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.
The human body usually occupies one of three positions: standing, sitting, or lying. The functions of the lower extremity are to afford support to the body and accomplish locomotion, therefore any disturbance of the normal relation of the extremities to the trunk interferes with the carrying out of those functions and proper support is not given and locomotion is imperfect. In such cases the positions assumed in standing, sitting, and lying are abnormal, often to an extent sufficient to constitute serious deformities, and locomotion, as in walking or running, is seriously impaired or rendered impossible.
The connection of the lower extremities with the trunk is through the means of the pelvic girdle and spinal column; therefore the pelvis and vertebrae above exert a marked influence on the extremities below and must be taken into consideration. The normal upright position of man is obtained by maintaining a proper balance. This balance can be disturbed either anteroposteriorly or laterally. The lower limbs are placed laterally, one on each side; this gives greater stability in that direction, so that when a person falls it is usually in a forward or backward direction rather than toward the side.
In the upright position the highest joint is that between the occiput and atlas and the lowest that of the ankle; to enable the body to be in a state of rest, in the upright position, with the use of the least amount of muscular exertion these joints are placed almost vertically one above the other. For the same reason if any part of the skeleton lies in front of a line joining the condyle of the occiput with the astragalo-scaphoid joint it is counterbalanced by a projection toward the opposite side. Thus the anterior curve of the cervical region is followed by the posterior curve of the dorsal; the anterior of the lumbar, by the posterior of the sacral. The hip-joint has its centre of motion slightly behind the centre of gravity as has also the knee. A vertical line through the center of gravity must fall within its base of support. This latter is formed by the arch of the foot; its two ends are the tuberosity of the calcaneum posteriorly and the head of the first metatarsal bones anteriorly. The body is in the position of greatest stability when the centre of gravity is midway between those two points, which is when it passes through the astragalo-scaphoid joint. As the line of gravity passes from the centre of the arch toward the ends, equilibrium becomes more unstable until, when it passes beyond them, it is lost and the body begins to fall. In maintaining a normal erect posture hyperextension of the hip-joint is prevented by the anterior or iliofemoral ligament; hyperextension of the knee is prevented by the lateral, posterior, and crucial ligaments (Fig. 494, A). The main muscular efforts required are those of the muscles of the back of the neck to hold the head level, owing to the head being heavier anterior to the condyles, and the muscles of the back of the leg to prevent the dorsal flexion at the ankle, due to the centre of gravity falling in front of the ankle-joint. When a person falls asleep in the erect posture the head drops forward and when a soldier is shot his calf muscles give way and he falls forward on his face.
Fig. 494. - Anteroposterior equilibrium.
Fig. A. - The body in the erect position; the centre of gravity c is about in the upper lumbar region; d-e is the base of support. The vertical line a-b through the centre of gravity c passes through the occipito-atloid joint above, in front of the sacro-iliac joint g, the hip-joint h, the knee-joint i and the ankle-joint j and falls between the points of support d-e, passing through the astragaloscaphoid joint. Hyperextension of the hip and knee is prevented by ligaments.
Fig. B. - When the trunk is inclined forward by bending at the hip-joint, the increased projection of the head and upper portion of the trunk in front of the centre of gravity is counterbalanced by the increased projection of the hips and lower portion of the trunk posteriorly. The vertical line through the centre of gravity still cuts the base of support d-e and the body remains in a state of equilibrium.
Fig. C. - When the body inclines backward, hyperextension at the hip is prevented by ligaments; therefore, when the vertical line a-b through the centre of gravity c falls beyond the base of support d-e, the body is in unstable equilibrium and it falls.
Fig. D. - If the body, as occurs in some diseases and injuries, is inclined so far forward as to bring the vertical line a-b through the centre of gravity c, in front of the base of support d-e, then it is in a state of unstable equilibrium and additional support is used, in the form of a cane, to prevent falling forward.
Deformities of the spine affecting its curves have already been alluded to (page 478). When the spine is the part affected it is usually the case that the secondary curve fully compensates for the increased primary one, hence there is no necessity for any change in the position of the joints below, and we find people with marked deformities of the spine who are normal from the waist down and who stand and walk perfectly well. Occasionally a case presents itself in which the secondary curve has not entirely compensated the primary one and then the body is bent at the hips until the centre of gravity is brought over the base of support (Fig. 494, B). If the deformity throws the centre of gravity too far back, by bending the hips it will be brought forward, but if for any reason, such as ankylosis, flexion is impossible, then it cannot be corrected at the hip-joint, and therefore in such cases equilibrium is unstable and the body falls (Fig. 494, C). If from deformity the centre of gravity is thrown so far forward as to fall beyond the base of support then a cane or crutches is required (Fig. 494, D).
Fig. 495. - Lateral equilibrium.
Fig. A. - The body being erect, a vertical line a-b through the centre of gravity c falls midway between the ankles or base of support d-e and the body is in stable equilibrium.
Fig. B. - The trunk being inclined to the right, the centre of gravity c is shifted to the right and a vertical line a-b through it falls still within the line of support d-e and the upright position can still be maintained.
Fig. C. - If the relative length of the two legs is altered, as by placing a block beneath one of them, the pelvis and upper portion of the body inclines to the opposite side, until a vertical line a-b through the centre of gravity C falls beyond the extremity of the base of support d-e and the body is in a position of unstable equilibrium.
Fig. D. - The body in a position of rest. The weight is borne mainly on the left leg; the right side of the pelvis falls until the iliotrochanteric and iliotibial bands are tense, when the position can be maintained without muscular effort.
When the hip-joint is involved it is never affected by hyperextension (the iliofemoral ligament prevents that), but always by flexion. This throws the centre of gravity forward; to bring it back a secondary curve is produced in the lumbar region, and we have a condition of lordosis established; if this is insufficient then the knees may be partly flexed, and if both are insufficient then artificial support or crutches must be used. This is the reason why flexion is sought to be avoided in the treatment of coxalgia, and why osteotomy is done when the hip is ankylosed in a flexed position. Practically speaking there is no efficient compensation occurring at the sacro-iliac joints, the pelvis moving with the lumbar vertebrae.