The Dynamic Function of Muscle
It is tempting, when we speak about anatomical design, to look first and foremost at our skeleton. All vertebrates possess a skeleton of some kind, whether it is the rather simple head and spine of a fish or the giant architecture of the dinosaur’s bones. Among the vertebrates we see a variety of designs fit for the book of marvels: small, rodent-like animals that burrow in the ground, animals that are fleet of foot, animals with long necks for reaching high branches, powerful hunters, huge dinosaurs with their massive leg bones and spines, dolphins and whales that swim in the water, and clever, big-eyed monkeys that swing in the trees. Of all of these, the human skeleton is in many ways the most remarkable, because it is the only one that is fully upright, with the spine vertically balanced over the legs. There are other animals that walk on two legs, but none that has gone fully upright, so that the arms no longer bear weight but can function independently. That one fact has made humans the most powerful creature on the planet, and enabled us to dominate all the other species.
To move and support the skeleton, muscles are of course required. To produce movement, muscles possess the unique property of being able to contract. Muscles attach to bones, which act as levers. So muscles are motor and bones are levers, and these two tissues, working together as a total system, support and move the skeleton. We have all seen the anatomical chart that shows the human skeleton, with the spine on the plumb line. The implication here is that muscles, acting on the skeleton, maintain posture just as they move parts of the skeleton. In other words, both posture and movement are the product of muscles, which support and move the skeleton.
And yet the action of muscles is not the most prominent nor the most fundamental feature of the human design. Muscles do of course contract to stabilize and move bones—that is their function, and the ability of muscles to produce movement is of course the most prominent feature of living things, which we identify as being alive by the fact that they move. But the contraction of muscles cannot fully explain movement or posture. Many of the primary postural muscles of the trunk, for instance, run vertically from vertebra to vertebra, or from the sacrum upward to the ribs. These are the main postural muscles that maintain the stability of the spine and trunk. To discharge this function, however, they must contract and pull on a vertical axis, which means that they will pull the structure down, which is exactly what we see in someone who is slumping and is quite literally pulled downward by their own muscles. This is clearly not how these muscles are meant to work. In a young child, who moves so gracefully and sits with perfect upright posture, the muscles do not pull down but, on the contrary, are functioning in such a way that the entire structure seems to lengthen and stretch. Yet these muscles are doing some kind of work—otherwise, the structure would fall down. How is it possible that vertically-positioned muscles do not pull the structure down but, on the contrary, maintain length?
The answer is simple. The muscles pull, but the skeletal parts to which the muscles attach go in the opposite direction so that the muscles are maintained in a lengthened state. A very clear example of this can be found in the muscles of the neck. These muscles, which attach at the vertebrae of the neck and run upward to the occiput, are designed to extend the head by pulling it down and back.
But this does not happen. Look at any young child, sitting naturally and automatically upright, and you will see that the muscles of the neck, far from pulling the head back and down, are lengthened. This is because the head is balanced forward and the muscles are kept lengthening between the head and spine. They are lengthened between the bony points to which they attach and, in that context, they contract and maintain tone.
Another structure that exhibits this dual tendency is the spine. Muscles must pull on the spine, but if this is all they did, they spine could not maintain its lengthened support against gravity. Instead, muscles are lengthened between vertebral attachments and ribs and, in this context, maintain the support of the spine.
This arrangement explains how muscles act on bones as a basic way of supporting the skeleton. In order to move bones, muscles must contract—something we see every day in humans and other vertebrates. To maintain support off the ground, however, muscles must lengthen between the bony structures to which they attach so that, even as they maintain support, the muscles are lengthened—a subtle and dynamic arrangement in which muscles and bones work together to produce upward support against gravity. In this way, muscles maintain support and move bones, but in the context of an overall structure that—in the case of humans—lengthens upward against gravity.