In any standing position, whether easy or forced, the weight of the body is borne by the lower tarsal joints and is transferred from them to the tuberosities of the os calcis rather than to the heads of the weaker and longer metatarsal bones. The line of gravity falls slightly outside the almost sagittal axis of the lower tarsal joints, and so tends to cause eversion of the foot, or if the foot is fixed on the ground causes the axis of the leg, reckoned from the ankle, to have an outward direction. This eversion becomes greater the nearer the feet are to each other, but it never becomes excessive, since the legs are fixed just the right distance apart at the hip joints. The line of gravity, the resultant of the equal weights resting on the two legs, generally falls just in front of the axes of the ankle joints, i.e., the legs have a forward direction from the ankle joint and form an angle of about 7° with the vertical. The ankle joints are fixed in position by the plantar flexors, especially gastrocnemius and soleus. But these muscles have to work but little, as muscular activity is again economised by the mechanical structure of the joint. This is such that while the axes of the ankle joints converge somewhat forward, the planes of dorsal flexion diverge somewhat outward, so that as long as the knees are more or less fully extended it is impossible for dorsal flexion to take place in both ankle joints at the same time. Further resistance to excessive dorsal flexion is caused by the broad anterior portion of the upper articulating surface of the astragalus, which prevents the corresponding articulating surfaces of the tibia and fibula from gliding far forward, since this would necessitate some separation of these bones and stretching of the ligaments between them.

All that has been said up to this point refers to a symmetrical standing position in which both legs share the body weight equally. In an easy standing position, however, we often support ourselves on one leg only. In this case the lower tarsal joints of this leg come into a position of maximum eversion, and the plane in which flexion in the ankle joint takes place, instead of being directed forward and outward, is directed straight forward or even forward and inward. The supporting leg is directed upward and outward from the ankle, is fully extended at the knee, and is fixed in the adducted position at the hip, and the upper part of the body forms an angle with it at the hip to prevent the line of gravity from falling too far out. The other leg may be placed in many different positions, and is used merely to maintain balance.

When we begin to walk we bring the foot of the supporting leg into a position of abduction in the lower tarsal joints by innervation of the muscles which evert the foot, this eversion being combined with a certain amount of dorsal flexion in the ankle. As a result the astragalus is rotated somewhat inward, and the axis of the ankle joint, which was previously directed inward and forward, becomes transverse, or is even directed backward and inward. When the lower leg is dorsally flexed at the ankle the flexion is not forward and outward, but straight forward, or even somewhat forward and inward, so that the line of gravity is easily transferred when the other foot becomes the supporting foot. The knee of the supporting leg is kept extended, whereby the inward rotation of the femur is somewhat increased. The calf muscles come slightly into play to control dorsal flexion at the ankle.

Simultaneously an impulse is sent to the abductors of the hip joint of the supporting leg, gluteus medius and minimus, which, as the foot is fixed on the ground, rotate the pelvis on its sagittal axis and so raise the hip of the moving leg. In order to maintain balance and prevent the line of gravity from falling too far out on the side of the supporting leg the extensor muscles of the back on the opposite side are brought into play, and the line of gravity then falls within the supporting surface.

In abduction the distance between the points of attachment of the ilio-femoral ligaments is shortened and the ligaments become lax, but are stretched again because the heavier part of the pelvis behind the hip joint rotates backward and the joint thus becomes further extended. As a result the moving leg, as soon as it is lifted from the ground, is carried forward quite independently of any flexion at the hip joint. The lifting of the foot from the ground on this side is not only brought about by the rotation of the pelvis on its sagittal axis, but even by the innervation of the long flexors of the knee (semi-membranosus, semi-tendinosus, and biceps) and by the flexion of the knee. A slight inward rotation of the leg accompanies this flexion, so that the foot has an almost sagittal direction.

In this way a change from standing to walking has been effected. In walking the centre of gravity of the body is continually being moved forward, and is first on one side of the sagittal plane and then on the other, each leg alternately providing support. This action of walking we divide into steps, and every step may be considered as consisting of three almost equal periods. An average step is 66 cm. and lasts 0.6 second.

With Dalla Rosa we may arbitrarily number the periods of a step in the following way: -

The first period lasts from the moment when the hanging and swinging leg has reached the vertical position and is in profile with the supporting leg to the moment when the heel of this hitherto swinging leg touches the ground.

The second period includes the time during which both feet touch the ground, however small the surface in contact.

The third period lasts from the moment when the posterior leg leaves the ground and becomes the moving leg to the moment when it reaches the vertical position in profile with the supporting leg.

During the first period the supporting leg with extended, or almost extended, knee performs a dorsal flexion at the ankle. At the same time the hip is extended to its maximum the abduction remaining in this hip from the standing position is overcome, and the pelvis rotates, at the hip of the supporting leg, on its vertical axis, so that the opposite hip is carried considerably forward. This movement is performed by the anterior portions of gluteus medius and minimus.* Dorsal flexion at the ankle, which up to this point has been straight forward, is now directed slightly inward, and the supporting leg rotates slightly outward, having previously been rotated a little inward. These changes in movement are due to the marked inversion in the lower tarsal joints which now replaces the preceding eversion. As the supporting leg leans farther forward the calf muscles are innervated to control the movement.