Forces acting in the forefoot during normal gait – an estimate
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).
Section snippets
Distribution of ground forces
After subdividing the foot sole into regions of interest, using a standard mesh configuration, forces acting in each of these areas were calculated from pressures that had been measured with a resolution of 4 sensors/cm2 [6]. During the push-off phase, at about 45% of the double-stride period, when the heel is off the ground with about 10° dorsal flexion in the MP joints, the ground force is shared to a great extent by the first and second metatarsal heads and by the big toe, these structures
Results
Fig. 3, Fig. 4 show explicitly the magnitudes of the forces that must act along the flexor tendons in order to equilibrate the turning moments effected by the ground forces F1 and F2, or G1 and G2, in each of the joints examined. These magnitudes, together with their directions as shown in Fig. 1, Fig. 2, and under the condition that the foot is dorsiflexed in the MP joints by 10°, lead to the forces within the structure of the forefoot as listed in Table 2. All values are presented as
General
Forces acting within the structure of the foot can either be determined directly, by means of implanted force transducers, or indirectly, by measuring external forces acting on the foot and estimation of the internal reactions to these external forces. Because any in vitro investigation necessarily involves the simulation of muscle forces, which in a complex structure like the foot would become very intricate, and because any in vivo experiment with implanted transducers is virtually impossible
Conclusions
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The first ray is the most heavily loaded structure of the forefoot. During the second force peak in the gait cycle, the flexor hall.long. and flexor hall.brev. (together with the abductor) muscles exert about 52% and 36% BW respectively, to enable the big toe alone to transfer 24% BW to the ground. The MP joint force amounts to about 86% BW.
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A total force of about 119% BW acts on the first metatarsal head. The first metatarsal bone is subjected mainly to axial compressive force, but also exposed
Acknowledgements
The author wishes to thank the anonymous reviewers for their constructive advice.
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