Relationships among self-reported shoe type, footstrike pattern, and injury incidence.
Goss, Donald L.
Gross, Michael T.
|Publication:||Name: U.S. Army Medical Department Journal Publisher: U.S. Army Medical Department Center & School Audience: Professional Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2012 U.S. Army Medical Department Center & School ISSN: 1524-0436|
|Issue:||Date: Oct-Dec, 2012|
|Topic:||Event Code: 310 Science & research|
Many healthcare providers have received questions regarding
barefoot or minimalist shoe running from clients with or without a
history of previous injury. In our recent review of literature, (1) we
noted that little to no injury comparisons exist between runners wearing
traditional running shoes and those running in minimalist footwear.
With annual injury incidence averaging 50% and the majority of injuries occurring at the knee, (2) several authors have suggested barefoot or minimalist shoe running as a possible method to reduce injury rates among runners. (3-6) Rearfoot striking runners in traditional shoes contact the ground with a relatively dorsiflexed ankle and extended knee. This initial posture causes rearfoot strikers to rely heavily on the knee extensors to attenuate vertical ground reaction forces using eccentric muscle activation. (7) Increased activation of the knee extensors may increase patellofemoral compression and tibio-femoral joint loading. Barefoot runners have demonstrated a more anterior foot strike with slight plantar flexion and increased knee flexion, (3,8,9) reduced initial vertical ground reaction forces, (10) reduced stride length, (9,11) and increased stride frequency (9,11) compared with rearfoot strikers in traditional shoes. Runners who wear minimalist footwear may demonstrate running mechanics similar to those involved in barefoot running, (9) or some runners wearing minimalist shoes may still use a rearfoot strike pattern. (12,13)
In an effort to assess injury incidence among foot strike patterns and shoe conditions, we developed and implemented an online survey instrument. The purposes of this survey were to assess the association of different shoe selection with reported foot strike patterns, to compare overall injury incidence associated with different shoe conditions and strike patterns, and to identify potential anatomic location differences in injury proportions between different shoe conditions. We hypothesized that barefoot and minimalist shoe runners would describe a more anterior foot strike pattern than traditionally shod runners. We also expected that experienced minimalist shoe runners would report fewer knee injuries and more foot and ankle injuries.
After receiving local approval from the institutional review board at our university and a local military facility, we recruited 2509 runners (1254 male and 1255 female) aged 18 to 50 who reported running at least 6 miles per week. The recruiting was accomplished by posting messages on university, military, and local running club listservs; posting flyers in local fitness centers; and by posting a link to the survey on several websites frequently viewed by runners. We made an intentional effort to recruit additional barefoot and minimalist shoe wearing runners to draw conclusions based on these groups.
[FIGURE 1 OMITTED]
The survey consisted of a maximum of 41 questions that participants completed based on responses to previous questions. We conducted pilot testing of the survey instrument with 10 physical therapy students and 10 military personnel. The mean survey completion time was 7 minutes. No questions were deemed ambiguous or offensive during pilot testing, so the composition of survey questions remained intact. We collected the survey data using an anonymous online survey posted on www.surveymonkey.com during a 16 month period from September 2010 through December 2011. Only "experienced" runners who reported using the same footstrike pattern and shoe condition for a minimum of one year were included in the injury comparisons.
We used chi-squared analysis to compare reported footstrike proportions among runners wearing traditional shoes, barefoot runners, and runners who reported wearing minimalist shoes. We used chi-squared analyses to compare retrospective lower extremity injury incidence rates overall and at each anatomic region between runners in traditional and minimalist shoes. We also used chi-squared analysis to compare lower extremity injury incidence rates among reported footstrike groups regardless of shoe selection. We calculated relative risk ratios for incidences of lower extremity injuries when chi-squared analyses were significant at the alpha=0.05 level. We also used chi-squared analysis to compare lower extremity injury incidence rates among 3 groups of runners with various levels of running experience. Finally, we described the proportion of runners who reported changing their footstrike pattern or shoe preferences over the preceding year. Even though these runners were not used in the injury analyses, we reported the reasons runners gave for making changes in their footstrike pattern or shoe preferences.
Of the 2509 individuals who initiated the survey, 2157 (86%) completed the survey. Not all participants answered every question. Some questions were skipped based on previous responses and some were skipped due to participant choice. Runners who reported changing their footstrike pattern or running shoe preference in the previous 12 months (n=1363) were excluded from further analyses. Since a large proportion of our sample reported changing their footstrike pattern or shoe preference or both, this reduced the size of our sample to 1146 for data analysis. From these runners, 904 completed all of the questions related to injury incidence and reported wearing traditional shoes or minimalist shoes, or running barefoot (Figure 1).
Seventy-three percent (662/904) of the participants in our survey reported using traditional shoes and not changing their footstrike pattern in the previous year, 25% (226 /904) reported wearing minimalist shoes and not changing their footstrike pattern in the previous year, and only 2% (16/904) reported running barefoot greater than 50% of the time and not changing their footstrike pattern for over a year. These 904 runners were used for comparisons of self-reported footstrike associations among shoe conditions. With only 16 primarily barefoot runners, we lacked sufficient statistical power to draw injury associations from this group. We examined injury associations between the remaining 888 runners wearing traditional and minimalist shoes. We report sample sizes for each question because sample size varied from question to question based on subject qualification and participation.
Runners in traditional shoes, minimalist shoes, and barefoot runners were similar in age, height, and mass. Most barefoot runner and runners who wore minimalist shoes were men. More minimalist shoe runners reported running faster and farther than those in other groups. Barefoot runners reported a greater number of total years running. Barefoot and minimalist shoe runners reported a shorter period of time in their current shoe condition (Table 1).
Irrespective of shoe type, 31% (280/904) of runners in our sample reported using a rearfoot strike, 43% (389/904) a midfoot strike, 20% (181/904) a forefoot strike, and 6% (54/904) reported being unsure of their footstrike tendencies. Fifty-eight percent (524/904) of runners surveyed reported using their preferred strike pattern exclusively. Twenty-three percent (208/904) of respondents stated that they varied their strike pattern and 19% (172/904) were unsure of the variability of their strike pattern.
Approximately 73% (662/904) of the runners in this sample reported wearing traditional running shoes (35.8% stability, 22.2% cushioned, and 15.1% motion control). Twenty-five percent (226/904) reported using minimalist footwear, and 1.8% (16/904) reported running barefoot greater than 50% of the time. Shoe selection was significantly associated with reported footstrike ([[chi square].sub.4]=143.4, P <.001). Barefoot and minimalist runners reported a more anterior footstrike than traditionally shod runners (Table 2, n=799).
We asked runners:
Runners who reported running greater than 50% of their mileage in bare feet represented a small portion of our sample (n=16). One third (5/16) of these runners reported wearing shoes for some portion of their weekly mileage. The mean length of time these runners reported running primarily barefoot in this group was 25 months [+ or -]17 months. Due to the small sample size of barefoot runners, we were unable to make any injury associations from this group.
Runners wearing traditional shoes were 3.41 times more likely to report injuries than experienced minimalist shoe wearers (46.7% traditional vs 13.7% minimalist, [[chi square].sub.1] = 77.4, P <.001, n = 888). Traditional shoe wearers were 2.64 times more likely to report foot injuries ([[chi square].sub.1] = 10.11, P=.001), 2.84 times more likely to report ankle injuries [[chi square].sub.1] = 7.24, P = .007), 3.2 times more likely to report lower leg injuries ([[chi square].sub.1] = 13.62, P <.001), 3.2 times more likely to report knee injuries [[chi square].sub.1] = 19.3, P <.001), and 9.8 times more likely to report hip injuries [[chi square].sub.1] = 15.36, P <.001) than experienced minimalist shoe runners (Figure 2). The mean length of time the experienced minimalist shoe wearers reported wearing these shoes was 26 months [+ or -]16 months. Injury incidence associations at the thigh and low back demonstrate a similar trend as with other anatomical regions, but chi-shared analyses were not statistically significant between groups, likely due to a small number of reported thigh  and low back  injuries among minimalist shoe runners.
We combined shoe preference groups to examine injury incidence rates among footstrike groups. Runners reporting greater than one year of using a rearfoot strike pattern reported a one year injury incidence of 52.4%, while experienced midfoot strikers 34.7%, and experienced forefoot strikers reported an injury incidence of 22.8% ([[chi square].sub.1] = 46.07, P <.001, n=881).
Injury incidence rates did not differ between high and low mileage groups. Respondents who ran more than 30 miles per week reported overall injury incidence rates of 41.4%. Those who reported running fewer than 20 miles per week reported injury incidence of 35.9% ([[chi square].sub.1] = 2.02, P=.16, n=765, power=0.79 for small effect size, 0.1). Injury rates were also similar among reported training pace groups.
Since minimalist shoe wearers and barefoot runners reported more overall years running, we conducted a secondary analysis to examine associations between running experience and injury incidence. Running experience was significantly associated with injury incidence ([[chi square].sub.2] = 6.17, p=.046, n=721). Runners who reported more years experience running reported fewer injuries. Injury incidence in runners who reported 1 to 5 years experience with running was 55% (219/398); runners with 6 to 10 years running experience reported injury incidence of 48.7% (92/189); 43.3% (58/134) of runners with greater than 10 years experience running and reported injuries (Table 3).
Reasons for Change of Shoe or Footstrike Preferences
The runners described in this section were not used in any of the previous analyses, but we believe it is noteworthy to describe the reasons given for runners' decisions to change shoe preference or footstrike pattern. Approximately 35% (866/2509) of the overall survey respondents reported changing their strike pattern in the previous 2 years. Eighty-two percent (707/866) of those who changed their footstrike reported formerly being a rearfoot striker. Of those who changed their strike pattern, 46% (397/866) did so because of injury. The 397 runners who changed their footstrike pattern due to injury reported a total of 500 injuries. Fifty-three percent of those injuries occurred at the knee among 66% of the runners who reported knee injuries (264/397).
Approximately 34% (848/2509) of runners in the overall survey reported changing their primary shoe type in the last 2 years. Eighty-three percent (702/848) of those who changed their primary shoe type had worn traditional running shoes previously. Forty-four percent (372/848) of runners who changed shoes reported that they did so because of injuries. The 372 runners who changed their footstrike pattern due to injury reported a total of 411 injuries. Forty-nine percent of these injuries occurred at the knee with 58% of these runners reporting a knee injury (214/372).
Little research has been conducted concerning injury trends that are associated with barefoot/minimalist shoe running or other alternative running styles. We hypothesized:
1. Barefoot and minimalist shoe runners would describe a more anterior foot strike pattern than traditionally shod runners.
2. Experienced minimalist shoe runners would report fewer knee injuries and a greater incidence of foot and ankle injuries.
Our data support our first hypothesis. Reported footstrike pattern was associated with shoe selection. Barefoot and minimalist shoe wearers reported using a more anterior footstrike pattern. The second hypothesis was partially supported. Experienced minimalist shoe runners reported fewer knee injuries without an increase in reported foot and ankle injuries. While we did not form a hypothesis concerning overall injury incidence between shoe groups, minimalist shoe runners reported fewer injuries. Our results indicate that experienced minimalist shoe wearers who have been wearing minimalist shoes for more than one year reported fewer injuries at the hip, knee, lower leg, ankle, and foot.
While significant associations were observed among reported footstrikes and injury incidence, the accuracy of self-reported footstrike has not been documented in the literature. Previous authors have reported a predominant prevalence of rearfoot strikers among traditionally shod runners. (3,14,15) With the recent attention to a more anterior footstrike and minimalist shoe selection in the popular media after the publication of "Born to Run" by Christopher McDougal1, (5) many runners have attempted to alter their footstrike tendencies or shoe preferences, or both. This point is evidenced by the many individuals who reported a change in their footstrike pattern or shoe preference in an attempt to reduce injuries, particularly knee injuries. Individual runners may or may not be able to report their own foot strike pattern accurately. We recently tested 87 runners using an instrumented treadmill and observed a self-reported accuracy of 69% for runners' ability to discern between a rearfoot and a more anterior footstrike pattern. (13) We readily admit that this is not excellent reliability, but hopefully it gives the reader a sense of context when interpreting our survey data.
Another limitation of this survey study is the reliability of self-reported shoe type worn by each respondent. In an effort to improve this reliability, we did not assess injury differences among motion control, stability, or cushioned shoe categories. Instead, we have combined these 3 groups into a "traditional shoe" category for comparisons to runners who wore minimalist shoes and to barefoot runners. We believe it is reasonable to assume that these runners knew if they were choosing to run barefoot, in a minimalist shoe, or in a traditional shoe since the average runner in this sample had been running for approximately 8 years. In our own recent laboratory sample of 87 runners, we have observed a self-reported accuracy of 98% for runners who reported wearing either traditional shoes or minimalist shoes. (13) The proportions of minimalist runners and anterior foot strikers in this study should not be taken as representative of the US population as a whole since running clubs consisting of primarily barefoot and minimalist shoe runners were specifically recruited for participation.
We chose to use self-reported injury data instead of medical diagnostic information for its ease of collection and because we believe that many runners will simply alter their training schedule rather than seek medical attention for minor "overuse" injuries. Runners may report these injuries in a survey, but the injuries may not appear in a medical record. While injury surveys are commonly published in the literature, (16-19) self-reported injury data may have limitations relative to the accuracy of the data. We chose to ask about injuries by body region and not trust reliability of self-report diagnoses (eg, "foot" injury vs plantar fasciitis or metatarsalgia; and "knee" injury vs patellofemoral pain syndrome or patellar tendonopathy). While this strategy limited the amount of information gathered, we believe keeping the choice regional and not diagnosis specific likely maintained some of the reliability of the self-reported data.
While age did not differ between our groups of traditional shoe and minimalist shoe runners in our sample, runners who wore minimalist shoes reported more years of running experience. This fact may be a potential confounder and may contribute to the lower injury incidence seen in minimalist shoe runners. Runners with more experience may have learned how to avoid injuries better than less experienced runners, and/or runners who are repeatedly injured over the first several years of running may choose to quit running and pursue an alternative form of exercise.
In our survey study, shoe selection and reported footstrike pattern were both significantly associated with reported injury incidence. We believe reported shoe selection is likely more reliable than reported footstrike pattern. These associations are preliminary in nature and do not imply cause and effect relationships. It is possible that this convenience sample of minimalist shoe wearing participants is biased and enthusiastic to share their experiences. Further research with prospective longitudinal samples is needed to investigate the injury prevention capabilities of alternative running styles with validated footstrike patterns compared to traditionally shod rearfoot striking running. For further description of the running mechanics of runners using different running styles, we refer the reader to our previously published review article. (1)
Self-reported footstrike pattern was associated with shoe selection. Barefoot runners and minimalist shoe wearers reported a more anterior footstrike than did traditionally shod runners. Experienced minimalist shoe runners reported fewer overall injuries and fewer injuries, specifically at the hip, knee, lower leg, ankle, and foot than did traditional shoe wearing runners. Runners who reported using a more anterior footstrike pattern reported fewer injuries than rearfoot striking runners. Additional prospective longitudinal research is needed to study injury incidence among runners using different footstrike patterns and shoe preferences.
We thank Dr Joseph Myers, Dr Charles Blake, Mr Amby Burfoot, Ms Shelly Schmidt, Mr Danny Dreyer, and Ms Renee Johnson for their assistance with subject recruitment.
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LTC Donald L. Goss, SP, USA
Michael T. Gross, PhD
LTC Goss is an Assistant Professor, US Army/Baylor University Doctoral Program in Physical Therapy, Fort Sam Houston, Texas.
Dr Gross is a Professor, Program in Human Movement Science, Division of Physical Therapy, University of North Carolina, Chapel Hill, North Carolina.
If an injury is defined as something that caused you to modify your training schedule for at least 1 week due to pain or discomfort (with or without formal medical care), have you experienced any lower extremity injuries in the past 12 months that you believe were caused by running?
Table 1. Demographics of questionaire respondents Shoe Preference Traditional Shoes Minimalist Shoes Number 662 226 Gender Male: 282, 42.6% Male: 156, 69% Female: 380, 57.4% Female: 70, 31% Age 37.2 [+ or -] 8.5 years 39.0 [+ or -] 8.4 years Height 1.74 [+ or -] 0.11 m 1.74 [+ or -] 0.09 m Mass 72.3 [+ or -] 13.6 kg 76.5 [+ or -] 10.2 kg Years Running 6.4 [+ or -] 5.7 years 9.2 [+ or -] 8.3 years Years in Current 4.5 [+ or -] 3.7 years 2.1 [+ or -] 1.4 years Shoe Type Years Using 4.2 [+ or -] 3.8 years 3.4 [+ or -] 3.4 years Current Footstrike 20.3% 6-10 mpw 20.8% 6-10 mpw 34.2% 11-20 mpw 31.7% 11-20 mpw Weekly Mileage 25.4% 21-30 mpw 24.6% 21-30 mpw 12.5% 31-40 mpw 9.5% 31-40 mpw 7.6% >40 mpw 13.4% >40 mpw 20.7% <6 mph 12.7% <6 mph 35.4% 6.1-7 mph 26.5% 6.1-7 mph Training Pace 28.2% 7.1-8 mph 33.9% 7.1-8 mph 11.2% 8.1-9 mph 17.7% 8.1-9 mph 4.5% >9 mph 9.2% >9 mph Shoe Preference Barefoot Number 16 Gender Male: 12, 75% Female: 4, 25% Age 37.3 [+ or -] 9.0 years Height 1.80 [+ or -] 0.08 m Mass 76.7 [+ or -] 10.1 kg Years Running 12.2 [+ or -] 10.5 years Years in Current 2.1 [+ or -] 1.4 years Shoe Type Years Using 2.7 [+ or -] 2.4 years Current Footstrike 12.5% 6-10 mpw 33.3% 11-20 mpw Weekly Mileage 29.2% 21-30 mpw 20.8% 31-40 mpw 4.2% >40 mpw 12.5% <6 mph 45.8% 6.1-7 mph Training Pace 33.3% 7.1-8 mph 8.3% 8.1-9 mph 0% >9 mph Descriptive statistics reported as mean [+ or -] SD. "mpw" indicates miles per week. "mph" indicates miles per hour. Table 2: Associations of shoe type and self-reported footstrike. Shoe Type Reported Footstrike Rearfoot Midfoot Forefoot Traditional 240 (41.4%) 266 (45.9%) 74 (12.7%) Minimalist 12 (5.9%) 99 (48.5%) 93 (45.6%) Barefoot 0 (0.0%) 9 (60.0%) 6 (40.0%) Table 3. Associations of running experience and injury incidence. Years Running Injury No Injury 1 to 5 years 219 (55.0%) 179 (45.0%) 6 to 10 years 92 (48.7%) 97 (51.3%) >10 years 58 (43.3%) 76 (56.7%) Figure 2. Self-reported injury rates by shoe type. Traditional Minimalist Foot 14 5.3 Ankle 8.8 3.1 Lower leg 12.7 4 Knee 17.1 5.3 Thigh 3.2 1.8 Hip 8.8 0.9 Low Back 3.3 0.5 Note: Table made from bar graph.
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