Forces acting in the forefoot during normal gait – an estimate

Abstract

Objective. To estimate forces acting along tendons and across the joints of the first and second rays of the forefoot during gait.

Design. Using recently published data on force distribution under the forefoot and relevant anthropometrical data, internal forces are calculated.

Background. It is of paramount importance to know the magnitude and direction of the forces acting within the most heavily loaded structures of the forefoot, especially when surgical treatment is envisaged. It can also be of major value in understanding the pathomechanics of certain disorders of the foot. As far as the author is aware, there is no such information presently available.

Methods. The ground force distribution during the second force peak of the stance phase was used with anthropometrical data (including lengths of lever arms of the tendons that cross the joints investigated) to determine conditions of equilibrium in the sagittal plane for each joint of the first and second rays.

Results. The flexor hallucis longus and brevis tendons exert about 52% and 36% body weight, respectively, and the peroneus longus muscle more than 58% body weight. The resultant force on the first metatarsal head amounts to about 119% body weight. The second metatarsal bone is subjected to a high bending moment with a resultant force of about 45% body weight acting on its head. The flexor digitorum longus and brevis forces are about 9% and 13% body weight, respectively.

Conclusions. The high forces acting along the flexor tendons of the heavily loaded first ray support the so-called longitudinal arch of the foot. The second metatarsal bone is also heavily loaded, but more in bending. If the first ray with its powerful toe be deprived of its function, be it through muscular fatigue, disease, or trauma, the second metatarsal bone will probably also fail.
Relevance

Such information is necessary to understand the physiological function of the foot. It might also explain the development and manifestation of certain foot pathologies. Furthermore, it is of importance when considering surgical procedures in the treatment of forefoot disorders.

Introduction

It would generally be agreed that any procedure involving surgery of a musculo-skeletal entity must necessarily take into account the influence of forces acting upon the relevant structure; and yet, in the case of the forefoot, there is an alarming paucity of such information available. Although few attempts have indeed been made to determine the forces acting along tendons and across the joints of the forefoot in normal walking, very little substantial data has become available for general use up to now. The aim of this presentation is therefore to analyse the forces acting along two principal rays of the foot, the first and the second, and to show the specific manner in which each of these two most heavily loaded rays perform in normal gait.

Such information is of paramount importance not only to understand the function of the physiological foot, but also to explain the development and manifestation of certain foot disorders. Knowledge of the physiological extent of loading of these forefoot structures would be of definite benefit for the treatment of such entities, especially when surgery is considered.

One of the first attempts to determine forces acting in the structure of the forefoot was carried out in 1927 by Abramson [1]. After measuring ground reaction forces under the metatarsal heads in standing, and assuming that muscular action alone maintained equilibrium, it was concluded that the second and third metatarsal bones were prone to fracture. However, probably the first attempt at estimating the forces that act across the metatarsophalangeal (MP) joints, and this in gait, was performed in 1979 by Stokes et al. [2]. Using geometrical data obtained from amputated feet, and by applying the ground reaction force that had been measured in six subjects during walking, the total force in the flexor tendons was evaluated. The joint force was calculated as the equilibrant of the measured ground reaction force under the toe and the corresponding force in the flexor tendons. These authors were the first to attempt estimating the forces in the flexors of the MP and interdigital (ID) joints, but they made such simplifications to the geometry of the MP joints, to the course taken by the flexor tendons, and to the points of application of the ground forces, that further investigations were deemed necessary to clarify the situation in the light of more exact anatomical data. Jacob has reported the results of such investigations using anthropometric data derived from four fresh autopsy unpaired foot specimens, and by measuring the forces during gait under the first, second and fifth metatarsal heads and under the pads of the first and second toes with piezoelectric sensors that were embedded in a flexible sandal; the gait test having been carried out on 11 healthy subjects (11 feet) with no foot disorders [3], [4], [5]. However, since the plantar surface of the foot immediately surrounding the force sensor also transmitted some ground force, by-passing the measuring device, the investigations were repeated, using the Novel-emed^® system, on 42 healthy subjects (84 asymptomatic feet) [6].

The distribution of vertical ground reaction forces under the foot as determined through this recent investigation (Table 1) will now be used to estimate the forces acting along tendons and across joint surfaces of the first and second rays of the foot during the push-off phase (second force peak during stance).