Muscular System: The Meat of the Matter

The joys of a massage or the relief of a chiropractic adjustment—and the stresses of physical work—play out partly in the muscles. Learn just what pumps the body up with Groupon’s guide to the muscular system.

The human body has more than 630 muscles keeping it upright and mobile. They make up almost half its weight and power the movements of the bones, the blood, and even the food in the stomach. Perhaps the most familiar muscles are those seen in bodybuilding contests and facial-expression contests: the skeletal, or voluntary, muscles. They’re attached to our bones and controlled by our brains, which zap them with electrical signals to cause their fibers to contract. During a muscle contraction, filaments inside the muscle fibers slide together, stacking up on one another so that the larger fiber shortens. In shortening, the fibers gain thickness—a phenomenon we notice as flexing. Whatever muscles do, they accomplish by this single pulling action. If a bicep (part of a category of muscles known as flexors) flexes to lift a barbell, it needs a tricep (an extensor), pulling in the opposite direction, to bring the arm back down.

The other two types of muscles are smooth muscles and cardiac muscles, and both are beyond our conscious control. Cardiac muscles control the beating of the heart, contracting the chambers to push blood throughout the body. But the blood doesn’t ride to the toes on that momentum alone. Lining the blood vessels are smooth muscles that help push it along. These also line the esophagus, stomach, and intestine to move food through the digestive track, and can even help regulate the body’s temperature by opening and closing capillaries near the skin surface, all without conscious effort. The subconscious brain is also happy to turn muscles to ends beyond their apparent purpose: for instance, what we experience as shivering from cold is simply the brain causing the muscles to spasm so they will generate heat and keep your blood and any baby chicks in your coat pockets warm.