The muscular system consists of 650 muscles in three main categories: skeletal, smooth, and cardiac muscles. It is controlled by the nervous system via two pathways: somatic and autonomic. Only skeletal muscles fall into the somatic category, meaning that they are under voluntary control; these are the muscles attached to the skeleton. They are striated in appearance and provide strength, balance, posture, movement and heat for the body.
Tendons are bands of fibrous tissue attached to the skeletal muscles that allow movement in the body. When a skeletal muscle contracts, it pulls on the tendon to which it is attached, causing the tendon to pull on the bone and resulting in movement. Ligaments, on the other hand, are the fibrous material that connects bone to bone and holds the skeletal structure together. Once injured, both require long healing times and are prone to weakness or re-injury. Rest is required, as scar tissue usually takes at least 90 days to form; once formed, the fibers can take up to another seven to nine months to reach maximum strength again.
Autonomic muscles contract and relax involuntarily and include both smooth and cardiac muscles. Smooth muscles are non-striated and are typically found in layers, one behind the other. They can be found in the walls of internal organs (excluding the heart), such as blood vessels, intestines, bladder, digestive system, and stomach. They are constantly at work, performing their functions throughout the body.
Cardiac muscles are specific to the heart and are also known as myocardium. These involuntary muscles contract to pump blood through the heart, then relax to allow blood to return after it has carried vital oxygen and nutrients to the body.