In running, strike patterns may be classified into three groups based on the foot-related initial contact point: initial rearfoot contact (IRFC), midfoot contact (IMFC) or forefoot contact (IFFC) patterns (5). Other separations with only two groups such as heel strike and non–heel strike are also possible. The way the foot initially makes contact with the ground influences the consecutive foot motion during stance. After an IRFC, the ankle–foot complex shows an initial ankle plantarflexion, whereas an IMFC and an IFFC are followed by an initial ankle dorsiflexion (35). It has also been shown that a habitual shod IRFC is characterized by higher loading, at least as defined by selected variables, when compared with a shod IFFC (37) or a shod IMFC (2,15), after habitual IRFC subjects were instructed to run with these altered strike patterns without changing the shoe conditions. This has led to hypotheses about associations between the initial foot contact pattern (IFCP) and the etiology of injuries, which are sometimes highly debated (6,7,25,29). The increased scientific interest for this topic raises the need for reliable methods for measuring IFCP.
The two most commonly used methods for IFCP determination are a kinematic determination of IFCP and the calculation of a strike index (SI) that uses both kinetics and kinematics. In field studies, the kinematic method has mostly been applied with video images. In IRFC, the heel or the rear one-third of the foot touches the ground first; in IMFC, the heel and the ball of the foot touch the ground nearly at the same time; and in IFFC, the ball or the front one-third of the foot touches the ground first and no heel contact is made (19). This method allows a quick screening of large groups of runners, even in competition. However, initial foot inversion–eversion, adduction–abduction, low-image resolution, and inadequate measuring frequency sometimes make it hard to define the exact instant and location of initial contact. It is especially hard to distinguish an IMFC from an IRFC or IFFC pattern (24). Altman and Davis (1) refined this kinematic method by measuring the foot segment angle at initial contact. The authors defined clear quantitative cutoffs, but others may use foot segment angles >0° or <0° to indicate a heel strike or a non–heel strike. This refined kinematic method provides a more continuous measure of the IFCP than a pure qualitative visual assessment and can be applied using high-speed video or motion capture systems.
Another frequently used method is the determination of an SI (5), which is based on the position of the center of pressure (COP) on the foot at initial contact. This method uses force plate data and kinematic data to locate the COP along the length of the foot. COP measurement using only force plate data is less accurate when only low ground reaction forces (GRF) are exerted (4), and this is the case at the instant of initial contact. Williams and Cavanagh (36) dealt with this issue by defining an SI at the instant when the vertical GRF reaches 10% of maximal vertical force, where already a certain amount of loading acts upon the foot. A good alternative would be to use a more sensitive high-speed pressure plate system that also allows a direct localization of the COP on the foot sole (11).
Several studies have determined IFCP in a large number of distance runners both in “competition” using the kinematic determination method (19,23,24) and in “laboratory” conditions using the SI method (5,22) or the kinematic determination method (7). In general, the currently available research suggests that when running shod at submaximal running velocities, approximately 75% of runners show an IRFC, approximately 20% show an IMFC, and 5% show an IFFC (5,19,22–24). However, the reported IFCP group percentage results are influenced by the IFCP determination method, running speed, and subject group characteristics.
A study by Nigg et al. (30) about shod heel-toe running reported that when running speed increases from 3 to 6 m·s−1, the shank and the rearfoot are more anteriorly tilted immediately before contact and that a flatter foot position is obtained at the highest running speed. These findings suggest that a subject’s SI will increase at faster running speeds. This assumption is supported by a study by Keller et al. (22) that investigated the IFCP group distribution in a wide range of running speeds from 1 to 7 m·s−1. IFCP group distribution changed from predominantly IRFC at running speeds up to 5 m·s−1 to predominantly IMFC at speeds higher than 5 m·s−1. This shows that running speed may influence IFCP. However, Keller et al. did not report within-subject IFCP alterations due to a changed running speed. Assessing these intraindividual alterations in SI, determined with a sensitive pressure plate, should provide a better insight in the relationship between running speed and SI.
Apart from running speed, the shoe or contact surface can influence running biomechanics (14,28,30,31) and in some extreme cases might even change ones strike pattern (33). If a study wanted to assess IFCP and intrinsic running technique differences, not confounded by differences in footwear but in realistic shod conditions, subjects should run in a neutral type shoe. This was not the case in the “marathon” studies (19,23,24) in which subjects wore their own running shoes that could have influenced IFCP differences between subjects. Also, Gruber et al. (18) found that when habitual rearfoot strikers ran barefoot on a soft surface, not all runners changed their IFCP, indicating it is not just the presence or absence of a shoe but also the contact surface properties that influence the chosen IFCP.
The goals of this study are, first, to accurately assess IFCP during steady-state (constant pace) shod running for a large group of long distance runners, wearing the same type running shoe, using high-speed plantar pressure measurements. Second, we want to assess the within-subject effect of running speed on the IFCP type over a wide range of relevant running speeds. Our hypotheses are that most runners show an IRFC, but with increasing velocity, some of these runners make a shift toward an IMFC or IFFC. Besides the previously stated main research purposes, a secondary intention was to look for differences in the peak vertical instantaneous loading rate (VILR) between IFCP because VILR is a variable that has been associated with musculoskeletal overloading in running.
METHODS
Subjects
Fifty-five runners (40 men and 15 women) of recreational and competitive level were recruited from local running clubs and the Ghent University and its surrounding community. For the male subjects, the mean ± SD age was 28.6 ± 8.1 yr; body mass, 71.9 ± 5.8 kg; height, 1.80 ± 0.05 m; training pace, 3.54 ± 0.34 m·s−1; weekly training volume, 41.2 ± 22.9 km; and years of running experience, 8.7 ± 5.9 yr. For the female subjects, mean ± SD age was 28.2 ± 8.3 yr; body mass, 59.4 ± 4.5 kg; height, 1.67 ± 0.05 m; training pace, 3.05 ± 0.30 m·s−1; weekly training volume, 36.3 ± 15.2 km; and years of running experience, 9.3 ± 5.2 yr. All subjects were aged between 18 and 58 yr, had a shoe size between U.S. men’s 6.5 and 11, and had a weekly training volume of 15 km or more. No runners were currently injured or had sustained any injuries that required a temporary or full cessation of running within 3 months before participation in the study. Written informed consent was obtained before participation in this study. Ethical approval for the study was obtained from the ethical committee of the Ghent University hospital.
Protocol and experimental setup
Before the running tests, subjects completed a questionnaire to assess running habits (e.g., weekly training volume, endurance run training speed, and years of running experience). After a short warming-up of 5–10 min, which also served as habituation to the test shoe and experimental setup, subjects were asked to perform several running bouts for a 25-m instrumented walkway at four speeds: 3.2, 4.1, 5.1, and 6.2 m·s−1. Those speeds were selected to represent the speed range in the training program of an endurance runner (13).
To counter a possible bias of shoe type or shoe construction on the subjects’ running style, all subjects wore the same shoes (Li Ning Magne, ARHF041). The shoes were modified for optimizing plantar pressure measurements by substituting a flat outsole and filling in the midfoot region of the midsole. The midfoot region was filled in with a standard EVA foam, with the same hardness as the original outsole, to level out the gap between the heel and the forefoot part of the shoe. A flat outsole was achieved by grinding off the original outsole profile and replacing it by a new nonprofile even outsole (shoe characteristics of U.S. size 10: forefoot width, 11.2; heel width, 9; sole length, 31.8; heel thickness, 2.9; heel-toe offset, 1.15 cm; impact testing results following ASTM F-1976-06 procedures, ∼950 N or peak g of ∼11.5g). No runners indicated feeling uncomfortable with the test shoes or with the selected speeds, and results from the questionnaire did not indicate a systematic difference in habitual shoe type between different IFCP groups.
Before every speed block, subjects practiced running at the selected speed by following pacing lights attached to the side of the runway. GRF (1000 Hz) and plantar pressures (500 Hz) were measured by a built-in 2-m force plate (AMTI, Watertown, MA) mounted with a 2-m pressure plate on top (Footscan; RSscan International, Olen, Belgium). Running speed was measured with a distance laser (1000 Hz; Noptel Oy, Oulu, Finland). During the experiment, subjects were given feedback on their running speed based on infrared timing gaits. For each speed and foot side, three successful trials per subject were collected. Trials were rejected if the speed measured from the timing gates was outside the ±0.2-m·s−1 range of the target speed, if the subject accelerated or decelerated during the measurement, if the feet were in contact with the edges of the pressure plate, or if it was obvious that the subject was targeting the force plate.
Data processing
Frequency analysis of GRF signals of running trials in our setup showed resonance frequencies higher than 80 Hz. GRF data were filtered using a Butterworth second-order low-pass filter with a cutoff frequency of 80 Hz. Residual analysis of GRF signals and qualitative assessment of the over/under filtering effect of different cutoff frequencies (range = 50–100 Hz, with 10-Hz intervals) on the force-time signals were performed to determine the optimal cutoff frequency. The pressures were dynamically calibrated with the vertical force signal. This means that the summed pressure of the entire contact surface was scaled to correspond with the GRF (Footscan 7 gait second-generation software).
Contact time was defined as the time when GRF was higher than 5 N. Peak VILR, which is a frequently used impact measure (6,8,21,29), was calculated as the maximal value of the first derivative, more than a 0.004-s interval, of the vertical GRF component during the initial contact phase (first 0.050 s of foot contact) and was normalized to subjects’ bodyweight (BW·s−1).
SI determination
The classic SI method uses force plate data for COP calculations and kinematic data to locate the foot on the force plate. In this study, COP data were obtained with a more sensitive pressure plate (sensor size 0.5088 × 0.762 cm) (Footscan; RSscan International). COP coordinates were expressed as a percentage of shoe length where the longitudinal axis of the shoe was determined by the Footscan 7 software. For normalization to shoe length, we assumed that the most distal COP point was at the normalized total foot length. On the basis of the COP position at initial contact, an SI was defined, and foot contacts were identified as IRFC (SI = 0–0.333), IMFC (SI = 0.334–0.666), or IFFC (SI = 0.667–1).
Statistical analysis
Intraclass correlation coefficients (ICC) were calculated for SI, VILR, and contact time using SPSS Statistics 21 (SPSS Inc., Chicago, IL) for trials within each foot side and condition. All ICC values were higher than 0.8, indicating low variability across trials. For further statistical analysis, these parameters were averaged per subject, speed, and foot side. If not all three trials could be assigned to the same IFCP group for a subject, the average value for statistical analysis was calculated based on the trials of the most frequent (two of three trials) IFCP. All further statistical procedures were conducted using MLwiN 2.27 statistical analysis software (University of Bristol, Bristol, UK) with a significance level set at P < 0.05. Multilevel linear regression models (three levels: participant, foot side, and measurement) were constructed to determine the within-subject effect of speed on SI and the between-IFCP group differences in VILR and contact time. These models combined the significant main and interaction effects (P < 0.05) of speed, foot side, and IFCP group. For the pairwise comparisons between the different speed conditions and the different IFCP groups, a Bonferroni correction was performed.
RESULTS
IFCP group distribution
On the basis of the SI, three IFCP groups could be classified: runners with an IRFC, an IMFC, or an IFFC. A visual representation of SI and the IFCP group for each subject in each speed condition is given in Figure 1. Some subjects showed a different color scale for their left and right foot indicating an asymmetry in IFCP (Fig. 1). An asymmetrical IFCP occurred in 11% of runners at 3.2 m·s−1, in 9% at 4.1 m·s−1, in 25% at 5.1 m·s−1, and in 31% at 6.2 m·s−1.
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FIGURE 1: SI per subject, per speed, and per foot side. SI is indicated by a color scale. Green cells indicate an IRFC. Yellow cells indicate an IMFC. Red cells indicate an IFFC. Each horizontal line represents data from the same subject for both left and right foot. Subjects are vertically sorted in such way that the upper horizontal line represents data from the subject with the highest mean SI and the lower horizontal line represents data from the subject with the lowest mean SI over both feet and all speed conditions.
Within-subject effect of speed on SI
A three-level linear regression model (subject, foot side, and measurement) was constructed with categorical parameters foot side (left as reference category) and speed (slowest speed as reference category) and SI as the dependent variable. The nonsignificant interaction terms that subsequently were deleted from the model, model parameters, and main effects are presented in Table 1. We found a significant main effect of speed on SI. Post hoc analysis revealed a significant difference in SI between all speed conditions (χ2 ≥ 22.6, P < 0.001), except between 3.2 and 4.1 m·s−1 (χ2 = 1.401, P = 0.237). This confirms an increase in SI when the running speed increases (Table 1).
TABLE 1: Results of the statistical analysis.
There is a group we called “transition” runners, in which an increase in speed caused a shift to another IFCP. Most runners shifted to a more anteriorly located SI. Some runners performed one such transition (37%), whereas others showed two (11%). However, there is a large group of runners that showed an IRFC over all running speeds (46%) as well as a small group of runners that showed an IMFC over all running speeds (6%). In these subjects, the increased running speed did not cause a shift toward another IFCP group (Table 2).
TABLE 2: Percentage of runners that show the same IFCP in all running speeds and percentage of runners that show a shift to another IFCP with an increase in running speed.
Atypical IRFC patterns
Most runners showed an IRFC when collapsed across running speed (68%). Typical IRFC show a COP trajectory with the initial contact at the posterior lateral shoe sole side, after which the COP moves rapidly toward the midline of the shoe sole. When processing the COP data, we noticed that some runners with an IRFC showed COP trajectories that clearly differed from the typical IRFC COP pattern (Fig. 2). A qualitative assessment was performed of the COP trajectory, plotted over the footprint in the Footscan software, of all foot contacts at 3.2 m·s−1 (55 subjects, three left and three right foot contacts per subject, minus three failed measurements, resulting in 327 foot contacts in total), which is the speed that best matches the runners’ training pace. An atypical IRFC COP trajectory was defined when initial contact was made in the rearfoot zone, immediately followed by a fast anterior COP movement along the lateral shoe margin into the midfoot zone, which was then followed by the COP moving medially in the midfoot zone (Fig. 2). On the basis of this qualitative assessment, 76 of 327 foot contacts were qualitatively selected as atypical IRFC, 193 as typical IRFC, and the remaining 58 as typical IMFC or IFFC.
FIGURE 2: Left foot COP trajectories from a typical IRFC trial, a typical IMFC trial, and an atypical IRFC trial.
Apart from such a qualitative assessment, we also wanted to be able to identify the atypical IRFC patterns using a quantitative functional measure. This was obtained from differences in the foot unroll timing parameters. The timing of a foot unroll can be subdivided based on some specific events during foot contact, such as initial foot contact, first metatarsal contact, initial foot flat contact, heel off, and last foot contact (10). In barefoot IRFC, the first metatarsal contact occurs at approximately 8% of contact time (10). When running shod, the foot touches the ground in a more dorsiflexed position (12) so the first metatarsal contact will probably occur later if plantarflexion velocity remains the same. The fast anteriorly moving COP in the atypical subjects indicates an initial fast shift of pressure toward the forefoot region with a first metatarsal contact occurring sooner in such patterns compared with the typical IRFC patterns, providing an objective measure to distinguish between the two categories. In IFFC or IMFC running, the first metatarsal contact concurs with initial contact.
The time of the first metatarsal contact was defined as time between initial contact and the instant of first plantar pressure in the metatarsal zone (10). For both the atypical IRFC and the typical IRFC foot contacts, the first metatarsal contact showed a double-peaked frequency distribution. This indicates that based on the time of the first metatarsal contact, two groups can indeed be identified in the IRFC foot contacts based on this temporal variable. The time of the first metatarsal contact also showed high significant ICC between the three trials of each foot (P < 0.001, left feet ICC = 0.910, right feet ICC = 0.936).
In the typical IRFC foot contacts (193 of 327 foot contacts), which were first identified by a qualitative assessment of the COP trajectories, the first metatarsal contact occurred at 11.8% ± 2.9% of contact time. In the atypical IRFC foot contacts (76 of 327 foot contacts), as first identified by the qualitative assessment, the first metatarsal contact occurred at 4.0% ± 2.0% of contact time. This means that when the first metatarsal contact occurs before 6.0% of contact time (mean time of the first metatarsal contact typical IRFC − 2 SD), there is a good chance that this pattern is an atypical IRFC pattern. Nevertheless, some atypical IRFC patterns showed the first metatarsal contact up to 8% of contact time (mean atypical IRFC + 2 SD). Consequently, for foot contacts with the first metatarsal contact between 6% and 8% of contact time, a visual qualitative assessment of the COP pattern is still needed for a correct classification. Of the 327 foot contacts, 35 had the first metatarsal contact between 6% and 8% of contact time. On the basis of the qualitative assessment of the COP trajectories, 18 were identified as typical IRFC and 17 as atypical IRFC.
At the higher running speeds, above training pace, the atypical patterns were also identified based on the qualitative assessment. If the atypical pattern was shown in more than one of the three trials per speed per foot, the runner was classified as an atypical IRFC runner for this specific speed and foot side. Forty-two percent of runners showed this atypical IRFC pattern at some point over all speeds. Even 9% of runners showed the atypical IRFC pattern in all speed conditions for one foot side, and 4% showed the atypical IRFC pattern in both feet in all speed conditions. At 3.2 m·s−1, 58% of runners showed typical IRFC and 24% an atypical IRFC; at 4.1 m·s−1, 55% of runners showed a typical IRFC and 24% an atypical IRFC; at 5.1 m·s−1, 44% of runners showed a typical IRFC and 20% an atypical IRFC; and at 6.2 m·s−1, 31% of runners showed a typical IRFC and 15% an atypical IRFC. An adjusted version of Figure 1, with an indication of the runners that showed the atypical IRFC pattern, is available as Supplemental Digital Content 1 (see Figure, Supplementary Digital Content 1, https://links.lww.com/MSS/A351, SI at initial contact per subject, per speed, per foot side, with an indication of the runners that show the atypical IRFC).
Between-group differences in VILR and contact time
On the basis of the SI and the assessment of the atypical IRFC patterns, we classified four groups of runners. Although not stated as one of the primary research goals of this study, between-group differences in contact time and VILR were assessed, also to help find the identification of the atypical IRFC as a distinct fourth IFCP. A three-level linear regression model (subject, foot side, and measurement) was constructed with the following categorical parameters: foot side (left as a reference category), speed (slowest speed as reference category), IFCP (typical IRFC as a reference category), and contact time (expressed in milliseconds) as the dependent variable. The results of this analysis are presented in Table 1. We found a significant main effect of speed and IFCP group on contact time. Post hoc analysis revealed a significant difference in contact time between all speed conditions (χ2 ≥ 536.411, P < 0.001), where contact time decreased with increasing speed. Post hoc analysis also showed that the typical IRFC group has longer contact times than the other IFCP groups (χ2 ≥ 31.027, P < 0.001) and that the IMFC group has shorter contact times than the atypical IRFC (χ2 = 4.275, P = 0.039) (Fig. 3).
FIGURE 3: Mean ± SD of the contact times (s) per initial foot contact pattern (IFCP) group per speed, collapsed over foot side. Notice that there were no IFFC at 3.2 m·s−1.
Another three-level linear regression model (subject, foot side, and measurement) was constructed with the following categorical parameters: foot side (left as a reference category), speed (slowest speed as reference category), IFCP (typical IRFC as a reference category), and VILR (expressed in BW·s−1) as the dependent variable. The results of this analysis are presented in Table 1. We found a significant main effect of speed (χ2 = 985.757, P < 0.001) and IFCP group (χ2 = 115.749, P < 0.001). Post hoc analysis revealed a significant difference in VILR between all speed conditions (χ2 ≥ 124.455, P < 0.001), where VILR increased when speed increased. Post hoc comparison between the IFCP groups showed significant differences in VILR between all IFCP groups (χ2 ≥ 22.420, P < 0.001), except between the typical IRFC and the IMFC groups (χ2 = 0.404, P = 0.525). The highest VILR were seen in the atypical IRFC group and the lowest VILR in the IFFC group (Fig. 4).
FIGURE 4: Mean ± SD of the maximal loading rates (BW·s−1) per initial foot contact pattern (IFCP) group per speed, collapsed over foot side. Notice that there were no IFFC at 3.2 m·s−1.
DISCUSSION
IFCP group distribution
The first purpose of this study was to accurately assess the IFCP during steady-state shod running for a group of long-distance runners using 500-Hz pressure measurements underneath the shoe, as such a system gives a more accurate COP at low vertical GRF and allows a direct localization of the COP on the plantar side of the shoe sole.
The resulting IFCP group distribution of 82% IRFC and 18% IMFC at 3.2 m·s−1 and 46% IRFC, 32% IMFC, and 22% IFFC at 6.2 m·s−1 were mainly in accordance with previous research that determined IFCP groups using the SI method (5,22). According to Keller et al. (22), more than 50% of our subjects showed an IMFC or IFFC when running at approximately 6 m·s−1. However, Keller et al. reported higher percentages of subjects showing an IMFC or an IFFC (86%) versus IRFC (14%) when running at 6 m·s−1. A possible explanation for this discrepancy could be that in the study by Keller et al. (22), not all subjects were able to run at 6 m·s−1. More subjects were presented at the slower running speeds than at speeds faster than 5 m·s−1. In our study, all subjects were able to run up to 6.2 m·s−1.
Within-subject effect of speed on SI
The second purpose of this study was to assess the within-subject effect of running speed on the SI. With increasing speed, SI significantly increased, indicating that when subjects ran faster, they tended to touch the ground more anteriorly on the shoe sole. This is supported by a previous study by Nigg et al. (30) in heel-toe runners (∼IRFC). Different kinematic changes may occur (e.g., more flexed knee or ankle), but foot angle would be a variable that specifically affects SI. Further research should clarify which specific speed-induced kinematic changes are related to a change in SI and if these speed-induced adaptations differ between the different IFCP groups.
In this study, 52% of runners showed the same IFCP over all running speeds whereas other subjects were termed “transitional” runners, which made the transition from an IRFC to an IMFC (24%) or IFFC (5%) or from an IMFC to a IFFC (9%). Some runners even showed a double transition from an IRFC over an IMFC to an IFFC (7%). These “transitional” runners create the complex task of a shoe design that is functionally adjusted for changes in IFCP, both at low and high running speeds.
Previous research reported larger percentages of IMFC and IFFC patterns during marathon and half marathon racing events in elite distance runners compared with recreational runners (19,23), suggesting there might be a performance benefit with IMFC or IFFC patterns. However, our results suggest that the greater percentages of IMFC and IFFC in elite runners might just be a consequence of their faster running speeds rather than these IFCP being beneficial for performance. This statement is supported by a research by Larson et al. (24), which found no significant differences in marathon time between the different foot strike pattern groups.
Atypical IRFC patterns
A main finding of this study was the splitting of IRFC into two groups: atypical and typical IRFC. On the basis of a qualitative assessment of the COP patterns, 22% of runners showed this pattern at two or more speed conditions and 18% of all runners showed this pattern with both feet in two or more speed conditions. This indicates that this pattern should indeed be considered as a distinct IFCP.
The atypical IRFC were characterized by an initial fast anterior displacement of the COP along the lateral shoe margin. These patterns have not been described before. At the lowest speed of 3.2 m·s−1, these atypical IRFC are characterized by an earlier first metatarsal contact (4.0% ± 2.0% of contact time) compared with the typical IRFC (11.8% ± 2.9% of contact time). This timing can be used as a criterion to distinguish between the atypical IRFC and the typical IRFC. However, for the foot contacts with a first metatarsal contact between 6% and 8% of contact time, a qualitative assessment of the COP trajectory is needed. Other criteria, based on kinematics and/or COP-based calculations, could also provide a good or even better classification into different IFCP categories.
On the basis of the previously reported higher loading variables in IRFC patterns, some authors have suggested that a switch from an IRFC to an IFFC or IMFC could possibly be beneficial toward external loading and/or injury susceptibility (27,32). Research that supports these suggestions reports a reduction in impact loading with an IMFC pattern (2,3) or an IFFC pattern (7,26). However, not all studies support these findings (5,17,25,32,34), and neither does our research. This discrepancy may partially be explained by atypical and typical IRFC.
As IFCP is frequently associated with the impact-like character of the foot contact, VILR was assessed. This showed that apart from the atypical COP patterns, at all studied running speeds, runners with an atypical IRFC were also characterized by higher VILR values than runners with typical IRFC. In the atypical IRFC the mean VILR increased from 153.0 BW·s−1 at 3.2 m·s−1 to 325.2 BW·s−1 at 6.2 m·s−1. In the typical IRFC, the mean VILR increased from 109.2 BW·s−1 at 3.2 m·s−1 to 259.6 BW·s−1 at 6.2 m·s−1. No significant difference in VILR was found between the typical IRFC and the IMFC and IFFC. The reported VILR values, which increased with running speed, are in line though with previous research (12,20,30). However, other studies (6,25) reported lower VILR. This discrepancy might be due to the lower cutoff frequency of 50 Hz that was used to filter GRF data in these studies. This might have caused more smoothing of the initial vertical GRF signal resulting in lower VILR. If the atypical IRFC patterns, with larger VILR, would not have been identified and instead treated as typical IRFC patterns, the entire IRFC pattern group would have showed larger VILR than both the typical IRFC and the IMFC.
It has been shown that increased stride length results in an increase in VILR (20). Such findings could support the reasoning that the lower VILR found in runners with an IFFC or IMFC might be due to the shorter stride lengths (and higher step frequencies) found in these runners (2,16). However, the reported differences in stride length and frequency between IRFC and IMFC or IFFC are much smaller (<2%) than the range of stride lengths that show an increase in VILR. Therefore, we believe that other factors than just the difference in stride lengths, such as segment and joint positions and speeds at touchdown, should be assessed to help explain the observed VILR differences.
When running with an IFFC or IMFC, the impact of running is partially attenuated by an initial ankle dorsiflexion movement (26). With a typical IRFC, this is performed by the cushioning properties of both the heel’s fat pad and shoe midsole (9). A possible explanation for the higher VILR associated with an atypical IRFC could be that when running with an atypical IRFC, neither of these “strategies” of impact reduction are fully used. Initial contact is made with the rearfoot, limiting the possible use of an “ankle-dorsiflexion strategy,” and the early first metatarsal contact and the fast anterior COP movement indicate the limited use of the cushioning properties of the heel partition. Future research may verify these hypotheses.
It was shown that the typical IRFC group has longer contact times than the other IFCP groups. A possible explanation for these differences can be found in the time between the initial foot contact and the first metatarsal contact. This first phase of foot contact is shorter in the atypical IRFC and absent in the IMFC and IFFC. Future research should assess if any other differences in running style, running kinetics, or kinematics exist between the IFCP groups and could explain the observed differences in contact time and VILR.
Finally, our study has the following limitations. All subjects wore the same neutral shoe to counter the possible bias of shoe type on IFCP. However, this means that some subjects had to run in a shoe type different from their habitual running shoe type. This could have influenced their “natural” IFCP. The time of the first metatarsal contact allows to distinguish between the typical IRFC and the atypical IRFC. However, for 10% of foot contacts (those with a first metatarsal contact between 6% and 8% of contact time), an additional qualitative assessment of the COP trajectory was needed. Other criteria based on kinematics and/or COP calculations might provide the same or even better classification.
This study introduced a refined SI determination method, with COP based on plantar pressure measurements, and identified a group of atypical IRFC runners that are characterized by a fast first metatarsal contact and high VILR values. This methodological refinement and IFCP group determination could help future research to reduce the lack of uniformity in the current research regarding the relationship between IFCP and variables of interest.
Summary
IFCP is influenced by speed as some subjects changed toward a more anterior located IFCP with increasing speed. Also, the presented methods allowed to discriminate an atypical IRFC versus a typical IRFC, characterized by different COP patterns in the initial part of stance. This resulted in higher VILR for the atypical IRFC group, but no difference in VILR between the typical IRFC and the IMFC groups. These findings challenge and underline the need for future research linking IFCP, measured accurately over a relevant velocity range, to injury susceptibility, performance, economy, or specific footwear needs.
For this study, the researchers received financial and product support from the Li Ning Company Ltd. We also thank Pieter Fiers for his help with the data collection and data processing.
The authors 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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