At the end of the first period when the heel of the swinging leg first touches the ground the ankle joint is slightly plantar flexed, the knee almost straight, and the hip somewhat flexed. The forward and inward rotation of the pelvis on the vertical axis of the hip of the supporting leg is compensated by the outward rotation at the hip of the swinging leg, so that the foot is directed straight forward.

At the beginning of this period the pelvis was higher on the side of the moving leg than on that of the supporting leg; at the end of the period the opposite is the case. During this period the pelvis as a whole, the upper part of the body and its centre of gravity have been lowered and have fallen somewhat forward, and the centre of gravity has passed the medial sagittal plane and lies over the forward foot. The movement has been carried out chiefly by means of gravity, and the muscles have controlled and directed the momentum so acquired.

* In this connection I would mention that gluteus maximus is essentially an extensor of the hip and only a weak abductor and outward rotator, that gluteus medius as a whole abducts, that its anterior portion rotates inward and flexes and its posterior portion rotates outward and extends, and that gluteus minimus has a similar effect, but is especially an abductor and inward rotator. Gluteus maximus comes strongly into action in hill-climbing, comparatively little in ordinary walking. The other two, on the contrary, are very active in walking, since they help to rotate the pelvis on its vertical axis and to fix it to the supporting leg in the third period.

Tensor fasciae latae is a flexor of the hip. During flexion it works with ilio-psoas, and by its power of inward rotation counteracts the outward rotating effect of the latter muscle.

The outward rotation of the swinging leg during the first period is performed by the posterior portions of gluteus medius and minimus. Flexion of the hip is produced principally by ilio-psoas, helped by the adductors, which are also to a certain extent flexors. Both ilio-psoas and the adductors aid outward rotation. The almost complete extension of the knee which occurs at the end of the first period is ascribed by Dalla Rosa to the vis inertiae of the mass movements, assisted somewhat by tensor fasciae latae, but for my part I consider that quadriceps femoris is especially active here.

The second period during which both feet touch the ground begins when the heel of the hitherto moving leg first reaches the ground. This leg, as already stated, is slightly plantar flexed at the ankle, extended at the knee, flexed and rotated outward at the hip. The foot of the posterior leg in many cases rests even at this point with the whole sole on the ground, is inverted at the tarsal joints, and dorsally flexed at the ankle; the knee and hip are fully extended, the hip at the beginning of this period standing a little higher than that of the other side. The rotation of the pelvis on the vertical axis of this hip has carried the other hip joint strongly forward, but the trunk has compensated for this by rotating in the opposite direction and is turned straight forward. The energy of the mass movements is checked by placing the foot on the ground (and is thus converted into heat), but is still present in the parts above the feet, and is greater in the parts farther removed from the feet. The centre of gravity during the second period also moves forward, but before the end of the period, at least in many cases, the upward movement begins, which however really belongs to the third period. The tarsal joints of the anterior foot now succeed the corresponding joints of the posterior foot as centres of the rotatory movements.

Some remains of the momentum of the first period undoubtedly influence the second period, but in my opinion this period entails a considerable amount of muscle work. The muscles of the forward leg take a specially active part here. The anterior group works strongly to prevent an increased and excessive plantar flexion at the ankle and the muscles which evert the foot act on the tarsal joints. The vis inertia? of momentum already acquired helps movement forward of the anterior leg, with its ankle dorsally flexed till it reaches a position at right angles to the foot placed on the ground. Quadriceps femoris must be brought into action to prevent flexion of the knee (which always takes place as soon as we place the foot on the ground) from becoming too great, and also later to extend the knee. I cannot agree with Dalla Rosa that quadriceps femoris does not assist in lifting the body provided this begins before the end of the period. Gluteus maximus prevents the rotation of the pelvis in the direction of the movement being continued from the first period, and, together with semi-mem-branosus, semi-tendinosus and biceps, prevents flexion of the hip. The extensors of the spine prevent the trunk falling forward in the direction of the movement.

The heel of the posterior leg leaves the ground during the second period (if it has not already done so, as sometimes happens in quick walking) by rotating on a transverse axis which passes through the heads of the metatarsals, the ankle being slightly plantar flexed. For my part I consider that this leg, although flexed at the knee, acts as a lever while flexion is maintained or its degree determined by the extensors and flexors working together, and that the special assistance given by the posterior leg during the second period, at least in walking with fairly long steps, begins before the alteration in the height of the hips has taken place.

During the third period the body once again is supported by one leg only, in this case by the anterior leg, while the other, the toes of which have just left the ground, swings forward to the vertical position in profile with the supporting leg. During this period the centre of gravity must again be raised to its maximum height by muscular action, i.e., by extension of the knee of the supporting leg. This raising of the centre of gravity, according to the brothers Weber, who have contributed much to our knowledge of the subject of walking, amounts on an average to about 32 mm., and with a body weight of 70 kilos involves 2,240 kgm. of work in 1,000 steps. I consider that it is chiefly quadriceps femoris that produces this extension, that the anterior leg muscles contract strongly to produce dorsal flexion at the ankle, and that the peronei maintain the position of eversion in the tarsal joints, which means, when the sole of the foot is on the ground, that they maintain the abduction of the leg at the tarsal joints. The femur, which in the second period was rotated outward, is again rotated inward by the complete extension of the knee, while the lower leg is flexed. Gluteus maximus is, in my opinion, the most active muscle in extending the hip, but semi-membranosus, semi-tendinosus, and biceps take part to some extent. The pelvis is simultaneously rotated at the hip of the supporting leg partly on its sagittal axis (by all the glutei), so that the other side is raised, partly on its vertical axis (by the anterior portions of gluteus medius and minimus), so that the side of the swinging leg is carried forward. The extensors of the spine on the side of the raised hip work to prevent the upper part of the body from leaning too far outward during the rotation of the pelvis on the sagittal axis, and also to maintain balance.