Chapter 18: Posture and locomotion
Posture
Standing involves a series of relatively immobile positions separated by brief intervals of movement during which swaying occurs. When a subject is in the easy standing position, few muscles of the back and lower limbs are active during the immobile periods. The position of the line of gravity, which is determined by the distribution of body weight, is important in determining the degree of muscular activity involved in maintaining all phases of posture. The line of gravity extends superiorly through the junctions of the curves of the vertebral column and inferiorly in a line posterior to the hip joints but anterior to the knee and ankle joints (fig. 18-1). When a subject is in the easy standing position, the hip and knee joints are extended and are in their most stable positions. Because the line of gravity passes posterior to the hip joint and anterior to the knee joint, the weight of the body tends to hyperextend these articulations. At the hip this is resisted by the iliofemoral ligament and at the knee by the ligamentous apparatus of the knee and the action of the hamstrings. Similarly, the generally anterior carriage of the weight of the body tends to cause dorsiflexion at the ankle joint, and this is resisted by contraction of the calf muscles. These muscles, along with paraspinal muscles are the only absolutely essential muscles for maintenance of the upright posture. Lateral stability in standing depends chiefly on the fascia lata, iliotibial tract, fibular collateral ligament of the knee, and tibialis anterior.
Locomotion
Locomotion is very complicated, is laboriously acquired, and becomes almost entirely automatic. Disturbances of gait are important signs in many disorders of the central nervous system.
When a subject is walking on level ground, the movements of the lower limbs may be divided into "swing" and "stance" phases, which are separated by heel-strike. The swing phase occurs when the limb is off the ground, and the stance phase when it is in contact with the ground and is bearing weight. A cycle of walking is the period from the heel-strike of one foot to the next heel-strike of the same foot.
At the time of heel-strike, the ankle is in slight dorsiflexion (fig. 18-2) and the anterior leg muscles contract in order to prevent the forefoot from slapping down. The muscles progressively relax to lower the foot and weight is transferred up the lateral side of the foot and then across the ball of the foot as the stance phase progresses. The last part of the foot remaining on the ground is the medial part of the ball of the foot and the great toe. This is termed “toe-off” and begins the swing phase.
At the time of heel-strike, the knee is slightly flexed on that side, with contraction of the quadriceps necessary to prevent collapse. As the stance phase progresses, this knee straightens, resulting in some upward motion of the trunk.
During the stance phase, the thigh abductors, particularly the gluteus medius and minimus, are active on the side of the leg that is in contact with the ground. This is in order to resist the tendency of gravity to cause downward movement of the hip on the opposite side (fig. 18-3). Additionally, the paraspinal muscles and lateral trunk muscles are active on the side of the swinging leg in order to resist downward movement of the iliac crest on that side.
The center of gravity moves upward and downward twice during each cycle, as is indicated by the bobbing up and down of the head. That is, the body is lifted as each limb is extended during its stance phase (fig. 18-2). There is also a slight side-to-side movement. The basic movements involved in walking are (1) flexion and extension at the hip, knee, and ankle joints and at the front part of the foot; (2) abduction and adduction, chiefly at the hip joint (fig. 18-3); and (3) rotation, mainly at the hip and knee joints.
Additional reading
Basmajian, J. V., Muscles Alive, 4th ed., Williams & Wilkins, Baltimore, 1978. Electromyographic findings, including chapters on posture and locomotion. Carlsoo, S., How Man Moves, trans. by W. P. Michael, Heinemann, London, 1972. Kinesiological studies, including discussions of standing, sitting, and gait.
Inman, V. T., Ralston, H. J., and Todd, P., Human Walking, Williams & Wilkins, Baltimore, 1981. Technical account, including kinematics, kinetics, and muscles.
Question
18-1 Where is the line of gravity in relation to the hip, knee, and ankle joints?
Figure legends
Figure 18-1 The line of gravity passes between the mastoid processes, in front of the shoulder joints, through or behind the hip joints, and in front of the knee and ankle joints. (Based on Carlsoo and on Basmajian.)
Figure 18-2 The swing and stance phases of the right lower limb.
Figure 18-3 Abduction at the hip joint, viewed from behind. A, Normally, when weight is borne on one (e.g., the right) limb (during the stance phase of walking), the pelvis tends to sag on the free, or swing (left), side (because of gravity). This is counteracted by abduction of the hip on the stance (right) side, chiefly by strong contraction of the (right) gluteus medius, which acts on the pelvis from a fixed femur. B, When abduction of the (right) hip is interfered with on the supported side (positive Trendelenburg's sign), e.g., by dislocation, fracture, or paralysis, the pelvis sags (arrow) on the unsupported side. (Based on von Lanz and Wachsmuth.)