The influence of the foot-ankle complex on the proximal skeletal structures.

2 Biomechanics Laboratory, University of California, San Francisco and Berkeley, and the Department of Orthopaedic Surgery, University of California School of Medicine, San Francisco, Calif. Since the human foot seems to have evolved concomitantly with bipedal locomotion, it seems unreasonable to study its functions isolated from those of the remaining segments of the lower extremity (3). It is my belief that a complete understanding of the foot can only be obtained by relating it to the other segments of the lower extremity. Walking is more than merely placing one foot before the other. There occurs a series of axial, rotatory movements which can be measured in a horizontal plane; many of these are familiar to any observant person. I call your attention to the obvious rotations of the pelvis and the shoulder girdle. What is not so apparent is that similar horizontal rotations occur in the femoral and tibial segments of the extremities. The tibiae rotate about their long axes internally during swing phase and into the first part of stance phase and rotate externally during the latter part of stance; this motion continues until the toes leave the ground. The amount of these rotations is subject to marked individual variations. The minimal amount of horizontal rotation of the tibia in space was recorded in our series of twelve male subjects to be 13 deg. and the maximal to be 25 deg., with an average of 19 deg. These rotations are particularly interesting when one reflects that a great portion of this rotation occurs when the foot is firmly placed on the floor; normally, the shoe does not slip but remains fixed. The rotations, however, generate a torque which can be recorded from force plates. This torque is of considerable magnitude, being measured at 60 to 75 in.-lb. We are now confronted with several observations which require an explanation. First is the measured horizontal rotation of the tibia of 19 deg. (plus or minus) about its long axis while weight is being borne on the foot. Second is the fact that the foot does not slip on the floor, although it may on a slippery surface. Finally, there is a measurable torque that may be recorded. A mechanism which would explain all three of these observations would have to have the following characteristics: It would permit the abovementioned rotations to occur, but it would offer resistance to them of a magnitude such that they would be transmitted through the foot to the floor and would be recorded on the force plate as torques.