Lessons from dynamic cadaver and invasive bone pin studies: do we know how the foot really moves during gait?

What have we learnt about foot and ankle kinematics?

A key finding is the considerable freedom of movement that exists at the ankle. For the frontal and transverse planes, respectively, Lundgren et al [26] reported a mean total range of motion of 8.1° and 7.9° during walking (n = 5) (figure 1), Arndt et al [27] reported 12.2° and 8.7° in slow running (n = 4), and using a dynamic cadaver model of stance, Nester et al [9] reported a mean of 15.3°, and 10.0° (n = 13). Whilst in almost all cases the range of sagittal plane motion was greater, the ankle is certainly not limited to the role of a dorsi- and plantarflexion provider, as was traditionally thought.

Furthermore, there is clear evidence that in some feet the ankle displays more frontal and transverse plane motion than the subtalar joint, which was traditionally perceived as the rearfoot joint most able to move in these planes. In the case of transverse plane motion, Lundgren et al [26] reported that the total range of ankle motion was greater than the equivalent subtalar motion in 3 of 4 participants (in walking). Nester et al [9] reported greater transverse plane ankle motion compared to subtalar motion in 7 of 11 cadaver feet, and Arndt et al [27] reported the same in all 3 of their participants (in slow running). In the case of frontal plane motion, Arndt et al [27] found ankle motion to be greater than the equivalent frontal plane subtalar motion in 2 of 3 participants for which data was available (slow running). Lundgren reported the same in 1 of 4 participants in walking [26] as did Nester et al [9] in 8 of 11 cadavers. Based on these data, the subtalar joint is certainly not the sole 'torque converter' described in many texts, and in fact the ankle and subtalar jonts share this function, with each adopting different roles for different individuals.

The inter-subject difference in how the ankle and subtalar joints move is also evident in the pattern of movement during stance. Lundgren et al's [26] subject-specific data illustrates that some people display adduction of the talus at the ankle (5–10°) in the first 20% of stance, with other participants showing little motion at all (figure 1). Similarly in slow running [27], 2 of 4 participants showed eversion of the talus at the ankle (> 10° in first 40% of stance), the other two showing little motion at all over the same period. The variation between subjects in the frontal and transverse plane 'role' of the ankle and subtalar joints suggests they could work in tandem to provide the motion required for each person. Certainly, we should never prescribe distinctive roles to these two joints as has been the case (ankle = sagittal plane, subtalar = torque converter) and we might consider them to have quite similar functional roles in the frontal and transverse planes.

Published data consistently illustrate the significant freedom of movement at the talonavicular joint (figure 2), and to a lesser extent the calcaneocuboid joint (figure 3). In the sagittal, frontal and transverse plane respectively, Lundgren et al [26] reported 8.4° (1.1°), 14.9° (6.1°), 16.3° (6.5°) total range of motion at the talonavicular joint during walking. Arndt et al [27] reported similar sagittal and frontal plane motion in slow running, but ~50% less transverse plane motion. Given the more angular articular facets it is no surprise that the calcaneocuboid joint demonstrates less motion than the talonavicular joint in most subjects studied. However, the mean total range of calcaneocuboid motion in stance (7.8°, 6.3°, 6.9° respectively [26]) is greater than the equivalent subtalar joint motion in some in-vivo subjects [26, 27] and cadaver feet [9], reinforcing its important role in overall foot function.

As with ankle and subtalar motion, there is no consistent pattern between people in the range of motion the talonavicular and calcaneocuboid joints display. For one participant of Lundgren et al [26] study, a total of 21° of motion was observed in the frontal and transverse planes during stance, yet only 5.2° and 6.0° in another participant. Remarkably, despite these stark differences, in the sagittal plane the same participants displayed 8.0° and 8.1° range of sagittal plane motion, respectively. Quite how such inter-subject variation is integrated into a clinical conceptual model of foot kinematics has yet to be determined. However, given these data are from asymptomatic feet, the data makes a mockery of any notion that a clinician should seek to alter the foot biomechanics of all patients such that their feet achieve some hypothetical mechanical ideal (i.e. one foot model fits all feet). It is far from fitting that in the year we celebrate the 150th anniversary of Darwin's 'discovery' of essential variations in nature, that foot health professionals continue to use a clinical model of foot function which seeks to eliminate all variation between our patients. Furthermore, remaining as a 'variation' of nature rather than a clone of the hypothetical 'Root' foot type is likely to be central to a person remaining symptom-free for most of their lives, since their own body will have adapted to adequately cope with its own variations.

Many clinical models of foot biomechanics combine the navicular and cuboid, but data from Lundgren et al [26] indicates that motion between these bones is comparable or greater than that at the subtalar joint (which we never ignore) (figure 4). Identifying this capability, and the fact that motion between the medial cuneiform and navicular is equal to or greater than motion at the talonavicular joint in some feet, is perhaps one of the most important findings from the recent dynamic cadaver and invasive foot kinematic studies. This is important because data demonstrate that the tarsal bones are able to make a significant contribution to the kinematics of the overall foot. Motion that was previously attributed to the midtarsal joint and rearfoot was most likely taking place between the cuneiforms, the navicular, and cuboid. These movements are invisible clinically due to overlying tissue and consequently are completely absent from most if not all clinical models of the foot.

For the forefoot, data have confirmed the greater stability of the first, second and third metatarsals compared to metatarsals four and five. The fourth and fifth metatarsals are functionally distinct from the other three metatarsals, in that they consistently displayed more motion during stance. Using a dynamic cadaver model, Nester et al [9] reported > 12° mean total range of motion in the sagittal and frontal planes between the fifth metatarsal and cuboid. These figures were broadly confirmed in subsequent invasive study (13.3° and 10.4° respectively [26]) (figure 5). Equivalent data for the other metatarsals was 5 to 8°.

Furthermore, the average total range of motion between the first metatarsal and medial cuneiform reported by Lundgren et al [26] was far less than the motion between the equivalent fifth metatarsal and cuboid (5.3°, 5.4° and 6.1° in the sagittal, frontal, transverse planes compared to 13.3°, 10.4° and 9.8°). This mobility on the lateral side of the foot is in addition to the motion between the cuboid and calcaneus (9.7°, 11.3° and 8.1° respectively) clearly demonstrating an infrequently discussed 'lowering' of the lateral arch of the foot.

There is an important observation from the slow running data reported by Arndt et al [27] and walking data from Lundgren et al [26], which is even more valuable since the data for the former study was collected on the same subjects and in the same session (day) as the latter study. The total range of motion at the subtalar, talonavicular, calcaneo-cuboid, cuboid-navicular, medal cuneiform-navicular, metatarsal 1-cunieform, and metatarsal 5-cuboid, was smaller in (slow) running than in walking. For the ankle, the range of motion during walking was far greater in the sagittal plane, and slightly greater in the frontal and transverse planes. Less foot motion suggests a stiffer structure, and given external forces are known to be greater during running, this suggests that greater muscle forces would be generated to control foot movements. One extrapolation from this observation is that foot orthoses for running need not be stiffer or have greater 'control' features (such as high levels of medial heel wedging) compared to orthoses for walking, since the motion taking place is already less.

Do we know how the foot really moves during gait?

Recent dynamic cadaver and invasive kinematic research has provided some useful insights. The rearfoot plays only a part of overall foot kinematics and we have consistently undervalued the contribution from mid- and forefoot articulations. This suggests that in order to control foot pronation, orthoses need to provide support across the entire rear- and mid foot and that the use of heel wedges alone is unlikely to produce the desired biomechanical effects on the foot. The forefoot undergoes a complex series of rotations which must influence the action of the intrinsic muscles of the foot, and researchers are only recently being able to investigate some of their functions [6].

Finally, variation between people in foot kinematics is high and normal. This includes variation in how specific joints move and how combinations of joints move. The foot continues to demonstrate its flexibility in enabling us to get from A to B via a large number of different kinematic solutions. Rather than continue to apply a poorly founded model of foot type whose basis is to make all feet meet criteria for the mechanical 'ideal' or 'normal' foot, we should embrace variation between feet and identify it as an opportunity to develop patient-specific clinical models of foot function. Clinicians should consider foot function in terms of the entire foot, and, given what we know about the variation between subjects, the general ranges of motion likely at specific joints, and what is observable clinically, rationalise the most likely kinematic solution for each patient. It is hoped that patient-specific conceptual models for foot biomechanics will lead to improved understanding of the role (if any) of foot biomechanics in causation of foot and lower limb problems, and improve our design of orthoses such that they have more precise and predictable biomechanical effects. With evidence of wide variation in foot kinematics from even small samples of participants, and of patient-specific response to orthoses [19], how can clinical practice continue to be so heavily based on the idea that one foot model should fit all, and that orthosis design and prescription is based on the ideal foot, rather than the dynamics of the foot of each patient?