Functional anatomy of the foot: A cybernetic model

Abstract

As a result of evolution and man's development of an upright posture, the centre of gravity of the body has been raised and at the same time there has been a reduction in the area of support on the ground. The foot can thus be regarded as an organ for the control of balance in order to help maintain an erect position and the mechanism of the deambulation. This is substantiated by the neurophysiological arrangement of the representation of the motor and sensory areas of the cerebral cortex and by the nervous proprioceptive representation of ligamentous and para-ligamentous structures particularly of the sinus tarsi. The sensory area of cortex representing the foot is larger than that for the hand (the foot as a sensory organ). This is in contrast to the motor cortex where there is a larger area for the hand. Thus there are more cortical afferent stimuli for the foot (constant antigravity control) than for the hand, and in contrast the corresponding motor activity is greater in the hand. The hand responds with a multiplicity of movement to a small number of afferent stimuli; in contrast the foot responds to control support as a result of many more afferent impulses. As shown by biomechanical research one of the essential movements of the foot is active eversion which stabilises the medial side so that load bearing is possible. The change to the upright posture required a major effort to overcome gravity and structural modifications in the supporting structures of the human foot. External or environmental forces and internal or genetic factors have an influence on the development of bone and joint surfaces. For the control of the position of the body and movement the flow of information from the ground enters through the receptors of the load bearing foot and are relayed to the cerebral cortex (the foot as a sensory organ). The forces derived from the motor response are exercised on the load bearing surface (the foot as a motor organ). It is the balance between extrinsic and intrinsic forces that induces variations in the function of the foot (the foot as a cybernetic organ). It is evident how shape and function of the foot are related. As foot surgeons we have to bear this in mind, both when we perform surgical operations and when we review our surgical failures. This is especially important in children where we can modify the development of the foot from normal to abnormal. To summarise, the foot becomes a functional biomechanical structure at the commencement of wheight bearing by the toddler whereas before that stage it was a sensory end organ for proprioceptive information. In the ambulant child or adult these two components becomes fused. As a consequence when one studies the many aspects of foot pathology it is essential to remember that the complexity of the situation cannot be tackled by the Foot Surgery only. The help a neurologist, histologist, biomechanical engineer and orthopaedic specialists who work on the knee, hip and spine must all collaborate for the analysis of the functional impairment and pathology.