How strong are you? Can your legs run fast? Can your arms lift heavy books? Your muscles make your legs run fast. Your muscles let your arms lift heavy books. Without muscles, you would not be able to move at all!

Muscles work by contracting and expanding. When it tightens it is called contraction. When it loosens the muscle relaxes. Your nervous system tells the muscles when to contract and expand.

Muscles are made of bundles of thin elastic strands called fibres. The muscle fibres are made of proteins. There are two types of muscle fibres. You have slow twitch and fast twitch. Fast-twitch muscle fibres contract quickly. These fibres give you bursts of power for things like sprinting.

Slow-twitch muscle fibres give you endurance. When you run a long way, your slow-twitch muscle fibres do most of the work. Some kinds of muscle have both fast-twitch and slow-twitch fibres.

There are three types of muscle called skeletal, smooth, and cardiac. Each type of muscle has a different job to perform.

Your skeletal muscles are attached to your bones. These muscles contract and relax to give movement to your bones. Usually, you have to think about making these muscles contract.

Smooth muscles move automatically. You do not have to think about these muscles to make them work. Your digestive system is surrounded by smooth muscle. Smooth muscles push food through your digestive system.

Your biggest blood vessels are surrounded by smooth muscle to make them strong. Your cardiac muscle makes your heartbeat. You do not have to think about moving your cardiac muscle as cardiac muscles contract automatically.

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