Bones and muscles

Some millions of years ago man’s ancestors began to walk on two legs.

Our upright posture has freed our hands for manipulating tools, but there are also some disadvantages associated with this posture. The back has to carry a much heavier load, which inevitably results in a considerable number of back complaints.

Movement is brought about by muscles which are co-ordinated by the nervous system. The pull of gravity means that numerous small adjustments have to be continually made in order to maintain position and movement.

The amount of muscle tissue a person has can vary enormously. A muscle that is not used can waste away almost entirely, whereas training can enlarge a muscle to five times its original size. That women have less muscle tissue than men is the result of hormones.

In addition to the well-known and clearly visible phenomenon of external movement caused by skeletal muscles, there is also ‘internal’ movement. This is not visible from the outside but is equally important.

In general, internal movement is brought about by a different kind of muscle tissue, over which we have no conscious control. Examples are the muscles in the intestinal wall, which move food from the stomach, and the muscles in the walls of blood vessels, which are able to constrict.

A very important muscle is, of course, the heart. This is largely responsible for the most important of the body’s movements, that of blood circulation.

Movement

Movement is a characteristic of all living animals and we admire particular forms of it in human beings. We compare the grace of the ballet dancer with the agility of the gymnast. We wonder at the timing and strength of the weightlifter.

We all need to move, walk, run, lift, carry, push and pull to do our daily work and continue life’s normal activities. But each phase of life is different. The uncertain gait of the child, walking with feet wide apart because his muscle groups are not yet fully co-ordinated, is a delight for a parent and amusing to the family, especially when gravity overpowers his muscles and he sits with a bump. The healthy adult has a good posture, demonstrating co-ordinated and smooth muscle action. Later, in old age, muscles weaken, joints become stiff, ligaments sag, and the back bends, so that movement becomes restricted. Also, the bones become brittle in old age as the collagen which gives the bone its elasticity disappears. If an elderly person falls down it is therefore easy for him or her to break a bone.

Although the basic pattern of our skeletal system can be traced from our predecessors in evolution, our upright posture has further developed the specific relationship between the muscles, skeleton and joints. It is important to appreciate the role of the brain as a whole in regulating movement, but especially the cerebellum and spinal cord which co-ordinate the posture, power and movements of groups of muscles.

Learning to move

We take for granted our ability to move and negotiate the world. In fact, learning how to co-ordinate our movements with sufficient expertise to get from A to B involves the same process as learning how to drive a car and, indeed, takes longer. A newborn baby is completely helpless, able only to grasp a finger and to feed. By about six weeks of age the infant has some measure of control of his neck, and by about ten weeks is able to roll over from his side onto his back. The next few weeks are spent gaining control of the upper body so that the baby is able, when lying on his stomach, to transfer his weight onto the arms and lift his chin up. This marks the first stage in motor development. By the second stage, at about five months, the baby has usually mastered the co-ordination necessary to sit up, and within another three months is able to stand if supported. Progress is now rapid and, in the third stage of development, the baby begins crawling on his stomach until, during the fourth stage at about twelve months, he has graduated to all fours.

At first, the baby holds a rail or the side of his cot to steady himself before taking a few tottering steps. By about eighteen months, however, he has gained enough control of his knees and legs to make a tentative attempt at walking on his own. At this stage, a baby often reverts to crawling again because for him it is a quicker mode of transport. This is only a minor setback, and the baby is soon back on two feet and walking without apparent difficulty. As we have seen, walking is a consciously learned ability achieved through trial and error, and it is possible to ‘forget’ the ability. Injury can render someone temporarily paralysed, and motor responses and muscle co-ordination have to be relearned.

Muscle tone

Muscle tone is the sum of the contracting and relaxing impulses from the spinal cord, cerebellum and the brain centres. Whatever position we adopt, muscles are in some state of co-ordinated contraction, maintaining posture and moving our chest and diaphragm to enable us to breathe. Damage to the central nervous system can destroy equilibrium. In paralysis the muscles receive no muscle impulses and cannot contract. In time they atrophy. The muscles go into spasm: the limbs adopt fixed positions and cannot be moved by voluntary action.

The brain can only direct movement if it has precise knowledge of the position of the limbs, joints and muscles, and damage to a nerve can cause loss of sensory input as well as the loss of motor control. This renders standing and walking difficult because movements are unco-ordinated.

Practice improves muscle control and trains the body to undertake many skilled actions at work or play. The skill of the mechanic, surgeon, musician or gymnast all depend on improving the control and the range of movement. Range of movement is limited by the connecting surfaces of the bones in the joints, the looseness or strength of the joint ligaments, and the size and condition of the muscles acting on that joint. Constant exercise by the young can lengthen joint ligaments, as happens with ballet dancers. Increased muscle power is then required to control that joint.

Forces

A particular movement depends on the type of joint. Muscles can rotate, extend or flex and this action is often reflected in their names. However, essentially all movement depends on the contraction of a muscle in a manner that mimics the action of a lever against a fulcrum. Because muscles are working against gravity they often function by controlled relaxing.

Common movements

Even when we are standing still numerous muscles are contracting to maintain our posture. Without these muscles acting on the bones connecting our joints we would collapse, as does a paralysed person. The line of the centre of gravity passes behind the hip joint, in front of the knee joint and in front of the ankle joints. To maintain our position our buttock, or gluteal, muscles, thigh muscles – the quadriceps – and the muscles in front of our legs are tensed. These are called the prime movers, and are balanced by the muscles, called antagonists, contracting at the back of the leg.

Sitting and walking

If we want to sit down, we bend slightly to tip our centre of gravity forwards and then our thigh muscles gently lower us into the seat, while our buttock muscles relax. Meanwhile, our back muscles and arm muscles are also assisting by maintaining the position of the head, arms and trunk. To stand we lean forwards and tighten buttock, back and thigh muscles. To walk, we hold our body balanced – let us say on the left leg – by the buttock muscles. The right leg swings forwards and the body’s centre of gravity moves forwards with it. The body is momentarily unbalanced and the right leg straightens to strike the ground with the heel, then rolls forwards cmto the ball of the foot. Balance is restored until the left leg swings forwards and the big toe of the right foot presses against the ground to propel the body forwards again and repeat the sequence. Thus, in order to walk we perform a complex series of co-ordinated muscular movements.