His muscle cells have experienced that led to production of

Anaerobic respiration; lactate

Explanation:

By utilizing the previous knowledge about respiration i am answering the question in one of the possible ways.

Anaerobic respiration is considered to be the process in which cells respire in absence of oxygen. During this, glucose molecules are broken down to produce energy along with the production of lactic acid. This process is used by small organisms as well as eukaryotic cells at the time of need such a s strenuous exercise  (when oxygen supply is low )to produce energy. When the production of lactic acid is high during anaerobic respiration, this leads to production of more energy in form of ATP which side by side causes pain in muscles and a condition called muscle cramps.

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