Final answer:
Muscle fatigue can be caused by fuel depletion, specifically the exhaustion of ATP reserves necessary for muscle contractions, and the accumulation of lactic acid which affects pH levels that are crucial for enzyme and protein activity.
Step-by-step explanation:
Muscle fatigue is a complex phenomenon that can occur due to multiple factors, one of which includes fuel depletion. This refers to the exhaustion of energy reserves, particularly ATP, which is crucial for muscle contraction. ATP is necessary for cross-bridge cycling in muscle fibers, as it provides the energy for the myosin heads to pull the actin filaments towards the center of the sarcomeres. When ATP levels diminish, muscle contraction efficiency decreases, leading to fatigue. Additionally, the accumulation of lactic acid due to anaerobic metabolism can also contribute to muscle fatigue because it lowers the intracellular pH, which in turn affects enzyme and protein activity. Other factors include imbalances in Na+ and K+ levels and potential damage to the sarcoplasmic reticulum (SR) and sarcolemma from prolonged exercise, impairing calcium regulation.