Skeletal and Muscular Systems

The Skeletal System

Human bones are not solid. They are more like hollow tubes that have been filled with jelly. This jelly like substance is a combination of nerves and blood vessels called marrow. New blood cells are produced in the marrow.

How many bones make up the human skeleton? A typical adult has 206 bones, although some people have an extra rib or extra bones in their fingers. The largest bones are the thigh bones, and the smallest bones are located in the ear. Thigh bones contribute about one-fourth of a skeleton’s entire weight.

Bones are made of living, growing tissue that has a unique set of needs. This is why a proper diet is important to maintain healthy bones. Bone-building nutrients include calcium and vitamin D.

Certain cells in the bone constantly break down old bone tissue, while other cells create new bone tissue. When a bone breaks, cells immediately begin to break down damaged bone tissue. Other cells begin to patch the break. Meanwhile, bone-building cells create new bone material near the break. With time, the bone can be as good as new.


As the body grows, the bones grow. The entire skeletal structure grows without ever interfering with other working body systems. With age, however, the bones may harden and become brittle.


Most bones are connected to one another with tough strands of tissue known as ligaments. The points of connection are called joints. Most joints move. Some swing back and forth like a door. These hinged joints are at the elbows, knees, jaw, fingers, and toes. They cannot be twisted without injury.


Ball and socket joints can be twisted. These joints, found in shoulders and hips, allow flexible movement in many directions. The gliding joints in the ankles, wrists, and the spinal column also allow movement in many directions, but the amount of movement is limited.

Other joints allow only a bit of movement. The ribs, for example are attached to the breastbone. If the ribs did not move, you could not breathe. Skull bones of adults do not move at all.

If bones were allowed to rub together at joints, the ends would be ground away. Instead, most bones are padded with cartilage. The cartilage acts as a shock absorber for the skeleton.

The Muscular System

The muscular system allows you to walk, breathe, talk, and swallow. Some actions, such as running, are controlled by voluntary muscles, which means they are under your control. Other actions, such as pumping blood through your body or churning the food in your stomach, are not under your control. These actions are controlled by involuntary muscles. There are three kinds of muscles in the body:

  • Cardiac muscles, which are involuntary, control the heartbeat.
  • Smooth muscles, which are involuntary, are found in the lungs, intestines, and bladder and are controlled without conscious thought.
  • Skeletal muscles, which are voluntary, allow you to act to change the position of your body.

How Muscles Work

All muscles work in the same way; they contract. Each muscle can only pull-not push-on the bones of the skeleton. Bending a joint, such as the arm, involves the contraction of one muscle. To straighten the arm again, the first muscle relaxes, and a second muscle contracts, pulling the arm down. Most joints in the body are controlled by pairs of muscles.


Importance of Exercise

When people hear the word exercise, they sometimes think its purpose is to make bigger, stronger muscles. However, exercise benefits all of the major systems in the human body, including the skeletal system. Exercise can increase the width and strength of bones, while a lack of exercise causes a decrease in bone density, size, and strength. Exercise and proper diet are especially important for older adults, whose bones can become weak and brittle.

Different kinds of exercises help the body in different ways. Some exercises, such as yoga or tai chi, increase flexibility and range of motion. This helps strengthen ligaments and reduces the strain on the joints. Other exercises, such as weight lifting, help build stronger muscle. Aerobic exercises, such as bicycling or swimming, help large muscle groups all over the body. This type of exercise also strengthens the heart and lungs.

To stay healthy and fit, exercise in each of these different ways. Just thirty minutes of exercise a day-even if it is merely a brisk walk-can lead to a longer, healthier life.