Muscles contract through the sliding filament mechanism, where actin filaments slide past myosin filaments. Contraction is regulated by motor neurons at the neuromuscular junction releasing acetylcholine, which triggers excitation-contraction coupling in the muscle fiber. Skeletal muscle contractions can be graded, tetanic, or isometric depending on stimulus frequency. Muscles require ATP generated through aerobic and anaerobic pathways to fuel contraction and fatigue results from an inability to continue contracting.