Aclient experiences burning muscle pain at the peak of high intensity physical training for an athletic competition. what is the best explanation for the cause of this occurrence?

Lactic acid is accumulated in the muscles which is causing burning muscle pain.  

Explanation:

During intense exercise our muscles shifts to anaerobic mode of ATP generation. So in anaerobic respiration muscles do not use oxygen but use lactic acid fermentation for energy production which don require oxygen.  

In lactic acid fermentation, the pyruvate produced during glycolysis is used and converted into lactic acid. The accumulation of lactic acid in the muscle causes the burning muscle pain during the peak of high-intensity physical training and after the training.  

Therefore lactic acid is accumulated in the muscles which is causing burning muscle pain.