ABSTRACT
We take contact of our body with the ground for granted. Except for brief moments during running or jumping, or falling from a tree or ladder, those of us who are not skydivers are rarely without physical contact between our body and the support surface of the earth. The acceleration of earth gravity pulls us downward and the reaction force provided by the surface of the earth or the floors of structures arrests our downward motion and provides a physical base on which stance and locomotion can be carried out. What a scale measures as weight is actually the contact force it provides in supporting the body against the pull of gravity. In the absence of an effective gravitational pull on the body, walking and running would not be possible 122unless one were stepping in place on a treadmill while being mechanically pulled toward its surface by elastic tethers. Locomotion on the moon, for example, is greatly constrained by the acceleration of moon gravity which is only one-sixth of that on earth. This has consequences for both walking and running. Walking is basically pendular in character and is limited by gravitational force. The maximum possible frequency of walking is proportional to the square root of the gravitational force. Running depends on being able to apply a backwards force with the foot without the foot slipping. The maximum backward thrust that can be exerted without the foot slipping is limited by the coefficient of friction and the normal force provided by gravity. The latter will be only one-sixth on the moon of what is on the earth. 1,2 Animals and people are bilaterally symmetric and their center of mass lies in or close to the median plane of their body. This symmetry considerably simplifies control of balance and locomotion. The upright body will not fall over during quiet stance so long as the center of mass is maintained above the support area defined by the feet (see Figure 5.1). 3 In this situation, the patterning of shear and normal forces on the soles of the two feet and their time history, when interpreted in relation to command signals to the legs and feet, provides information about the direction of body sway and the relative position of the center of mass of the body (see Note 1). Individuals who lose vestibular function as adults initially have great difficulties with balance and locomotion. During their recovery period, they learn to use the tactile cues on the soles of their feet to control their balance. This becomes easier with continued practice; however, they continue to experience difficulties when they walk on uneven or compliant surfaces under conditions of reduced illumination. 4
