Chapter seven describes smooth, cardiac, and skeletal muscle structure and function. Smooth muscle fibers (cells) are found in the walls of hollow internal organs and are uninucleate, spindle-shaped, and under involuntary control. Cardiac muscle fibers comprise much of the heart and are uninucleate, branched, striated, and also under involuntary control. Skeletal muscle fibers are multinucleate, cylindrical, striated, and under voluntary control. Muscle contraction maintains posture, allows for the movement of the body and internal organs, and generates heat to warm the body. Skeletal muscle structure and contraction are discussed. The contractile elements of skeletal muscle fibers are myofibrils which contain actin and myosin filaments. During muscle contraction these filaments slide over each other, causing the entire muscle fiber to shorten. Muscle fibers contract in an all-or-none fashion. The importance of exercise in maintaining good health is discussed as are muscle twitch, summation, and tetanus. Muscle contraction can be isometric or isotonic. During isometric contraction muscles do not shorten and no movement occurs. Muscle names often contain information regarding size, shape, attachments, location, and action. The origins, insertions, and actions of major muscles are both listed in tables and explained in the text.