This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
As locomotion is one of the main functions of the lower extremity, derangements of this function are to be explained by a knowledge of the normal action of its mechanism. The means by which support is accomplished have already been explained in the maintenance of equilibrium. Locomotion embraces walking, running, jumping, etc. Of these walking is the fundamental movement, and the others are only amplifications and modifications of it. In slow normal walking on a level surface the thigh moves on the pelvis, the leg on the thigh, the foot on the leg, and the toes on the rest of the foot. These movements are almost solely in an antero posterior direction, there being almost no lateral or rotary movements; these begin only when the actions become violent and irregular, such as are necessary in running, overcoming obstacles, etc. It is for this reason that a person may have no limp when walking slowly, but a very perceptible one when walking rapidly. There is always a small amount of lateral motion present which varies with the individual and the sex.
As slow walking necessitates mainly anteroposterior motion, it can be explained by viewing the body laterally.
In ordinary walking the body inclines forward 5 degrees, in fast walking 10 degrees, and in running about 22 degrees (Weber). In walking (Fig. 499, A) the body is inclined forward and at the same time one leg begins to advance (the right). This causes flexion of the left ankle and flexion of the right hip (Fig. 499, B and C). As the right foot touches the ground it extends and the right knee flexes to avoid the shock of impact (Fig. 499, D), the left knee begins to flex and flexes more than the right in order for the left foot to swing clear of the ground while being advanced; if this was not done it would be necessary to raise the limb by tilting the pelvis up on that side. The left continues to advance flexed while the right gradually extends (Fig. 499, E), and finally when the right is fully extended the left is likewise fully extended (Fig. 499, F) and strikes the ground with the foot about at a right angle to the leg.
The object of flexion of all three joints is, first, to avoid shock in impact, and, secondly, to raise the free foot and allow it to swing forward clear of the ground. The object of extension is to push the body forward.
The hip-joint. flexes to an extent proportionate to the length of the step (Fig. 499, A). If this joint is put out of use by being ankylosed, first, the shock of impact is more severe, no flexion being possible; second, the limb can only be brought forward by bending the pelvis on the opposite hip, and, to a certain extent, the trunk above backward; third, to aid still more to advance the foot forward the pelvis will be rotated laterally on the opposite hip. This causes a swaying of the trunk backward and forward and a side swing or waddle of the pelvis. Fourth, the forward propulsive force is weakened by the loss of the hip extensors. The knee, like the hip, lessens the shock of impact by flexing. It raises the foot clear of the ground as it is swung forward, and it aids propulsion by extension. If ankylosed, shock is increased, onward propulsive force is lost, and it is necessary to tilt the pelvis upward in order to raise the foot from the ground and allow it to swing forward. This abducts one or both legs and causes marked waddling. The ankle also reduces shock and gives propulsion; if ankylosed, shock is increased and propulsion weakened. This is the least necessary of the three joints and to substitute it artificial appliances are useful, so that in quiet walking limp may be almost lacking, but violent and complicated movements are to a large extent impossible. The toes, especially the big toe, aid in propelling the body forward.
Fig. 499. - Walking. Tracings from photographs by Muybridge.