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The Bundles of Energy

Science Factfile

• Muscles are bundles of cells and fibers.
• Muscles work in a very simple way. All they do is tighten up--that is, contract--and relax.
• You have two sets of muscles attached to many of your bones which allow them to move.
• There are 630 active muscles in your body and they act in groups.
• Muscles can only pull. They never push.