The New Childrens Encyclopedia

276

Mighty muscles

Muscles are tissues that move parts of the body by

contracting, or getting shorter. You have around

650 skeletal muscles layered over your skeleton

and these make up about half of your weight.

They are attached to bones by stringy tendons.

TYPES OF MUSCLE

There are three types of muscle: skeletal muscles move bones

when you want them to. Most muscle is skeletal muscle.

Cardiac muscle keeps the heart beating. Smooth muscle is

found inside hollow organs such as the digestive tract. You can’t

control the actions of smooth muscle—they are automatic.

HOW MUSCLES WORK

Skeletal muscles get shorter and fatter

when working—they stretch when relaxed.

They work because your brain tells them to.

If you want to reach out to grab something,

your brain tells your arm muscles to work.

The muscles shorten, pulling the arm bones.

Muscles work in pairs, because they work

by pulling. So in your arm, your biceps

works to bend your arm and your

triceps straightens it.

This large, powerful

muscle, the gluteus

maximus, straightens

the hip when you,

walk, run, stand up,

or climb a hill.

The extensor digitorum

longus straightens your

toes and helps to lift the

foot up when you walk.

The calf muscle

(gastrocnemius)

bends your foot

downward when

you point your toes.

Muscles cover the

skeleton and give

the body its shape.

Biceps

contracted

Triceps

contracted

Muscles need oxygen to

make energy. If starved of

oxygen, perhaps during a burst of

activity, they produce energy without

it and a waste product called lactic

acid builds up in the muscle cells.

This can cause painful

muscle cramps.

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THE HUMAN BODY