Muscle cells need food and oxygen to make them workt. Inside the cell the oxygen is used to release energy from the food. The energy is used to make the fibres slide and carbon dioxide is produced.

When muscles are at work they are being stretched and contracted. Stand up, touch the side of your leg with your fingers and walk. You should be able to feel the muscles moving with each stride. The muscles need a great deal of energy for their work and are well supplied with blood vessels called capillaries to bring the cells food and oxygen in the blood.

A muscle has the power to contract but it does not have the power to stretch. All it can do is relax and let another muscle pull it back to its original length. The muscles are arranged in pairs on the skeleton so that they can help each other stretch after they have contracted. One pair of large muscles are in the upper arm. They are called the biceps and triceps. The biceps is on the front of the upper arm and the triceps is on the back. When the biceps muscle contracts it raises the lower arm and the triceps relaxes and stretches. The biceps stays firm as long as you want to keep your lower arm raised. When you want to lower your arm your triceps is ready to help. It was stretched when the biceps contracted. When the triceps contracts the lower arm goes down and the biceps is stretched. This makes the biceps ready for the next time you want to raise your lower arm.

Muscles are attached to bones by tendons. These are made from a material which is very tough and does not stretch. The tendons are like ropes or cords holding the muscles to the bones. If the muscle is close to the bone it is moving like the muscles in the arm the tendon is short. If the muscle is at some distance from the bone it is moving the tendon is long. For example, your finger bones are moved by muscles in your lower arms. There are no muscles in your fingers so the tendons are very long. Hold out your hand, palm down, and wiggle your fingers. You should see some of the tendons move under your skin.