The muscular system is composed of over 600 muscles that make up half of our body weight. Muscles have properties of contractility, extensibility, elasticity, and irritability that allow them to contract and relax in response to stimuli. There are three main types of muscles - voluntary skeletal muscles that are attached to bones and move the skeleton, involuntary smooth muscles found in organs, and involuntary striated cardiac muscle that makes up the heart. Tendons attach muscles to bones to facilitate movement.