810 muscles? (2)

Some muscles, believe it or not, have red and white fibers.  The red fibers work more slowly that the white ones, but can work for a longer time.  The white fibers provide your burst of speed.  A hummingbird‘s wing muscles move more than 100 times per second!  Even when you aren’t moving, dozens of your muscles are working.  For example, those in your neck hold your head erect.  When you doze off, your neck muscles relax.

Muscles use oxygen, sugars, and fatty acids as fuel.  They give off heat to keep you warm.  When you run fast they give off so much heat that you perspire to cool off.



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