The muscular system is composed of specialized cells called muscle fibers in which their predominant function is contractibility. Muscles, attached to bones or internal organs and blood vessels, are responsible for movement and nearly all movement in the body is the result of muscle contraction.

Reference: https://training.seer.cancer.gov/anatomy/muscular/#:~:text=The%20muscular%20system%20is%20composed,the%20result%20of%20muscle%20contraction.