According to “A Special Report on Trail Running 2010,” a publication produced through a partnership project of Outdoor Foundation and Montrail, there were 4.8 million runners who participated in trail running in 2009, accounting for 153.7 million “runs.” This represents approximately 11% of all runners. A simple review of the content of current runner's magazines, both in print and on-line, demonstrates a similar level of popularity of trail running as a sport. Two popular runner's magazines, “Runner's World” and “Running Times,” have entire on-line sections devoted exclusively to trail running, whereas 1 runner's magazine, “Trail Runner,” has focused entirely on trail running since its inception in 1999. However, current scientific literature on trail running does not reflect the growing interest or number of trail runners currently participating in the sport: a PubMed literature search (accessed January 14, 2013) on trail running yielded only 0.3% of the literature available on running as a whole.
Foot strike has been a well-studied topic in scientific literature, as well as a popular topic in runner's magazines. However, a limited number of recent “in-race” foot strike studies exist for traditional road marathon races, in which rear-foot strike (RFS) prevalence of between 87.8 and 93.7% has been observed (9,10). Neither study revealed a statistically significant difference between foot strike and sex. Larson et al. and Kasmer et al. were divided on the association of foot strike and performance: Larson concluded there was no relationship, whereas Kasmer et al. found that RFS was less common in elite runners. Neither study provided sufficient analysis of shoe type to determine its association with foot strike or performance.
The goal of this study was to fill the void of foot strike prevalence studies in long-distance trail running, while further studying the associations between foot strike, shoe type, and performance. It was hypothesized that (a) RFS prevalence would be lower in trail ultramarathon runners as compared with road marathon runners; (b) there would be no relationship between foot strike and sex; and (c) minimalist shoe type would be associated with a non-RFS foot strike pattern, but not improved performance; while a non-RFS foot strike pattern would be more common in more elite runners. The objectives of this study, therefore, were to (a) determine the RFS prevalence in a 50-km trail race and compare with traditional road marathon races; (b) determine if there is a relationship between foot strike and sex in a 50-km trail race; and (c) determine if there is a relationship between foot strike, shoe type, and performance in a 50-km trail race. Furthermore, by accomplishing the objectives of this study, we aimed to assist in the prevention, diagnosis, and treatment of injury in trail runners by publishing foot strike prevalence data specific to trail runners, while investigating possible training modifications to improve their race performance.
Experimental Approach to the Problem
To collect the foot strike pattern of trail runners, we videotaped the 2012 Ice Age Trail 50-km race, La Grange, WI, USA (May 12, 2012). Setup was similar to previous work (9) with some exceptions, most notably improved camera resolution, decreased perpendicular offset distance to the filming zone, and decreased width of the filming zone. These changes enabled the analysis of both foot strike and shoe type for each runner. Overall RFS prevalence was determined by video analysis for comparison with previous studies. Foot strike was analyzed by sex to determine whether a more common foot strike pattern existed among either sex. Foot strike was also analyzed by shoe type to determine if a non-RFS pattern was more common among minimalist runners. Finally, performance was analyzed by foot strike and shoe type to determine if either variable was associated with improved performance, and as such be considered as a possible training modification to improve race performance.
There were 165 runners videotaped at the 8.1-km mark of the 2012 Ice Age Trail 50-km race. Runners were excluded from the further analysis if either (a) the runner was walking (n = 2) or (b) foot strike was visibly altered by runners passing in the opposite direction (n = 2). The remaining 161 runners were included for foot strike classification. The study was granted an exemption from institutional review board oversight by the Medical College of Wisconsin Institutional Review Board in accordance with 45CFR46.101(b) (4) and did not require informed consent be obtained. The race director was contacted and granted permission to videotape participants of the race.
All race participants were videotaped at 8.1 km from the starting line, 0.8 km before the first aid station. The 8.1-km mark was within a section of the course that was single track for approximately 2 km before and 0.8 km after the filming zone, which naturally forced single-file running. The filming zone was level, approximately 46 cm wide, and without significant terrain irregularity. Thus, there was no preparation of the site or prior indication of the oncoming filming zone on race day to capture the most “natural,” foot strike.
Video recording was collected at 240 frames per second (Casio EX-ZR 200, Tokyo, Japan). The camera was positioned perpendicular to the trail, offset 1.4 m from the center of the filming zone, secured to a tri-pod on a firm, level surface at a height of 40 cm. Bib number and sex were sequentially collected to match video data.
Foot strikes were captured for each runner: 2 foot strikes for 35 runners (21.7%), 3 for 119 runners (73.9%), and 4 for 7 runners (4.3%). Foot strike classification and shoe type classification were then completed for each foot strike, simultaneously, but independently by a 3-member panel, using frame-by-frame analysis provided by Apple QuickTime Version 7.6.9 (Apple Inc., Cupertino, CA, USA). The 3-member panel consisted of an MD, a certified pedorthist, and an MD-candidate. Each member of the panel had prior research experience in foot strike classification of runners. In the event of a disagreement on foot strike classification, the classification agreed on by 2 of the 3 examiners was assigned. In no case did all 3 examiners disagree on classification. Foot strike classifications included fore-foot strike (FFS), mid-foot strike (MFS), and RFS as defined by Lieberman et al. (11). An additional classification of split-strike was identified and defined as asymmetry between the foot strikes of the left foot and right foot. Shoe type was classified as minimalist, having a known heel lift of ≤4 mm, or traditional.
Overall prevalence of each foot strike classification was calculated. The asymmetrical foot strike classification of this study and other previously published work was excluded from further analysis, while FFS and MFS classifications were merged into a single classification, non-RFS. A Fisher's exact test was used to compare foot strike prevalence with the results of Larson et al. (10) at the 10-km site of a marathon. A χ2 analysis, in lieu of a Fisher's exact test because of a large n, was used to compare foot strike prevalence with the results of Kasmer et al. (9) from the 8.1-km site of a marathon. Fisher's exact tests were used to evaluate the relationship between foot strike classification and sex, as well as foot strike classification and shoe type. Mann-Whitney tests were used to evaluate the association between shoe type and performance, as well as foot strike and performance, as neither data passed the D’Agostsino and Pearson omnibus normality test and could not be normalized. The association of shoe type and performance and foot strike and performance was further evaluated by separating runners into groups of 20 by sequential rank and analyzing data with χ2 tests. Statistical significance was set at p ≤ 0.05 for all analyses. All statistical analysis was completed using GraphPad Prism 5.02 for Windows (GraphPad Software, San Diego, CA, USA; www.graphpad.com).
There were 165 runners filmed at the 8.1-km site of the 2012 Ice Age Trail 50-km race. One hundred fifty-five runners successfully finished. Foot strike pattern was classified unanimously for 152 (94.4%) of 161 runners (interrater kappa coefficient, κ = 0.85). Rear-foot strike prevalence was 85.1%. There were no runners classified as FFS. Foot strike prevalence is included in Table 1.
The runners in this study demonstrated a significantly lower RFS prevalence compared with the runners observed by both Larson et al. (p < 0.01) and Kasmer et al. (p < 0.01) (Figure 1). There was no relationship found between foot strike and sex (p = 0.60). There was a significant effect of shoe type on foot strike, with RFS being less common among runners in minimalist shoes (p < 0.01) (Figure 2). There was no association between foot strike and performance, determined by rank at 8.1 km (p = 0.83) (Figure 3) or separated by groups of 20 by sequential rank (p = 0.79). There was, however, a significant effect of shoe type on performance, as determined by rank at 8.1 km, with faster runners more likely to be wearing minimalist shoes (p < 0.01) (Figure 4). The significance persisted after separating into groups of 20 by sequential rank (p < 0.01).
The RFS prevalence observed among trail runners participating in this study was found to be significantly lower than the RFS prevalence among road racers in similar distance races (Figure 1) (9,10). No single explanation for this observed difference exists; however, several theories can be offered.
First, impact transients have been defined previously as the collision forces generated during heel contacts (11) and may be related to injury (1,2,7,13,17,18). As such, some runners may preferentially avoid an RFS pattern to avoid impact transients and reduce the cumulative effect of these impact transients, as previously suggested (12,20). The runners in this study were classified as ultramarathon runners, defined as runners engaging in any distance longer than a marathon. As such, these “ultramarathoners” may be more accustomed to running longer distances than that of typical marathon runners and, therefore, more accustomed to adopting strategies, including a non-RFS pattern, to preserve their bodies from the larger number of cumulative impact transients.
Second, minimalist shoe type was found to be associated with decreased RFS prevalence (Figure 2). This is the first “in-race” study to investigate the relationship between shoe type and foot strike pattern by evaluating all runners' shoe type, although an observation of apparent decreased RFS in minimalist shoes has been reported previously (9). It is possible that the proportionally large number of trail runners observed wearing minimalist shoes in this study (16.4%) was greater than in the road runners observed in previous studies and, therefore, contributed to the decreased RFS prevalence observed in this study. This theory supports the claim that minimalist shoe type promotes a non-RFS pattern.
However, it is important to note that minimalist shoe type did not force a non-RFS pattern, as 66.7% of runners wearing minimalist shoes were observed using an RFS pattern. This observation of RFS prevalence in runners wearing minimalist shoes was greater than that previously observed by Goss and Gross (6), in which RFS prevalence of experienced minimalist runners wearing minimalist shoes during a 5-minute treadmill run was 34.3%. Thus, a proportion of runners wearing minimalist shoes, including experienced minimalist runners, have been observed using an RFS pattern. The increased RFS prevalence of minimalist runners in this study may represent (a) a transition by some runners with a non-RFS pattern initially to an RFS pattern before the 8.1-km mark as a result of increased muscle fatigue associated with increased muscular demands of a non-RFS pattern (5) or (b) a more “natural” sample of runners' foot strike patterns in “in-race” conditions, less affected by the “sampling effect” previously described by Morin et al. (14).
Third, the difference in RFS prevalence in this trail ultramarathon compared with traditional road marathons may be explained by potential differences between long-distance running on a road and on a trail. As suggested by Morin et al. (15), the different and often variable running surface encountered during a mountain trail ultramarathon run as compared with a traditional road or treadmill run may induce different foot strike adaptations.
Previous studies by Larson et al. (10) and Kasmer et al. (9) are divided on the relationship of foot strike pattern on performance. Larson et al. (10) observed no association between foot strike pattern and performance, whereas Kasmer et al. (9) observed a decreased RFS prevalence in the more elite runners. When studying a group of elite half marathoners, Hasegawa et al. (8) also described a decreased RFS prevalence to be found in this sample of elite runners. In the present study, no such relationship between foot strike pattern and performance was supported (Figure 3). Thus, the decreased RFS prevalence observed in this study is unlikely to be attributed to a field of more elite runners, which is further supported by finishing times spanning between a 3.7 m·s−1 pace (7.16 minutes per mile) and a 1.5 m·s−1 pace (18.3 minutes per mile).
Although this study did not support a relationship between foot strike pattern and performance, it did suggest the presence of an association between shoe type and performance: minimalist shoe type was more common among the more elite runners (Figure 4). This finding suggests that either (a) minimalist shoe type is associated with a better running economy or (b) more elite runners prefer minimalist shoe type. The former was first suggested by Divert et al. (3), who concluded that decreased shoe mass was associated with decreased oxygen consumption. More recent studies have demonstrated mixed results. Perl et al. (16) concluded barefoot running was associated with increased efficiency in both FFS and RFS patterns, whereas Franz et al. (4) concluded that shod running was more efficient in MFS running. In the research of both Perl et al. and Franz et al., Vibram FiveFingers were the shoe type used to simulate the “barefoot” running condition, as supported by previously published work by Squadrone and Gallozzi (19). There has been no similar published research comparing barefoot running with other minimalist shoe types. Hence, the ability to generalize the findings of either Perl et al. or Franz et al. to this study, where all of the minimalist runners observed were in minimalist shoe type other than Vibram FiveFingers, has yet to be supported scientifically. Thus, the combination of findings in this study (i.e., improved performance associated with minimalist shoe type but not with a non-RFS pattern) suggests that the observed improved performance in runners in minimalist shoe type is more likely because of mass effect than to foot strike pattern. Similar to previously published research, there was no relationship observed between sex and foot strike (7,8).
This study represents the initial attempt to classify the foot strike patterns of runners during a trail race of approximate marathon distance. It provides insight into RFS prevalence, as well as the relationships between foot strike, shoe type, and performance. As such, there are limitations of this study. First, the definition of minimalist shoe type used in this study was strictly based on heel-toe drop. One may argue that there is more to the minimalist definition than heel-toe drop, specifically amount of support. In addition, as shoe type evolves, both minimalist and traditional, the distinction between the 2 may become increasingly ambiguous. Second, each runner's prior training and level of conditioning were not obtained, but may have contributed to his or her foot strike adaptations over the course of the race. Third, as this study only analyzed runners in 1 ultramarathon trail race, it may not be generalizable to a broader runner population or among varying running conditions. One may suggest that the mileage difference between a 50 km race and marathon, the difference between a trail race and a traditional road race, or both may represent a selection bias for a unique type of runner or unique running strategy compared to most traditional runners.
A possible limitation of this study is the definition of minimalist shoe type. The definition used in this study was strictly based on heel-toe drop. One may argue that there is more to the minimalist definition than heel-toe drop, specifically amount of support. In addition, as shoe type evolves, both minimalist and traditional, the distinction between the 2 may become increasingly ambiguous.
Further research in trail running is necessary to confirm the decreased RFS prevalence in this study of trail runners and to further investigate the result of fatigue on foot strike pattern, as previously demonstrated by Larson et al. (10). The relationships investigated in this study will need further support to determine if they are generalizable to a greater runner population, specifically the increasing number of trail runners.
As trail running grows in popularity, it can be expected that more trail runners will seek medical professionals (MD, Physical Therapist (PT), Certified Athletic Trainer (ATC), Certified Strength and Conditioning Specialist (CSCS)) for performance improvement, injury prevention, and injury treatment. This study suggests that RFS prevalence in trail runners is lower than in road runners. However, similar to road runners, most trail runners prefer an RFS pattern. Biomechanically, certain injury patterns may be more prevalent for certain foot strike patterns. Knowledge of a runner's specific foot strike pattern and subsequent biomechanical consequences may assist in understanding injury prevention, mechanism of injury, and most importantly, treatment or therapy recommendations. Furthermore, this study suggests that minimalist shoes may represent a reasonable training modification (for those runners wearing traditional shoes) to improve performance.
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Keywords:Copyright © 2016 by the National Strength & Conditioning Association.
running; endurance; gait biomechanics; running economy; injury