In standing the line of gravity falls inside, and as nearly as possible through the centre of the supporting surface. This surface is bounded posteriorly by a line drawn through the tuberosities of the os calcis, anteriorly by a line through the heads of the metatarsals, and laterally (under normal conditions) by lines drawn from the head of the fifth metatarsal on each side to the tuberosity of the corresponding os calcis. The centre of gravity of the trunk falls, with very slight variations, within the thorax in front of the tenth dorsal vertebra,* and equilibrium is easily maintained in different positions by means of small movements in the spine and extremities.

The erect position, with more or less extended spine, and with the hip, knee, and ankle joints fixed, is maintained partly by muscular action, partly by means of mechanical arrangements designed to economise this action.

Let us now consider the position of a person standing on both legs with the weight of the body equally distributed between them.

In an easy standing position the centre of gravity of the trunk falls behind the transverse axis of the hip joints. The weight of the trunk acts on the pelvis in the region of the promontory of the sacrum. The posterior part of the pelvis is rotated downwards, the anterior part upwards, so that the lower extremities are fully extended at the hip, or, in other words, the pelvic tilt + is reduced to its minimum. The stretching of the ilio-femoral ligaments to the maximum causes the minimum tilt to be reached before the pelvis has rotated too far backward, and it is generally held that the flexors need not come into action at all to fix the hips in an easy standing position. Since the line of gravity falls far back in this position, we stand with knees slightly bent and with feet somewhat dorsally flexed in order to bring the line inside the supporting surface.

* The centre of gravity of the body as a whole is situated in the second sacral vertebra.

+ By pelvic tilt is meant the angle between the horizontal plane and a plane through the upper aperture of the pelvis, or that plane which passes through the upper edge of the symphysis pubis, the ilio-pectineal line, and the promontory of the sacrum. This angle varies considerably, and is smallest in an easy, greatest in a forced, standing position. It depends also on the position of the legs, because the distance between the points of attachment of the ilio-femoral ligaments varies with different degrees of extension or flexion, abduction or adduction, rotation outwards or inwards of the hips. This distance is least when the thighs are neither rotated outwards nor inwards and are abducted to form an angle of 20° with one another. In this position the ligaments are least stretched, the pelvis can most easily rotate backwards, and the pelvic tilt can reach its minimum of 40° or 50°. The inclination commonly varies between 60° and 65°. By rotation outward or inward, combined with abduction or adduction and a fully extended position, the pelvic tilt can reach its maximum of about 100°.

In a strict military standing position the tilt of the pelvis is greater, the upper part of the body is inclined forward, the concavity in the cervical and lumbar regions of the spine is increased, the convexity in the dorsal region is decreased. The centre and line of gravity are moved forward, so that the latter in some cases may fall in front of the transverse axis of the hip joint. The lower extremities are fully extended at the knee, and their axes fall vertically downward and sometimes a little forward from the hip joints. The ilio-femoral ligaments are not stretched, nor are the hip joints extended, to the maximum, but their extensors must work to a certain degree to prevent flexion and a falling forward of the trunk.

Except in a forced position the line of gravity falls somewhat behind the transverse axis of the knee, and the weight of the body, though working with very short leverage, tends to cause flexion. Dalla Rosa * considers that in an easy standing position flexion is entirely or almost entirely prevented by the mechanical structure of the knee joint, for, the articulating surface of the inner condyle of the femur being longer than that of the outer, the femur is rotated inwards when the knee is fully extended with the foot and leg fixed. But flexion cannot take place in the knee until this rotation has been overcome, and, as the ilio-femoral ligaments are stretched and prevent outward rotation, gluteus medius and minimus must be innervated to produce this action. In my opinion, however, the knee, though very nearly, is not fully extended in an easy standing position, and I believe that one must bring quadriceps femoris into play to prevent flexion even in this position. This conclusion is arrived at by palpation of this muscle group. In this palpation, as in all other kinds of examination of muscles, it must be remembered that there are all possible intermediate stages between complete muscle relaxation (as under chloroform), or the weak muscular tone of a comparatively inactive muscle on the one hand and a muscle in a state of the strongest possible contraction on the other.

* Of modern writers who have thoroughly discussed walking, Dalla Rosa most deserves study. With regard to the mechanical and dynamical details relating to this subject, I have in my comparatively short essay generally followed Dalla Rosa's most valuable treatise, but nevertheless I differ from him on some points. To a somewhat greater extent do I differ in regard to the question of the general motor forces in walking. On this point Dalla Rosa's view seems to me a paradox; in any case, his way of formulating his ideas is likely to give rise to totally false impressions.

In a strict military standing position, when the knees are fully extended and extension maintained and simplified both by the line of gravity being moved forward and by the inward rotation of the femora, one can easily prove for oneself that quadriceps femoris again comes into action.