Your body is composed of three types of muscle: skeletal muscle, smooth muscle, and cardiac muscle. Skeletal muscle makes up 40 percent of your total body weight, and smooth muscle and cardiac muscle each make up 5 percent of total body weight. Your muscles must contract in order to facilitate vital bodily functions. Skeletal and smooth muscles are located in different areas in your body to serve their distinct purposes.
Your skeletal muscles are attached to your skeleton by tendons and contract to allow your body to move. Skeletal muscles span your joints, attaching bone to bone, which allows you to flex, bend and twist your body. Your skeletal muscles can be controlled both voluntarily and involuntarily. You can voluntarily choose to lift a cup or throw a ball. Involuntary skeletal muscle contractions are also called reflexes. Your brain can send a signal to your muscles to trigger muscle contraction even if you do not consciously choose to do so.
Unlike skeletal muscles, smooth muscles do not function to move your skeleton. Smooth muscles make up the walls of your blood vessels, digestive tract and airway. The smooth muscles of your blood vessels can contract or constrict to propel blood throughout your body, and the smooth muscles of your digestive tract allow you to swallow food and move food from your stomach through your intestines. You cannot consciously control smooth muscle contraction, and your brain releases hormones into your blood that control smooth muscle contraction.
Although skeletal muscles and smooth muscles serve very different functions, both muscle groups use the same mechanism of contraction called the sliding filament model. Both muscle fibers are composed of thick filaments called myosin and thin filaments called actin. When the muscle fiber is excited, myosin and actin slide past one another, grabbing onto each other as they slide. This generates tension that can shorten or lengthen a muscle.
Smooth muscles contract very slowly, whereas skeletal muscles can contract at different rates. Skeletal muscles can be composed of fast-contracting fibers, slow-contracting fibers or a mixture of both. As you age, your skeletal muscles may become predominantly composed of slow fibers causing slower movements and reflexes, whereas the rate of smooth muscle contraction will not change with aging.