The ability to move about enables us to react to our surroundings, initially of important survival value for the human race – for example, to search for food, escape from dangerous situations, and to seek shelter from the elements.
In most four-limbed creatures both the front- and hind-limbs are used for locomotion. The hind-limbs are usually stronger than the fore-limbs, although in birds, it is usually the fore-limbs – wings – that are stronger. Humans are unusual in this respect. Their hinds-limbs suffice to move them about. This bipedal method of locomotion has freed the arms and hands to become grasping and manipulative organs. The arms and hands have developed into instruments of great precision, enabling us to use tools and make things. It is possible to make a greater range of movements with the arms than with the legs. The fingers have developed differently from the toes, as required by their different function.
Shoulder and arm
Strong muscles connect the shoulder blade (scapula) and collar bone to the body. They give strength and mobility, but the mobility is at the expense of stability as the shoulder is susceptible to dislocation. The upper arm bone fits into a shallow socket in the scapula; this type of ball-and-socket joint allows a greater range of movement. The lower arm is hinged to the upper arm, and allows movement in two planes – backwards and forwards, and sideways – and also allows the limb to rotate. As a result of these movements of the shoulder and arm, the hand can be placed in the large number of different positions needed to perform its multivarious functions.
The human hand is one of the most highly developed and specialized ‘paws’ in the animal world. Although its pentadactyl (five-digit) structure is similar to that of many other mammals, the specialization of the muscles enables it to do far more. The hand is able to grasp because the thumb can be opposed to each of the other fingers. This occurs only in humans and apes and is essential for working with tools. The skin of the palm is ridged and firmly attached to the underlying muscle and tissue, which improves the grasping and feeling surfaces. Embedded in the skin at the tips of the fingers are tactile corpuscles that can detect touch, pressure, temperature and pain with great sensitivity. This enables us to discriminate between the feel of various materials and objects. Blind people using Braille can even read with their fingertips.
A complicated pattern of ridges on the skin of the fingers improves our gripping ability, and is also unique to the individual. No duplicate patterns of fingerprints have been detected in millions of worldwide investigations.
Hip and leg
The pelvis provides a strong, firm support for transferring the weight of the body to the legs. Furthermore it provides a strong fulcrum for the often rapid movements of the legs. Because the hip joint requires strength and stability, it lacks the shoulder’s wide range of movement.
The powerful gluteus maximus, the muscle of the buttock, is attached to the back of the pelvis, to help maintain an erect posture.
The leg is the running, jumping and standing limb. The bones are long and strong and form effective levers on which the powerful muscles act. The knee is a hinge joint that allows movement only in one plane, that is. Backwards and forwards, to accommodate any irregularities of surface, and to adjust the position of the foot, as in kicking.
One interesting ‘design feature’ of the leg is that the large and powerful muscles of movement are concentrated as near as the trunk as possible, with long tendons transmitting the pull to the relevant bones. These tendons can be felt rippling under the skin when the ankle is bent to raise the foot. This arrangement reduces the inertia needed to swing the limb backwards and forwards while running, compared to a leg in which the muscle bulk is concentrated near the end. Similarly the toes are moved by muscles in the lower leg, which are connected to the bones in the foot by long tendons.
In theory the function of the foot, which is to provide a base for the upright skeleton and the leverage necessary for walking, could be fulfilled by a simple strong bone. However this would make walking on a bumpy surface impossible.
The foot is segmented which provides the necessary flexibility. It is therefore arched – otherwise it would not be stable. The foot has three arches, one along each side and one across its width, held together by strong ligaments. The soles of the foot are constructed like the palms of the hand, and are very sensitive. Walking barefoot induces the formation of calluses, which protect the sole of the foot.