The term "normal foot" has many different interpretations. This article reviews some of the ways in which the word "normal" has been used historically to describe the foot. Also discussed are the problems of attempting to determine what should constitute a normal foot and proposed criteria for distinguishing between the normal and the pathological.
The authors present a methodology to measure the frontal plane angular and linear displacement and the transverse plane angular displacement of subtalar joint movement. This method is combined with a modification of the Kirby method for determining the transverse plane projection of the subtalar joint axis onto the plantar foot. A mathematical model is then used to construct the subtalar joint axis into a three-dimensional linear equation. Data are obtained from an in vivo series of 62 feet that indicates that within acceptable clinical errors of measurement the subtalar joint is a ginglymus type of joint that moves around a single fixed axis. Results also indicate that the subtalar joint axis is more superior and lateral to the neutral foot than any previous studies on cadaver feet have shown. Finally, the authors show that once the subtalar joint axis can be accurately located, the torque on the joint axis produced by ground reactive forces and muscular forces can be computed.
The etiology of hallux abducto valgus has been the topic of much study. The authors of this study have attempted to investigate an association of anomalous insertions of the tibialis posterior tendon and their relationship to hallux abducto valgus. The authors present evidence that such anomalous insertions are normal, but may contribute to the development of the deformity because of variations in those insertions. A biomechanical model for that contribution is proposed.
Motion in the ankle, subtalar, midtarsal, and first metatarsophalangeal joints has been well documented. However, motion in the first metatarsocuneiform, the first cuneonavicular, and the first interphalangeal joint has not been addressed. Motion in these joints has not been documented because many believe that little motion occurs at these joints, and because of the difficulty in assessing motion at these joints. Using two-dimensional motion analysis, the authors present sagittal plane ranges of motion occurring in the first metatarsophalangeal joint, the first metatarsocuneiform joint, the medial cuneonavicular joint, and the first interphalangeal joint during the propulsive period of gait. This pilot study indicates that sagittal plane motion between the navicular and calcaneus and between the first metatarsal and first cuneiform are very mild and inconsistent. However, plantarflexion motion between the first cuneiform and the navicular is significant and comprises most of the plantarflexion motion of the first ray during propulsion. Motion in the first interphalangeal joint is slight during the first 80% of the propulsive period but shows slight-to-moderate dorsiflexion during the last 20% of the propulsive period of gait.
The effects of muscular activity on the distribution of forces under the foot, as well as within the foot, are of great importance for determining the mechanisms of foot pathologies. Limited data exist concerning muscle forces during the gait cycle and the effects of muscle forces conveyed to the ground-reactive forces of the foot. The authors developed a cadaveric loading system to determine the effects of force applied to the Achilles tendon on the forefoot-to-rearfoot loading relationship in eight cadaveric specimens. The study indicated that, during axial loading of the tibia, force was inherently transferred from the rearfoot to the forefoot. However, the observed forefoot-to-rearfoot loading relationship did not match the predicted loading relationship from a rigid-body diagram, as would be observed in a class I lever. The results indicated that, as the force was increased on the Achilles tendon, the change in loads on the forefoot and rearfoot was not linear. Specimens with calcaneal inclination angles greater than 20 degrees demonstrated a more linear increase as compared with those with inclination angles less than 20 degrees.
One of the reasons that high heels may contribute to the formation of halux valgus is that the wearers pronate during propulsion. This pilot study was performed to determine whether relocation of the heel under the counter of a fashion high-heeled pump could change the degree of pronation of the foot during the gait cycle. The authors report that more foot stability was experienced by the subjects when the center of the heel was offset between 2 and 4 mm medial to the center of the heel counter. This study is designed to promote further research into whether the shoe industry should change the design parameters of high-heeled fashion shoes in order to improve foot function.