Final answer:
Skeletal muscle fatigue during prolonged sub-maximal exercise is caused by accumulation of metabolic byproducts, glycogen depletion, and electrolyte imbalance as the muscle requires continuous energy which can be hindered by limited oxygen delivery, leading to reliance on inefficient anaerobic pathways.
Step-by-step explanation:
The cause of skeletal muscle fatigue during three hours of sub-maximal exercise is primarily a result of the accumulation of metabolic byproducts (like lactic acid), depletion of muscle glycogen, and disturbances in the muscle's electrolyte balance. During prolonged exercise, the body relies on aerobic respiration for energy; however, as the exercise continues, oxygen delivery to muscles may become limited, resulting in an increased reliance on anaerobic pathways. This can lead to an accumulation of metabolites that interfere with muscle contraction. Furthermore, glycogen stores can become depleted, and without enough glycogen, muscle cells cannot maintain their energy supply. Lastly, sweating during long periods of exercise can alter the balance of sodium and potassium, which are key to the proper functioning of muscle cells.