The foot orthosis also reduced calcaneus eversion relative to the tibia at 50% of stance for subjects 1, 2, and 3, but not subject 4 or 5 (Fig. 2). Subject 4 showed reduced eversion from 60% to 100% stance. Reductions in eversion at 50% of stance were achieved via changes almost entirely at the ankle joint in subject 1 (+4.0° inversion position). By contrast, subjects 2 and 3 showed greater changes at the subtalar than the ankle joint (1.4° less subtalar eversion, subject 2; 3.1° less subtalar eversion, subject 3). Subject 4 demonstrated 2° less everted position at both ankle (at 70% stance) and subtalar joints (80% stance).
The peak and/or period of calcaneus–tibia eversion velocity during the first 10% of stance was reduced by the foot orthosis in subjects 1–4 (Fig. 3). Subject 4 experienced increased ankle inversion velocity (22°·s−1 increased to 37.4°·s−1), and subject 1 experienced ankle inversion rather than eversion velocity (−23°·s−1 eversion velocity at 13% stance changed to 5.2°·s−1 inversion velocity). Subject 2 showed evidence of reduced peak ankle (−24.7°·s−1 reduced to −13.8°·s−1 at 25% stance) and subtalar eversion velocity (−108.6°·s−1 reduced to −66.3°·s−1 after initial contact). For subject 3, the orthosis prevented the rapid transition from subtalar eversion to inversion velocity in 0%–10% stance. At the ankle, the inversion velocity was increased (from 30.7°·s−1 to 39.8°·s−1), but subsequent eversion velocity also increased (from −21.5°·s−1 to −34.1°·s−1). Subject 5 displayed no changes in frontal plane velocity.
Transverse plane effects
With the exception of subject 5, the orthosis reduced the range of and/or peak in abduction of the calcaneus relative to the tibia. The timing of changes was subject specific (Fig. 4). Subject 5 displayed a small increase (maximum increase, 0.7°) in calcaneus–tibia abduction throughout stance. The effect for other subjects was not present throughout all stance. For subject 4, the ankle was unaffected at initial contact, but the subtalar joint was less adducted (3.2° compared to 5.8°). By contrast, for subject 1, peak ankle abduction was reduced by 2.7°, and the subtalar joint showed a trend toward more abduction (+2.6° abduction at 25% stance). Opposite ankle and subtalar effects were also evident for subjects 2 and 3.
Changes in transverse plane angular velocity were not systematic (Fig. 5). For subjects 1, 2, 4, and 5, there were no significant changes. Subject 3 demonstrated reduced calcaneus–tibia abduction velocity in early stance (from −70.7°·s−1 to −28.6°·s−1).
The antipronation orthosis reduced the peak and range of rearfoot eversion and, to a lesser degree, abduction relative to the leg in all subjects. However, contrary to clinical paradigms, these changes were inconsistent between subjects and occurred to varying degrees at different times of stance. Eversion and abduction velocity were affected less consistently than angular motion. Overall, the kinematic changes were generally small and concurred with the only other report of orthotic effect using bone anchored markers (14). The reduced eversion of the calcaneus relative to the leg (mean, −2.0°; Table 1) was within the range defined through meta-analysis of the literature (from Ref. (5): 95% CI, 1.42°–3.07°; from Ref. (9): 95% CI, 0.72°–3.53°). The smaller transverse plane effects concur with Mills et al. (8).
In contrast with clinical paradigms, the orthosis induced kinematic changes at both ankle and subtalar joints. However, these effects were not systematic across subjects. At heel strike, load is first applied to the subtalar joint via the calcaneus, and its response to the external loads, the distribution of which are changed by foot orthoses (10), will influence the load transferred to the ankle via the talus. Subtalar kinematics that occur in response to external loads will be influenced by constraints imposed by osseous contact forces, ligament and capsular restraints, and tendon forces, and these will be person specific. Many of the structures at the subtalar joint also cross the ankle, and thus, the properties and kinematic response of the ankle will affect the subtalar kinematic response. It seems plausible that feet with distinctly different ankle and subtalar structures, such as “pes planus” and “pes cavus” foot types, might be associated with specific patterns of response to a foot orthotic. Furthermore, because the initial effect of an orthotic is to influence load distribution under the sole of the foot, the orthotic effect could be sensitive to the underlying pattern of plantar load distribution. Plantar pressure patterns are known to differ between different foot types) (2).
The strong interaction between the mechanical behavior of the subtalar and ankle joints and the multiple (often common) factors/structures affecting each joint may explain the subject-specific responses observed. For example, in one subject, a relatively stiff subtalar joint may exhibit minor modifications to its kinematics because of the foot orthosis. In this case, the external forces altered by the orthosis may be largely transferred to the ankle. Depending upon its own mechanical constraints, the ankle may demonstrate a greater kinematic change than the subtalar joint. In another foot, however, a more compliant subtalar joint might allow for a greater change in its kinematics. However, depending upon the nature of the change in forces under the foot created by the orthosis (which itself will be variable between people, dependent upon dynamic foot shape and tissue properties, orthosis shape, and material properties), this might still result in transfer of forces to the ankle comparable with those in the case of the stiffer subtalar joint. Thus, a large kinematic response at the ankle might also be observed, even in the case of a large subtalar response. Furthermore, even if a subtalar joint demonstrates little kinematic response, there may also be little kinematic response at the ankle if the constraints are sufficient to resist the forces altered by the foot orthosis (see subject 2, ankle response was negligible and subtalar response the least of the five subjects). In this case, the forces would be more strongly referred to the tibia and knee, and we know this can occur (3). It should also be remembered that joint stiffness is dynamic and varies as joint position and motion velocity change.
There are many interrelated variables affecting subtalar and ankle kinematics, and therefore, a systematic kinematic response to a foot orthosis seems unlikely. The role of the orthosis cannot be to adjust foot position or motion to achieve a single optimum kinematic pathway or foot position as current orthotic paradigms suggest (based on concepts described in Root et al. ). Rather, the orthosis influences the external loads that determine the person-specific foot kinematics, and as a result, the orthosis causes the joints of the foot to pass through adjusted versions of a person’s underlying kinematic pathways.
It is not clear whether the small alterations in joint kinematics we report can lead to changes in clinical symptoms. The changes observed are presumably representative of those that would occur over many repetitive gait cycles each day, and there may be potential for accumulative benefit from even small kinematic changes. Also, we have only reported on effects across two rearfoot joints, whereas most foot structures span multiple joints. Furthermore, we do not know what feature (or features) of foot kinematics needs to change to elicit a clinical response, and in this study, changes were highly person specific. Peak heel eversion and range of heel eversion are the most commonly reported parameters (8), but their clinical relevance is unproven. Identifying which parameter a foot orthosis must change remains a significant challenge because plantar loading, joint moments, and joint motion cannot be modified separately by the orthosis, and thus, changes in each will always be coupled.
Understanding how the observed changes in separate ankle and subtalar kinematics might lead to clinical benefit is also complex. Any orthotic-induced change in the stress experienced by injured tissues might vary depending upon how each of the ankle and subtalar joints respond to the orthosis. For example, the moment arms of tendons that span both the ankle and subtalar joints will differ depending upon where movement is occurring and where centers of rotation lie. A change in ankle kinematics might affect stress in a specific tendon or ligament, or joint contact forces, differently than if the same kinematic change occurred at the subtalar joint.
The data are from a small sample of asymptomatic individuals, and thus, the orthosis has been tested in a limited range of types of feet. Indeed, the subject who showed the least response (subject 5) displayed minimal rearfoot motion and perhaps reflects a foot that might not normally receive an antipronation foot orthosis. Also, it is inappropriate to draw conclusions about how foot orthoses result in clinical benefit from such small asymptomatic samples. However, the validity of the data (direct measurement of bone motion) and measurement of separate ankle and subtalar kinematics offers new insight into how orthoses can affect feet. This research is therefore suited to identifying new avenues for understanding orthotic effect. The effects of a foot orthoses are linked to the footwear in which they are used. The forces applied to the base and sides of the orthoses are via the sole and shoe upper, respectively. In this study, the upper was largely removed, and although not evaluated formally, the shoe had reduced stiffness. The effect of an orthosis will be strongly related to its geometry and material properties, and results here will not apply to orthoses with significantly different properties.
The antipronation foot orthosis produced small and unsystematic reductions in eversion and abduction of the heel relative to the leg at various times during stance. This was achieved via complex changes at the ankle and subtalar joints that were specific to each subject tested. These changes contradict existing orthotic paradigms and are indicative of a strong interaction between the ankle and subtalar joints.
No funding was received for the work contained in this article.
The Intellectual Property (IP) for the insoles tested in this work is owned by the University of Salford and licensed to a University spin off company. Nester is a Director of that company. No payment was received by Nester for the work in the article submitted.
Liu, Jones, Lundgren, Kundberg, Arndt, and Wolf declare no conflict of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2012The American College of Sports Medicine
FOOT ORTHOTIC; REARFOOT PRONATION; INSOLE; FOOT BIOMECHANICS