November 23, 2012 10:59 am

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 wouldn’t be able to move at all!
Muscles work by tightening and loosening. Tightening is called contraction. Loosening is called relaxing. Your nerves tell muscles when to contract.
Suppose you see a cookie on a table. You want to pick up that cookie and eat it. Your brain sends out a signal. Nerves carry the signal from your brain to your arm and hand muscles. The signal tells muscles in your arm to contract. Your arm reaches out for the cookie. Then the signal tells muscles in your hand to contract and grab the cookie. Muscles in your arm contract to bring the cookie to your mouth. Your hand pops the cookie into your mouth. Your jaw muscles contract and relax so you can chew the cookie. Yum.
Muscles are bundles of thin strands called fibers. The muscle fibers are made of substances called proteins.
There are two types of muscle fibers. The two types are slow twitch and fast twitch. Your fast-twitch muscle fibers contract rapidly. These fibers give you bursts of power. When you suddenly jump or run fast while playing tag, your fast-twitch muscle fibers are hard at work.
Slow-twitch muscle fibers allow you to keep doing exercises. They give you endurance. When you run a long way, your slow-twitch muscle fibers are doing most of the work. Some kinds of muscle have both fast-twitch and slow-twitch fibers.
There are three types of muscle called skeletal, smooth, and cardiac. Each kind of muscle has a different job to do.
Your skeletal muscles are attached right to your bones. These muscles contract and relax to make your bones move. These are the kind of muscles you use to run or swim or reach for cookies. You usually have to think about making these muscles contract.
Smooth muscles move automatically. You do not have to think about your smooth 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 stronger.
Your cardiac muscle makes your heart beat. You do not have to think about moving your cardiac muscle. Your cardiac muscle contracts automatically.
Exercise makes muscles stronger. Lifting heavier and heavier weights makes your skeletal muscles stronger. Many people lift weights to make their arm and leg muscles stronger. A gym teacher or special trainer can show you how to lift weights safely.
Running, walking, swimming, and jumping rope are exercises that can make your heart muscle stronger. Any exercise that makes your heart beat faster makes your heart muscle stronger.
Muscles can get weaker if they are not used. Using your muscles every day keeps them strong and healthy.
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