Muscular system: movement

Raise your hand to wave. It feels like a single action, but several parts of your body had to cooperate: a muscle, a tendon, a bone, and a joint. Movement comes from this teamwork.

Skeletal muscles attach to bones by tendons. When a muscle contracts, it shortens and pulls on a bone. The bone then pivots around a joint, acting like a lever, with the joint as the pivot point. Because a muscle can only pull, never push, the body needs a second muscle to pull the bone back. So muscles work in pairs that oppose each other. In your upper arm, the biceps is on the front and the triceps is on the back. When the biceps contracts, the elbow bends, which is called flexion, and the triceps relaxes. When the triceps contracts, the elbow straightens, which is called extension, and the biceps relaxes. You can feel this by placing a hand on each muscle while you move your arm.

How does one muscle shorten? Muscle fibers are packed with tiny protein strands that slide past each other, pulling the ends of the fiber closer together. This sliding takes energy, which mitochondria release from food using oxygen delivered by the blood. This is one reason breathing and heart rate rise when you exercise: your muscles need more oxygen. Some of the energy released is lost as heat, which is why you warm up when you move, and why shivering, a rapid series of tiny contractions, helps warm you when you are cold.

Muscles respond to how you use them. With regular exercise the fibers grow thicker and stronger. With little use, muscles shrink and weaken, a process called atrophy. Muscles also tire. When they work harder than the blood can supply oxygen, they can no longer keep contracting at full strength, and they need rest to recover.

Common mistakes: thinking muscles push, and thinking muscle can turn into fat. Muscle and fat are different tissues. A person who stops exercising loses muscle size and may gain fat, but one does not change into the other. Next lesson: the nervous system, which tells muscles when to contract.