It has previously been shown that a variation of abduction–adduction of the foot (in the transverse plane) could contribute to a variation of eversion–inversion (5). For that purpose, calcaneal abduction between touchdown and midstance was additionally calculated using KineMat. The results showed that the mean values between each shoe condition (based on three repetitions) varied individually only by 1–2° (e.g., smallest in subject 3 between 3° and 3.5° and largest in subject 1 between 5° and 7°). Thus, overall, the subjects ran relatively constant, which suggests that the shoe modifications did not influence calcaneal abduction (and consequently eversion) substantially.
It is possible that shoe eversion results may depend on the shoe marker configuration. In a small additional study (one subject, three shoes, three repetitions each), the more anterior marker configuration S1-S2-S3 was compared with the more posterior configuration S1-S4-S5 (Fig. 1). The results showed that total shoe eversion with the anterior configuration was 2–4° larger than with the posterior configuration, suggesting that anteriorly placed markers are likely to include midfoot rotations compared with posteriorly placed markers. Thus, the present shoe eversion results may be dependent on the position of the shoe markers and should be investigated systematically in future studies.
The results of this study showed that the shoe sole modifications did not produce the expected systematic effects on the test variables. This is in contrast to the expectations I and II of this study. Evidence was found that a relationship or coupling effect between shoe and bone eversion occurred during running, which is in accordance with expectation III. Large lateral heel flares were found neither to increase foot eversion velocity nor internal tibial rotation velocity compared with reduced heel flares. Similarly, neither maximum nor total eversion and tibial rotation were increased with large heel flares.
It was expected that the large lever of the flared shoe sole would produce the largest eversion velocity and that the decreased lever of the round sole the smallest. This was not the case and is in contrast to previous investigations using shoe markers, where it was concluded that prominent lateral heel flares cause an increased initial eversion or eversion velocity (6,10,29,30). It was further expected that the flare of the shoe sole would increase maximum shoe eversion and the round shoe sole would decrease maximum shoe eversion. However, all subjects had the largest maximum eversion with the round shoe and the smallest maximum eversion with either the flared sole or the straight sole. It is speculated that i) the prominent lateral flare compressed considerably during touchdown hereby reducing the acting lever and that ii) the hard outer sole of the round shoe deformed very little, which may have favored a rolling action of the foot, resulting in a large maximum eversion. Future investigations may want to establish the change of lever length over time of various shoe sole modifications to clarify this issue.
The results on total internal tibial rotations were found consistent with those reported by Lafortune et al. (22), who found no significant differences in internal tibiofemoral rotation between normal and varus wedged shoes. Total internal tibial rotation of this study was found to be smaller than in previous studies measuring internal tibial rotation during running (8.9–11.1°(26); 15°(35); 22°(31)) using external markers as well as during walking (6–8°(23); 11.1°(33)) using bone markers and during walking using external markers (7.5°(33)). This suggests that the present tibial rotation values were either comparatively small, that previous results based on external markers overestimated tibial rotation, and/or that tibial rotation during running may be of about the same order of magnitude than during walking.
The above discussed results may be influenced by the application of local anesthesia, which was necessary because of the invasive character of the study. It is possible that due to the anesthetic the proprioceptive feedback and, consequently, possible adaptations of the movement pattern to different shoe conditions may have been changed. To test this, Reinschmidt et al. (33), using the same subjects at the same test date, compared three trials with and without bone pins in subjects 2 and 4. It was concluded that pre- versus postoperative knee and ankle joint rotations showed graphs that were similar in shape and magnitude, the maximum differences being 2°. Thus, it is unlikely that the local anesthesia had a substantial effect on the results, and it remains speculative whether the subjects would have adapted their individual running patterns toward the test shoes if the local anesthesia was not present.
Running is a movement pattern which may be difficult to alter by interventions such as shoe sole modifications. More specifically, locomotion is thought to be controlled by a central pattern generator (44). During running, a basic locomotor-like pattern is modulated by input from supraspinal centers and motion related feedback (44). One may therefore argue that a running pattern is predetermined and that muscular activity during running is used to adapt to shoe modifications. Although muscle activation (i.e., EMG) was not measured in this study, muscular activity as a response to shoe modifications may have been present during testing. This possible explanation is supported by the following argument: A number of authors have suggested that for a given task, there may be various solutions with respect to the rotations between different segments of the lower extremity (12,22,24). Thus, a specific movement, such as running, may be associated with individual movement patterns such that an external input (i.e., shoe sole modifications) may have only small and varying effects on the kinematics of the calcaneus and tibia.
Previously reported differences in maximum eversion between shoe and skin were between 2° and 4°(7,14,28,37), which is considerably smaller than the differences found in this study (between 5° and 20°, Table 2 and 3). This increased shoe movement may explain the high eversion velocity that occurred early in the stance phase (Table 3). One possible explanation for this discrepancy (between the present data and previous investigations) may be the differences in the methodologies used. The previous studies used shoe and skin markers and a two-dimensional approach, whereas the present study used a three-dimensional approach and shoe eversion was calculated relative to the tibia using bone markers. The observed relative movement between the bone and shoe is suggested to consist of slipping inside the shoe and of fat pad and shoe material deformations.
The large shoe eversion of the round shoe was not reflected by the bone eversion results. Thus, the increased shoe eversion (possibly induced by the round sole construction) may have been compensated by muscular activation such that the kinematics on the bone level remained unchanged.
Mean shoe sole effects were found to be small (less than 1°) but differences between subjects were found to be large (up to 7°). Thus, on the bone level, shoe sole effects on tibiocalcaneal movements may be small and unsystematic.
Total shoe eversion and shoe eversion velocity were found to be approximately twice as large as the respective bone level measurements, the correlation being r = 0.88 (between total shoe and bone eversion) and r = 0.79 (between maximum shoe and bone eversion velocity). This suggests that there may be a relationship or coupling effect between the shoes and bone. This relationship is possibly influenced by the shoes and the configuration of the markers attached to the shoe.
Simultaneously measured shoe markers showed no systematic shoe sole effects on shoe eversion, which is in contrast to previous studies. It can be argued that if systematic shoe sole effects were present at the shoe level, then bone level effects could be expected. However, since the present results do not support this argument, it is possible that local anesthesia, individual muscular responses and/or the test shoe construction influenced the calcaneus and tibia kinematics during running.
This study was supported by the Swedish Defense Material Administration, the Swiss Federal Sports Commission (ESK), the Olympic Oval Endowment Fund of Calgary, and ADIDAS America. The help of N. Murphy, R. Lawson, H. Strebel, J. Waser, E. Avramakis, A. Ming, and T. Haag at various stages of the project was greatly appreciated.
Address for correspondence: Alex Stacoff, Laboratory for Biomechanics, Dept. of Materials, ETH Zürich, Wagistrasse 4, 8952 Schlieren, Switzerland; E-mail: Stacoff@biomech.mat.ethz.ch.
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