This document provides an overview of the muscular system including: 1) Types of muscle such as cardiac, smooth, and skeletal muscle which makes up 40% of total body mass. 2) The microscopic structure of skeletal muscle including sarcomeres which contain actin and myosin filaments responsible for muscle contraction. 3) The sliding filament theory of muscle contraction involving interactions between actin, myosin, calcium, and ATP. 4) How nerve impulses trigger muscle contraction through the process of excitation-contraction coupling. 5) Causes of muscle fatigue including reduced ATP, lactic acid buildup, dehydration, and decreased nerve signals.