Muscles move bones
Take three fingers of your left hand and place them on your left knee. Now run the fingers down the front of your lower leg. You should feel the sharp edge of your shinbone and to the left of it your shin muscle. Push your fingers into the calf muscle and raise the tips of your toes. You should feel your calf muscle move and become hard. In fact it is not the movement of the toes that has made the muscle go hard. It is the movement of the muscle that has made your toes flex. Your toe bones, as with all the bones in your body, can do nothing without muscles.

Put both your hands round the back of your left leg while your toes are pointing upwards. Feel the calf muscle then put the foot back on the floor and make it stand on tiptoe so that you raise your knee. You should feel the calf muscle change shape and become harder. Again the movement of the bones in the foot was not due to themselves but due to the actions of another muscle.

The shin muscle and the calf muscle work together to raise and lower your toes. All over your body are similar pairs of muscles that are arranged to move other bones. Muscles are arranged in pairs because of the way that they bring about movement. They can only make a bone move when they get shorter. This shortening action makes the muscle pull on the bone. A muscle can never push on bone.

However, by the muscles arranging themselves in pairs they can help each other. When your shin muscle shortened and pulled on your toe bones, your calf muscle lengthened. Once the toes were raised the shinbone could not bring about any movement but the calf muscle could. It shortened and pulled the bones down so hard that your foot stood on tiptoe. While the calf muscle shortened the shin muscle lengthened so it could be ready to raise the toes again if you wished.

The shortening and lengthening of muscle occurs because of how they work. Muscles are made of fibres. When you wish a muscle to move a bone, energy is used to slide the fibres along each other and lock them together to hold the bone in place. When the bone is to be moved another way the second muscle pulls on it and also pulls on the shortened muscle. The pulling force makes the fibres unlock and slide back. When a muscle does this it is said to be relaxing. All the work is being done by the other muscle.

Some bones have more than two muscles attached to them. The bones in the thumb have seven muscles attached to them. This arrangement allows the thumb to be moved into many different positions so it can grip objects with the fingers. It is even possible to make your thumb go round in a circle. Just straighten your thumb and try it.

Muscles are not directly attached to bones. They are held onto bones by tough, non-stretchy strings called tendons. You can see the tendons that make your fingers move if you hold out your hand palm downwards and wiggle your fingers. If you want to hold the largest tendon in your body, feel the back of your ankle. There you will find the Achilles tendon. It attaches the calf muscle to a large bone in the ankle.

You may think that you could easily make a model of the skeleton and its muscles by using wire and elastic bands. The wire could be used to make a framework with moveable limbs and the elastic bands could be connected to the limbs to make them move. If you did make such a model you would find that it had very jerky movements compared to the movement of real body muscles. The reason for this is that an elastic band releases all its pulling power at once while a muscle behaves differently. A muscle uses a different number of fibres at different times. If a slow, weak pulling force is needed only a few fibres in the muscles will slide and shorten the muscle but if a fast strong force is needed, they all slide at once. Although we can explain simply how two muscles may move a bone, when we look at all the different ways muscles move the body we can see that many muscle movements are more complicated than this.

Is a muscle attached to just one bone?
No. It is attached to two. If a muscle were attached to just one bone it would be useless. It would simply pull on the side of the bone then remain short because there would be no way to lengthen it. A muscle is attached closely to one bone and a tendon runs across a joint and attaches the muscle to another bone. When the muscle shortens, the bone to which it is closely attached does not move but the bone to which it is attached by a long tendon does move. For example, the calf muscle is attached closely to a bone in the lower leg. This does not move when the calf muscle shortens. The Achilles tendon connects the calf muscle across a joint to a bone in the ankle. When the calf muscle shortens it is the bone in the ankle that moves to make you stand up on tiptoe.

Fingers feel bony. Where are the muscles that work them?
The muscles are in the forearm. They are connected to the fingers by long tendons that cross the wrist. You can see the tendons of muscles which straighten the fingers on the back of your hand.

Why are the muscles not around the finger bones?
The muscles would have to be short and thick to make the finger bones work. This would cause the fingers to be too thick to make delicate movements.

The ribs seem bony. Do they have any muscles attached to them?
Yes, they do. The muscles are between the rib bones. There are two sets. When one set of muscles shorten or contract the ribs are raised. The second set of muscles relax and lengthen at this time. When the second set of muscles contract the ribs are lowered and the first set of muscles relax and lengthen at this time.

Why are the ribs raised and lowered?
The ribs are attached to the spine and to themselves in a special way. When the ribs are raised, the way they are attached makes the inside of the chest larger. This makes more space for air in the lungs and you breathe in. When the ribs are lowered, the way they are attached makes less space inside the chest and you breathe out.

Do animals have bones to move their muscles?
Yes, they do. In fact if you eat meat, you eat muscle. The parts of the fish that you eat are the muscles in its back and sides which bend its backbone to make it swim.

How do you change the expression on your face?
There are muscles under your skin on your face that make you change your expression. For example, there are muscles around your mouth that can pull it into a smile or a sulk. It takes seventeen muscles in your face to make you smile and forty to make you frown.

How do our eyes move?
You have six muscles attached to your eyes. They are arranged in pairs and allow you to move your eyes up and down, side to side. They can even make your
eyes go round in an action called rolling your eyes. This is similar to the action of making the thumb go round.
Are there any muscles inside the body?
Yes, there are, but they are different from the muscles that move your bones. The heart is made from a special muscle called cardiac muscle. This is unlike muscles which move bones. Muscles which move bones need to rest at some time. The heart muscle never rests. It always keeps shortening and lengthening to push the blood round your body. The other kind of muscle is called smooth muscle. It is found in the walls of the stomach. When it shortens and lengthens it churns up the food in your stomach. Smooth muscle is also found in the intestine. When it shortens and lengthens it pushes food along.

Do the muscles which move bones have a name?
Yes, they do. Because they move the skeleton they are called skeletal muscles. Each skeletal muscle has its own name like the calf muscle or shin muscle.