Muscles

Your body moves, stretches, and even pumps blood, all because of muscles. These tissues make up the muscular system, which includes all the muscles working together to keep you moving.

Each muscle is comprised of long cells called muscle fibers, which are packed with tiny units containing protein strands called actin and myosin.

When the brain sends a signal, these protein strands slide past one another, causing the muscle to shorten. This is called contraction.

After contraction, the muscle returns to its original shape, called relaxation.

Your body has different types of muscles. Some, like the ones that help you lift your arm, are under your control—these are called voluntary muscles. Others, like the muscles that control your heartbeat, work automatically. These are called involuntary muscles.

Based on their function and location, muscles are grouped into three types.

First, there are skeletal muscles that attach to bones and help with voluntary movements like jumping.

Next, there are smooth muscles, which are involuntary and found in organs like the stomach.

Lastly, cardiac muscle is also involuntary. It is found only in the heart and is responsible for pumping blood.