absence of oxygen.
Lactic acid fermentation is a form of anaerobic respiration in which phosphate and glucose with in the muscles is broken down to produce energy and a byproduct, lactic acid.
Lactic acid causes intense pain and fatigue in the muscles due to which muscles can not rely on anaerobic respiration for a longer period of time.