The increasing number of runners and consequently of running injuries of the last decades has produced an interest in studying the effects of shoe sole constructions on the kinematics of running and their effects on the development of running injuries (2,4,5,13,28). Biomechanical factors that have been associated with the development of running injuries include excessive foot eversion and excessive tibial rotation (8,19,27,34,41). The effects of shoe sole modifications, specifically the change of sole geometry on the lateral side of the rearfoot, are thought to be important with respect to eversion and consequently with respect to running injuries (6,10,15,29,30,36). Cavanagh (4) pointed out that early running shoes have been noted to show a prominent lateral heel flare producing excessive eversion that may be associated with running injuries.
When running with heel landing, the lateral aspect of the shoe sole touches the ground typically first with a touchdown angle of 5–10° (as seen in the frontal plane). It has been postulated that a prominent and hard heel flare would increase the lever about the subtalar joint, causing an increased initial eversion and/or maximum eversion velocity which has been confirmed experimentally (10,29,30,36). However, during midstance, kinematic effects of lateral heel flares have been reported to be small and dependent on the midsole hardness, which lead to controversial results (6,15,29). It was suggested that the discrepancies in the results might be because of differences in the methodologies used (29). Thus, heel flare effects are expected to be substantial at touchdown but small for total eversion.
The biomechanical factors that have been used describe these kinematic effects have been defined between touchdown and midstance of running (6,11,28). They describe maximum eversion and tibial rotation, the ranges of motion, and the maximum velocity of eversion and tibial rotation (Fig. 1 and Table 1). Additionally, during the stance phase of running, the everting movements of the calcaneus are transferred to the tibia by a coupling mechanism (16,17,18,24,31). Consequently, it has been suggested that excessive eversion may be transferred into excessive tibial rotation (8,20,34,40). Generally, tibial rotation depends on foot eversion, the vertical force, plantar- dorsiflexion, ligament integrity, and muscle-tendon forces (17). Shoe sole constructions may influence the movement of the foot and/or the orientation of the subtalar joint axis, which may change the movement coupling in the ankle joint complex. This may affect tibial rotation, resulting in an increase of loading at the knee. Therefore, to understand the effects of shoe sole modifications on internal loading, one has to study movements of the calcaneus and tibia during running.
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FIGURE 1: General definitions of the study variables. The variables for the tibia and the shoes were defined similarly.
Table 1: Definition and functional explanation of variables used in this study; the shoe variables were defined accordingly.
Previous studies of shoe sole effects during running are based on shoe or skin mounted marker settings. It has been shown recently that externally mounted markers overestimate the movements of the underlying bone (3,33). Nevertheless, it can be assumed that there exists a relationship between shoe eversion and bone eversion. This relationship is currently unknown. The present study was designed to quantify the effects of shoe sole modifications on tibiocalcaneal eversion and tibial rotation using bone pins, and to compare eversion measured at the bone level with eversion determined from shoe mounted markers. More specifically, it was expected that:
I. Large lateral heel flares increase maximum foot eversion velocity and maximum internal tibial rotation velocity systematically compared with reduced heel flares.
II. Large lateral heel flares increase maximum foot eversion and maximum internal tibial rotation systematically compared with reduced heel flares.
III. An increase in shoe eversion variables (maximum foot eversion velocity and total foot eversion) is related to an increase in bone eversion.
METHODS
General Project Description
The experiments were performed at the Department of Orthopaedics, Karolinska Institute at Huddinge University Hospital, Stockholm, where previous bone pin studies have been carried out (21,24). The project was part of a larger study (32,33). Ethical approval for the experiments was obtained from the Ethics Committee of the Karolinska Hospital and by the Medical Ethics Committee of The University of Calgary. The experimental set-up, testing procedure, data analyses, and data reduction have been described previously in more detail (32,33,38).
Five healthy male volunteers participated in this study (28.6 ± 4.3 yr, mass 83.4 ± 10.2 kg, height 185.1 ± 4.5 cm); they were all injury free at the time of the experiments and had no previous injury history that may have influenced their locomotion patterns. The subjects gave their informed consent to participate in the study. Intracortical Hofmann pins with reflective marker triads were inserted under standard local anesthetic (Citanest 10 mg·mL-1), which was active for 2–3 h, leaving enough time for the experiments. Two bone pins were drilled into the posterior lateral aspect of the calcaneus and the anterior lateral aspect of the tibial condyle and reflective marker triads were screwed onto each of these pins (Fig. 2). Three markers were glued on the test shoes, one at the posterior lateral aspect of the calcaneus and two in the midfoot. The effect of shoe marker configuration on eversion was tested on one subject using auxiliary markers 4 and 5 (Fig. 2), identified by felt pen marking at the shoe over the calcaneal region.
FIGURE 2: Bone pin marker positioning: at the tibia from T1 to T3, at the calcaneus from C1 to C3, at the shoe from S1 to S3, and the posterior shoe markers S4 and S5.
Experimental Set-up and Testing Procedure
Three high-speed cine cameras (LOCAM) were set around (in umbrella form) and focused on a force platform (KISTLER) that was mounted flush with the runway. The camera speed was set at 200 Hz. Three LEDs, triggered by a threshold detector connected to the force plate, were used to synchronize the cameras and to determine the time of contact on the force plate. The synchronization was accurate within one frame, corresponding to 5 ms in the worst case. A calibration frame with six control points (0.5 × 0.5 × 0.5 m3) was used for the three-dimensional reconstruction.
The subjects performed heel-toe running trials with a relatively low running speed of between 2.5 and 3.0 m·s-1. The running speed was monitored by photo cells placed 0.7 m in front and behind the force platform. Each of the test conditions was repeated three times, and trials were repeated if the subjects did not land with their right foot on the force plate and/or if an obvious modification of the gait pattern occurred.
Test Shoes
The tests were performed with standard shoes (Adidas Equipment Cushioning, 1994) where the rearfoot geometry was systematically changed. The original sole was changed to a single density (Shore A45) midsole and was modified at the lateral side to a wide flare, a neutral flare or straight sole and a rounded sole (Fig. 3). The flat outer sole was constructed with a hard material of Shore A 65 and was 3 mm thick. These shoe sole modifications were thought to produce different lever arms during the initial landing phase: The flared shoe sole with the largest lever, the round sole with the smallest, and the straight sole with an intermediate lever. Besides, the heel counter of all shoes had a lateral cutout to prevent impingement with the calcaneal bone pin during running.
FIGURE 3: Test shoes used in the study. “J” depicts the assumed position of the axis of rotation at the ankle joint complex located approximately at the proximal end of calcaneus, viewed in the frontal plane. The close-up shows the expected effect of shoe sole modifications on the length of the touchdown lever.
In addition to the study variables, eversion of the shoe relative to the tibia was also determined, with the standing trial of each shoe condition being used for the definition of the neutral position for this purpose. The shoe variables were determined to compare the results of this investigation with previous studies using external markers and to quantify the relative movement of the shoe and the calcaneus caused by slipping of the heel inside the shoe. However, it has to be kept in mind that two of the shoe markers were placed at the midfoot. Thus, strictly spoken, shoe eversion of the present study was a combination of shoe eversion at the calcaneus and at the midfoot.
Data Analysis and Reduction
KineMat, a set of programs written in MATLAB™, was adapted from Reinschmidt et al. (33) for the specific needs of this investigation, which allowed the reconstruction of the three-dimensional position of the markers and the calculation of the relative segmental movements. The barefoot standing trial was used as the neutral position to define the segment-fixed coordinate systems of the calcaneus and tibia. For that purpose, the subjects were instructed to stand with straight knees, the ankle in the neutral position of 90° dorsiflexion and the feet aligned parallel to the force platform, representing the laboratory coordinate system. The standing trials with the respective shoe conditions were used for the shoe marker analysis. The intersegmental rotations were calculated for the stance phase of all test conditions as Cardanic angles using a joint coordinate system approach (JCS) at the ankle joint complex. Inversion–eversion was calculated with the following sequence of rotations: 1) plantar–dorsiflexion about a tibia fixed mediolateral axis, 2) foot abduction–adduction about the floating axis, and 3) inversion–eversion about the anteroposterior axis of the foot (after Cole et al. (9)). Tibial rotation (corresponding to abduction–adduction in the above sequence) was calculated with a different sequence to avoid calculations about the floating axis having limited anatomical meaning: 1) tibial rotation about a tibia fixed proximal-distal (longitudinal) axis, 2) inversion–eversion about the floating axis, and 3) plantar–dorsiflexion about a calcaneus fixed mediolateral axis (after Nigg et al. (31)).
The accuracy of the spatial reconstruction between two marker triads was determined i) based on the residuals of the DLT equations averaged over the entire stance phase for all markers and ii) based on the deviations of the intermarker distances of the same trials. The mean error based on DLT residuals was found to be in the order of ± 4°, which included noise error and lens distortion error. The mean error based on marker distances (RMS) was found to be ± 1.0° including noise error only. Thus, for the present study, a realistic estimation of the error was likely between the two errors given above. The error of the shoe data was about ± 1.0° higher than that at the bone, because it included inaccuracies of different standing trials with different shoes.
The biomechanical factors, i.e., the test variables which have been associated with specific running injuries are listed in Table 1. Excessive eversion has been suggested to force the Achilles tendon to bend laterally, hereby producing an asymmetric stress distribution across the tendon, which could lead to Achilles tendon problems (8,34). Excessive eversion velocity has been associated with overloading and injury of the muscles of the posterior tibial compartment, e.g., medial tibial stress syndrome (10,34,36,41). Excessive tibial rotation has been associated with changes in the tracking of the patella, hereby changing the contact pressure and possibly the friction of the articulating surface of the patella, which may be related to the occurrence of the patellofemoral pain syndrome (39). Tibial rotation is thought to take place as a result of the movement coupling from the calcaneus to the tibia. All these variables indirectly describe the movement at those structures of interest, but do not directly describe the load within these structures. However, they are relatively easy to quantify.
RESULTS
The general patterns of eversion and tibial rotation are presented in Figure 4 (single curves of a typical subject) and Figure 5 (mean curves of each condition for each subject). In all subjects, eversion and internal tibial rotation took place from touchdown until midstance; thereafter, the movements reversed to inversion and external tibial rotation until take-off. These general movement patterns were found to be consistent for all subjects and test conditions.
FIGURE 4: Example of calcaneal inversion and eversion and tibial rotation (subject 1 with the straight shoe sole). Thin lines: three repetitions, thick lines: mean curve. Labels on the vertical axes indicate movements in the positive direction.
FIGURE 5: Mean curves of calcaneal inversion and eversion and tibial rotation of all conditions and all subjects: (···) straight, (—) flared, (- -) round shoe sole. The standard deviation during the stance phase was on average ± 1.1° for eversion and tibial rotation.
Inversion at Touchdown (βo)
Results based on bone pin markers.
All subjects consistently lowered their feet toward the ground in an inverted position. The differences between the shoe conditions (straight, flared, round) within each subject were small (ranging between 0.17° in subject 1 and 1.79° in subject 2, Table 2); the differences between the subjects were as large as 10°.
Table 2: The results and SD of the study variables based on bone pin data; positive values represent eversion and internal tibial rotation; negative values denote inversion.
Results based on shoe markers.
Shoe sole modifications showed no systematic differences in touchdown inversion. Subjects 2 and 4 showed the largest inversion with the flared sole, subject 3 and 5 with the straight sole, and subject 1 with the round shoe (Table 3). The smallest inversion was found with the round sole in subjects 3, 4, and 5 and with the straight sole in subjects 1 and 2. Thus, there was no consistent pattern of shoe inversion at touchdown across the five subjects.
Table 3: The results and SD of the study variables based on shoe mounted markers; positive values represent eversion and internal tibial rotation; negative values denote inversion.
Variables of Maximum Velocity (˙βmax, ˙ρmax)
Results based on bone pin markers.
The differences in maximum eversion velocity (˙βmax) between the heel flare modifications (between 23°·s-1 in subject 5 and 98°·s-1 in subject 4) were smaller than the differences between the subjects (smallest in subject 5 with 68–90°·s-1 and largest in subject 1 with 144–191°·s-1, Table 2). The flared shoe showed enhanced eversion velocity in subjects 1, 2, and 4, but not in subjects 3 and 5, compared with the straight shoe condition. The round shoe showed a reduced eversion velocity only in subject 4, but an increased velocity in all other subjects, compared with the straight shoe condition. Hence, measured on the bone level, there were no systematic effects on maximum eversion velocity because of the heel flare modifications.
The differences in maximum internal tibial rotation velocity (˙ρmax; between 6°·s-1 in subject 2 and 26°·s-1 in subject 1) were smaller than the differences between the subjects (smallest in subject 3 with 44–54°·s-1 and largest in subject 5 with 85–107°·s-1). The flared shoe showed enhanced internal tibial velocity in subjects 2 and 4, but not in subjects 1, 3, and 5, compared with the straight shoe condition, and the round sole was found with reduced internal tibial velocity in all subjects except subject 2.
Results based on shoe markers.
The flared shoe was found with an increased maximum eversion velocity in subjects 2 and 4 only (Table 3). Subjects 1 and 3 showed the largest velocity with the round shoe and subject 5 with the straight shoe. Thus, shoe modification effects on maximum eversion velocity were unsystematic across the five subjects. The comparison of the maximum eversion velocity measured at the shoe (˙βmax/shoe) with that at the bone (˙βmax/bone) is shown in Fig. 6. Maximum eversion velocity of the shoe varied between 160°·s-1 and 450°·s-1. The shoe eversion velocity was about twice as large as that at the bone (varying between 70°·s-1 and 225°·s-1). Increased shoe eversion velocities correlated with increased bone eversion velocities (r = 0.79;Fig. 6 : mean subjects 1–5), showing a good relationship between internal (bone) and external (shoe) movements.
FIGURE 6: Maximum eversion velocity of the shoe relative to the bone. Diagrams “Subject 1–5” show each trial of all subjects. Diagram “Mean subject 1–5” shows the mean values of each shoe condition of all subjects. (Bone values may differ slightly from
Table 2 because of different standing trial results from different shoes.)
Variables of Maximum (βmax) and of Total Movement (Δβmax, Δρmax)
Results based on bone pin markers.
The differences in maximum (βmax) and total eversion (Δβmax) between shoe sole modifications were in the order of 1–3°, but the differences between the subjects were up to 7° (Table 2). The flared shoes showed an increased total and maximal eversion in subjects 1 and 4, a decreased eversion in subject 3, and an inconsistent result in subjects 2 and 5. Total and maximal eversion was decreased with round soles in subjects 2 and 4 but increased in all other subjects. Hence, there were no systematic shoe sole effects with respect to maximum and total eversion on the bone level.
Total internal tibial rotations (Δρmax) between shoe sole modifications were in the order of 1–2°, and the differences between the subjects were in the order of 0–3.5°. Thus, there were no systematic shoe sole effects with respect to total internal tibial rotation.
Results based on shoe markers.
Although considerable shoe modification effects were found on shoe eversion, the results were not as expected (Table 3). All subjects had the largest maximum eversion with the round shoe (and not with the flared shoe as expected) and the smallest maximum eversion with either the flared sole (subjects 1 and 4) or the straight sole (subjects 2, 3, and 5). Thus, the main shoe sole effect was unsystematic across the five subjects.
The comparison of total eversion measured at the shoe (Δβmax/shoe) with that at the bone (Δβmax/bone) is shown in Figure 7. Eversion at the shoe level varied between 11° and 26°. Shoe eversion was found to be about twice as large as bone eversion (r = 0.88;Fig. 7 : mean subjects 1–5), and the two movement patterns varied between touchdown and take-off. Immediately after heelstrike, the shoe moved considerably more than the bone, then toward midstance the shoe and bone moved together until shortly before take-off (Fig. 8).
FIGURE 7: Total eversion of the shoe relative to the bone. Diagrams “Subject 1–5” show each trial of all subjects. Diagram “Mean subject 1–5” shows the mean values of each shoe condition of all subjects. (Bone values may differ slightly from
Table 2 because of different standing trial results from different shoes.)
Shoe eversion as a function of calcaneus eversion. Presented are three trials of subject 1 with the straight shoe. HS, heel strike; TO, takeoff.
Calcaneal Abduction and Shoe Marker Configuration
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.
DISCUSSION
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.
Eversion and Tibial Rotation
Generally, the movement patterns of this study (Figs. 4 and 5) were found to be similar to previous investigations using external markers in running (1,26,31,35,39), using bone markers in running (25), as well as bone markers in walking (22,23). Shoe eversion velocities of this study were found comparable to previous studies using shoe markers (Clarke et al. (7) : 532°·s-1; Williams and Ziff (42) : 475°·s-1; van Woensel and Cavanagh (43) : 408°·s-1), considering the slightly faster running speed of these studies (3–4 m·s-1), compared with the present one (2.5–3 m·s-1).
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 test shoes had a cutout in the lateral heel counter (Fig. 2), which was necessary to prevent impingement with the calcaneal bone pin. This cutout may have reduced heel counter rigidity and the fit of the heel inside the shoe, although the coupling between the calcaneus and the shoe seemed relatively constant throughout the stance phase (Figs. 6–8). On the other hand, van Gheluwe et al. (14) provided evidence that heel eversion is independent of the rigidity of the heel counter. Thus, whether or not heel counter rigidity or lateral cutouts had a systematic effect during testing cannot be answered conclusively.
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.
Shoe Eversion
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.
In an additional study (with the same test subjects at the same test date (38)), the results of a normal shoe were found to be very similar to those of the straight shoe. The normal shoe was constructed with softer material on the lateral side (Shore A35). This indicates that the subjects may have altered their muscular activity not only to shoes with different sole geometry (as in present study) but also to shoes with different midsole hardness.
CONCLUSIONS
In this in vivo study shoe sole modifications did not change tibiocalcaneal rotations substantially but demonstrated a significant correlation between shoe and bone movement patterns during running. The following conclusions can be drawn from the results of this study:
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: [email protected].
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