Lateral Equilibrium

In the upright position the centre of gravity falls midway between the ankles of the two feet. The fact of there being two points of support adds to the stability, which increases as the feet are separated. Hence it is that falls in an anteroposterior direction are more common than sideways. In standing the weight is transmitted from the spine through the femorosacral arches (page 490) to the hip-joint, thence downward through the femur and leg-bones to the astragalus. Here we have to deal with straight lines and angles rather than curves. The spine is normally straight; the line from the spine to the hip-joint is practically straight (no bending being possible), and from the hips to the feet is likewise straight, and the centre of gravity falls midway between the ankles (see Fig. 495, A).

The two innominate bones and the sacrum form practically one solid bone, therefore the two hip-joints always maintain the same relative position to each other. When the leg is completely extended there is no lateral movement at the knee-joints. There is a marked more or less lateral movement in the subastragaloid joint which allows the leg to be inclined to one side without moving the foot. From these facts it is evident that lateral equilibrium can be disturbed by a deviation of the spine above the sacrum to one side (Fig. 495, B), and also by anything that affects the length of either leg (Fig. 495, C). The femorosacral arch is rarely affected, the most usual affection being sacro-iliac disease, or fracture, or relaxation of the sacro-iliac joint, especially in pregnancy.

The lateral equilibrium is maintained almost solely by muscular force except when a position of rest is assumed. The hip-joint is capable of both abduction and adduction, and in the erect position the ligaments on both the upper and lower surfaces of the joint are lax and do not contribute any support. When, however, a position of rest is desired the hips are moved laterally so that the centre of gravity falls on one leg, which is kept extended, the opposite hip then descends until further adduction is stopped by the ligaments on the top of the hip of the other side (Fig. 495, D). These ligaments are the outer limb of the iliofemoral (Y) ligament and the reinforcing tendinotrochanteric band, an offshoot from the rectus tendon, the iliotrochanteric band, and by the iliotibial band from the crest of the ilium to the outer tuberosity of the tibia.


For the movements of the body to be properly performed a definite normal relation of the parts to one another must be maintained, whether the body is in a state of motion or at rest. During movement the position of the bones is controlled by the muscles; when at rest, the muscles relax and the position of the bones is controlled by the ligaments. The weight of the body acts as a constant force pressing downward. For this constant pressure not to do harm it is nicely balanced on the bones and ligaments aided by the muscles. If any one of these three is disturbed the balance is altered and disability and ultimate deformity results. A distortion of a bone, as a badly united fracture, throws the weight and muscular action too much to one side and first the action of the part is impaired and then, if use is persisted in, deformity increases. When a person who is standing becomes tired they assume a position of rest, that is, their muscles relax, their joints are extended and the weight is borne on the ligaments. If, now, as in adolescents, these ligaments are weak, they give way. If in the foot, flat-foot results; if in the knees, then knock-knee; if in the back, scoliosis or lateral curvature. If it is the bones which are the primary cause of the lack of proper balance, the surgeon by osteotomy, excisions, etc., will restore them to their proper direction. If it is the muscles, as in infantile and other paralyses, transplantations, or the taking of a tendon from the strong side and placing it on the weak side, will be resorted to. If it is mainly the ligaments, these will be aided in their function by the use of apparatus, while by means of exercises the muscles are aided in regaining their normal power. The conservative surgery of the extremities has as its underlying principle the restoration of equilibrium to a part whose balance has been disturbed.