The sensation of fatigue in the muscles after prolonged strenuous physical work is primarily caused by the accumulation of lactic acid. During intense physical activity, the muscles require a high amount of energy, which is mainly supplied by the breakdown of glucose through a process called glycolysis. However, when the energy demand exceeds the oxygen supply, as is the case during intense exercise, the muscles switch to anaerobic metabolism, leading to the production of lactic acid as a byproduct.

As lactic acid accumulates in the muscles, it can interfere with muscle contraction and contribute to the sensation of fatigue. It can decrease the pH level in the muscles, leading to muscle acidosis, which can impair muscle function. The accumulation of lactic acid also contributes to the feeling of muscle soreness and stiffness experienced after intense exercise.

While other factors such as a decrease in the supply of oxygen, minor wear and tear of muscle fibers, and depletion of glucose can also contribute to muscle fatigue, the accumulation of lactic acid is considered one of the primary factors responsible for the sensation of fatigue during prolonged strenuous physical work.