Muscles are the fleshy part of the body and consist of long, thin fibres, bound together in bundles by connective tissue, and supplied with blood and nerves. What is remarkable about these fibres is that they can become shorter in response to a stimulus. The shortening is caused by protein filaments inside the cells which pull against each other; on relaxation, the muscles are pulled back into their original position, by gravity or by the action of other muscles. Sometimes the cells are unable to relax their hold – this involuntary contraction of a muscle is called a spasm and may be caused by pain.
There are, roughly speaking, two types of muscle: the involuntary and the voluntary. The heart and the hollow organs (digestive system, uterus, blood vessels, etc.) are of the first sort. They work without conscious control while life lasts – you do not need to instruct your heart to beat. The voluntary muscles are mostly under conscious control, so that it is for you to decide to move your limbs, for example; but you do not, of course, have to plan the movements of each muscle. When you decide to bend your knees, for instance, reflex actions determine the different movements of the several different muscles which this entails, and you neither know, nor need to know which ones they are.
The voluntary muscles also respond to various stimuli through reflex actions – when you touch a hot stove, messages racing along the nerves will jerk away your hand faster than thought.
Because they can be controlled, the voluntary muscles can be trained to work more efficiently. With suitable training, the nervous system learns how to recruit muscle fibres more rapidly and more precisely, and the muscles themselves become stronger and bigger and more capable of clearing away the waste products of their activity, so that they can continue working for longer. If they are not exercised, they waste away quickly.
The action of a muscle is to develop tension between two points on the skeleton, so as to draw them together or prevent their being pulled apart or control the rate at which they are being pulled apart. Without muscular control, the spine is much less stable – as in an unconscious person.
The muscles which control the spine are those of the back and neck and the abdominal muscles.
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