Characterization of Foot-Strike Patterns: Lack of an Association With Injuries or Performance in Soldiers

Transcription

1 MILITARY MEDICINE, 180, 7:830, 2015 Characterization of Foot-Strike Patterns: Lack of an Association With Injuries or Performance in Soldiers MAJ Bradley J. Warr, SP USA*; Rebecca E. Fellin, PhD*; Shane G. Sauer, MS*; LTC Donald L. Goss, SP USA ; Peter N. Frykman, MS*; Joseph F. Seay, PhD* ABSTRACT Objectives: Characterize the distribution of foot-strike (FS) patterns in U.S. Army Soldiers and determine if FS patterns are related to self-reported running injuries and performance. Methods: 341 male Soldiers from a U.S. Army Combined Arms Battalion ran at their training pace for 100 meters, and FSs were recorded in the sagittal plane. Participants also completed a survey related to training habits, injury history, and run times. Two researchers classified FS patterns as heel strike (HS) or nonheel strike (NHS, combination of midfoot strike and forefoot strike patterns). Two clinicians classified the musculoskeletal injuries as acute or overuse. The relationship of FS type with two-mile run time and running-related injury was analyzed ( p 0.05). Results: The Soldiers predominately landed with an HS (87%) and only 13% were characterized as NHS. Running-related injury was similar between HS (50.3%) and NHS (55.6%) patterns ( p = 0.51). There was no difference ( p = 0.14) between overuse injury rates between an HS pattern (31.8%) and an NHS pattern (31.0%). Two-mile run times were also similar, with both groups averaging 14:48 minutes. Conclusion: Soldiers were mostly heel strikers (87%) in this U.S. Army Combined Arms Battalion. Neither FS pattern was advantageous for increased performance or decreased incidence of running-related injury. INTRODUCTION Running is a popular, low cost, and proven method for maintaining aerobic fitness. Nineteen million Americans report running at least 100 days per year, 1 and studies have reported the benefits of running, including decreased pain, disability, and health care costs. 2,3 Running is the primary mode of required aerobic training utilized in the U.S. Army because of the fitness-related benefits and minimal requirement of equipment and resources. However, some of the benefits of running are offset by the risk of injury. Training-related injuries are often studied in the military. Although these studies do not separate injuries as a result of running from other activities related to training (load carriage, etc.), lower extremity musculoskeletal injuries account for a large percentage of these injuries seen in recruits and infantry. 4 In civilians, a comprehensive review reported that the incidence of lower extremity injuries among runners ranged from 19% to 79%. 5 Within the Army, there are many physical demands in addition to running, with combat arms military occupational specialties (MOS) the most physically demanding. Maintaining a high level of fitness in these occupations is imperative to *U.S. Army Research Institute of Environmental Medicine, Military Performance Division, 15 Kansas Street, Natick, MA Keller Army Community Hospital, 900 Washington Road, West Point, NY Army Medical Department Center and School, 3630 Stanley Road, Fort Sam Houston, TX Any citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement of approval of the products or services of these organizations. The views expressed in this manuscript are those of the authors and do not reflect the official policy of the Department of Army, Department of Defense, or the U.S. Government. doi: /MILMED-D Soldier and unit success. With the inherent risk of musculoskeletal injury associated with running, coupled with demands of combat arms occupations, it not surprising that Combat Arms Soldiers are more likely to experience soft tissue knee injuries than any other MOS. 6 To potentially decrease injury risk and increase performance, many runners attempt to alter their foot-strike (FS) pattern The mechanical goal of this alteration is to switch from a pattern where the heel strikes the ground first during a running stride (heel strike [HS] pattern) to a pattern where the midfoot or forefoot strikes the ground first. The midfoot and forefoot strike patterns can collectively be referred to as non-hs patterns (NHS), and runners may look to barefoot or alternative running styles (Chi or Pose running) to facilitate an NHS pattern. 11 Rather than being a function of what is worn (or not worn) on the foot, the injury implications may have more to do with what part of the foot impacts the ground first. Recent publications have implied that habitually running with an NHS pattern may reduce training-related injuries by reducing impact forces transmitted through the lower extremities. 12,13 In one study involving collegiate crosscountry runners, NHS runners suffered from about half of the running-related injuries that the HS runners incurred. 10 Once broken down by sex, only females had significantly fewer injuries with an NHS pattern, and there was no difference in injuries for male runners between the two patterns. However, this cohort averaged nearly 45 miles per week of running, 10 which calls into question whether these results apply to recreational or novice runners. In specific cases, modification of FS patterns have alleviated symptoms of some running-related injuries, such as anterior compartment syndrome 9 ; however, numerous reviews have found little to no evidence that supports changing FS patterns in order to prevent injury. 12,14 16 Most of these 830

2 reviews suggest prospective studies are needed to thoroughly describe the relationship between injury risk and FS patterns. Because the current distributions of FS patterns in the military are unknown, it is difficult to predict how or if the NHS pattern influences military injury rates. However, since a portion of runners already habitually run using both FS patterns, accurate characterization of current FS patterns will allow for the evaluation of injury outcomes related to each pattern. Therefore, the purpose of this study was to characterize FS pattern distribution, injury, and performance in a cohort of U.S. Army Combat Arms Soldiers. We hypothesized that the majority of our cohort would utilize an HS pattern, similar to reports from the civilian literature. Furthermore, we hypothesized that there would be no difference in proportion of retrospective injury incidence and 2-mile run time between groups. METHODS Participants A total of 341 healthy U.S. Army enlisted men were enrolled in this study (mean ± SD; 24.7 ± 5.1 years old, ± 8.5 cm, 81.0 ± 11.4 kg, 4.2 ± 4.1 years of military service). Command staff at Fort Carson, Colorado, permitted recruitment of these Soldiers from the 1-66th Armor Regiment. The volunteers gave informed consent. Ethical approval was granted by the U.S. Army Research Institute of Environmental Medicine s institutional review board. The investigators have adhered to the policies for protection of human subjects as prescribed DoD Instruction and the research was conducted in adherence with the provisions of 32 CFR Part 219. Individuals were excluded from this study if they currently suffered pain while running, were not between the ages of 18 and 40, or were on a medical profile that prevented them from running at a self-selected pace. Experimental Setup A level, straight paved area approximately 100 m long was utilized as a runway for this study. The starting and finishing lines were identified with traffic cones. At the approximate midpoint of the runway, a video capture volume was established. This volume included a high-definition video camera (Vixia HD40, Canon, Melville, New York) filming at 30 Hz mounted approximately 0.45 m from the ground. The camera was operated via Dartfish software version 5.5 (Dartfish, Fribourg, Switzerland) with a laptop computer (Precision M4400, Dell, Round Rock, Texas). A Mini- Mac 1000 W studio flood lamp (Bardwell & McAlister, Hollywood, California) was set up near the camera to illuminate the filming volume during early morning data collections. Approximately 8 m opposite the camera was a 0.6 m high by 7.3 m long white plastic backdrop. The length of the filming volume was designed to capture at least two full gait cycles from the waist down as the volunteer ran. After the finish line, a station was located where surveys could be completed. Protocol During the morning of their participation in the study, volunteers arrived with their unit to the testing location. The unit preformed a shortened physical training (PT), which typically consisted of a brief run (approximately 10 minutes) as warm up for the study. The unit was then briefed on the research study, and volunteers read and signed an informed consent document, which summarized the two tasks of running and completing the survey. To prevent data capture down time and because the time window with the Soldiers was limited, some volunteers completed the survey while the rest of the cohort ran. On arrival at the starting line, volunteers were individually assigned a subject number and instructed to run the length of the runway at the same speed at which they would typically perform their two-mile run for PT testing. As the volunteer drew near to the filming volume, cones funneled him to within 1 m of the backdrop. The backdrop provided contrast within the video to help identify FS patterns more easily. The volunteer was instructed to maintain his pace until after he passed the finish line at the end of the runway. The survey included questions on demographics, military training, running history, running-related injuries, days modified because of training in the past year and 2-mile run time from the most recent PT test. Self-reported running-related injuries were defined as any injury over the course of the participant's life that caused you to modify your training scheduleforatleast1weekduetopainordiscomfort,(withor without formal medical care) that occurred as a result of running. The survey asked for an injury description, the location involved, the date of the injury, and length of time to recovery. Study staff reviewed finished surveys to ensure their completeness before dismissing volunteers. Time to complete the survey and run was approximately 15 to 20 minutes. Data Processing Surveys from a previous study 17 were modified for use in this study. Completed surveys were digitized using Remark Office OMR8 software version 8.4 (Gravic, Malvern, Pennsylvania) and a Canon DR-9080C Scanner (Canon, Melville, New York) for the purpose of coding and review. The software highlighted any scanning issues for manual review. After review the data were exported into a Microsoft Excel (Microsoft, Redmond, Washington) spreadsheet. Within the spreadsheet, the data were sorted to allow for review of any missing or outlying data points. A subset of the surveys (n =15)wasreviewedinfullbyhandtoensureaccuracyof the encoding process. Self-reported running-related injury data were independently reviewed and classified as acute or overuse by a physician assistant and physical therapist with over 28 years of combined experience. Overuse injury was defined as an injury that likely occurred progressively from repetitive use, whereas acute injury was defined as occurring from a single 831

3 TABLE I. Descriptive Breakdown by Volunteer s Self-reported Average Weekly Running Mileage for Average Run Times, Cases of Reported Acute or Over-use Injury, and the Number of Individuals Who Reported no Injury < traumatic event. In many cases, the diagnosis was accurately provided by the respondent (e.g., patellar tendonitis), whereas in other cases, the classification decisions were based on a free texted injury description, duration of time to recovery, and body region. If these data were unclear or missing, the injury was not categorized. These data were supplemented with an analysis of the reported days of modified training. In the instance of disagreement between clinicians, a discussion occurred and agreement was achieved for all cases. Because the average time in service was 4.25 years, only injuries that were documented as occurring in the previous 5 years were included in the injury analysis. Video data of FS patterns were reviewed by two blinded independent investigators and categorized each volunteer as utilizing an HS or NHS pattern. An HS pattern was defined as one in which the heel of the shoe clearly made contact with the ground before the rest of the foot. An NHS pattern included both forefoot pattern, in which the ball of the foot contacted the ground first, and a midfoot pattern, in which the heel and ball contacted the ground simultaneously. During video data review, only a clear FS was used for identification and each data point contained at least two footfalls. Agreement between investigators was 93% and discrepancies were reviewed as a team until consensus was achieved for each volunteer. These data were manually incorporated in to the MS Excel spreadsheet with the survey data. Data Analysis The association between FS pattern and the occurrence of a running-related injury was analyzed using a χ 2 test. Kruskal Wallis tests were used to evaluate the relationship between days of modified training because of injury in the last year as well as 2-mile run time between Soldiers with HS and NHS patterns. For all statistical tests, we used α = >30 Total HS NHS HS NHS HS NHS HS NHS HS NHS HS NHS HS NHS N Average Run Time N/A Acute N/A 45 3 Overuse N/A Uncategorized N/A 6 1 No Injury N/A Average run time = average self-report 2-minute time; Injury categories are reported in number of observations (N), and are described in the text. time, cases of acute injury and overuse injury reported in the past 5 years, and reported weekly mileage categories is provided in Table I. There was no significant difference in the percentage of Soldiers utilizing an HS or NHS and any history of a running-related injury ( p = 0.51), one or more overuse injuries in the past 5 years ( p = 0.89), or one or more acute injuries in the past 5 years ( p = 0.37, Fig. 1). No significant differences were observed between groups in the reported number of days of modified training (HS, 13.9 ± 49.5; NHS, 21.8 ± 78.4, p = 0.77). There was no significant difference in reported 2-mile run time between NHS pattern (14.80 ± 1.6 minutes) versus HS pattern (14.76 ± 1.7 minutes, p = 0.75). There was no significant difference in weekly training mileage between runners utilizing an NHS and HS ( p = 0.71). The majority of our cohort ran 15 miles per week (79.5%), and the majority of that group (53.5%) ran 6 to 10 miles per week. The three most described injuries by the Soldiers included knee pain, low back pain, and shin splints. In the 13 forefoot strikers, five reported overuse injuries that included a single case of shin splints, knee pain, ankle pain, achilles tendonitis, and two cases of low back pain. DISCUSSION The purpose of this study was to characterize FS pattern distribution in a cohort of U.S. Army Soldiers in a combat arms unit and determine if FS patterns were related to selfreported running-related injuries to the lower extremity and RESULTS Of the 341 male Combat Arms Soldiers who participated, 296 (87%) were characterized as landing with a HS pattern and 45 (13%) were characterized as utilizing an NHS pattern, with only 13 (4%) using a forefoot strike pattern. A descriptive breakdown of number of volunteers, average run FIGURE 1. Percentages of volunteers for each FS category who reported 1 or more injuries. 832

4 performance. Not surprisingly, most Soldiers (87%) from this Combined Arms Battalion ran with an HS pattern. We alsofoundnosignificant differences with respect to injury rate or 2-mile run performance between FS groups, supporting our second hypothesis. To our knowledge, this is the first study to address the relationship between FS patterns and injury in lower mileage runners. Results from our retrospective study suggest that there is no relationship between FS pattern and runningrelated injury. Daoud et al. 10 reported higher overall rates of repetitive injury among runners utilizing an HS pattern, but when evaluated by sex, there was no significant difference between FS patterns in repetitive injuries amongst males. This finding is consistent with this study using an all-male cohort. It may be that potential benefits of an NHS pattern demonstrated in the cohort of collegiate runners only occur when women with higher weekly training mileage are studied. More research is needed to determine if male and female Soldiers with higher weekly training mileage would elicit differences in injury rates between FS patterns. The percentage of Soldiers reporting that they have experienced a running-related injury in our study (51% 56%) falls within the range of what has been reported in previous publications (19.4% 79.3%). 5,10 Even during a short 13-week training program intended to prepare runners for a 10 km race, 30% of the participants reported an injury. 18 This rate is lower than that reported in this study. It is plausible that if the runners from that particular study reported injuries for a longer duration than 13 weeks, then the percentage of runners reporting injury would be similar to the current results. When compared to trained, high mileage collegiate cross-country runners, Soldiers from this study reported far fewer injuries (52% vs. 74%). 10 Running is clearly associated with a risk for injury, but it does not appear that this cohort of Combat Arms Soldiers is exposed to higher levels of risk than civilian runners. It was not unexpected that the top three most reported injuries included knee pain, low back pain, and shin splints. 18 There was no difference in 2-mile run times between FS patterns in this cohort; in fact, the mean times were nearly identical (Table I). Because the 2-mile run time is assessed twice per year and is directly linked to promotion, Soldiers are consistent in recalling their most recent run time. 19 Even in long distance races, reported performance differences between FS patterns has been contradictory. One study reported a greater percentage of NHS runners in faster runners during a half-marathon (based on finishing times). 20 However, the current findings are supported by reports of no significant difference between runners who utilized HS and NHS patterns at the end of the 2009 Manchester City Marathon. 21 On the basis of these previous reports, a performance difference in reported 2-mile run times was not expected because of the short distance of the 2-mile run. The ratio of HS to NHS runners in this cohort of Soldiers is similar to that demonstrated in other literature where FS pattern has been recorded in mid-to long-distance civilian runners This is the first study the authors are aware of which reports FS pattern in a military cohort. Although one other study has reported 69% of runners used an HS pattern, those data were based on a small cohort of collegiate crosscountry runners (n = 52) who may have self-selected into competitive running. 10 Soldiers in this study ran fewer training miles per week than runners who have been previously studied. Since 68% of our cohort ran between 5 and 15 miles per week, they were likely more representative of the typical recreational athlete (Table I). Although these findings are consistent with previous results, there are several limitations. Typically, people underreport injuries, and Soldiers tend to report slightly faster 2-mile run times as compared to their record score. 19 Even with the limitations inherent with self-report data, which is systematic between both groups in the cohort, there were clearly no differences in injuries and 2-mile run times between FS groups. Additionally, the assumption was made that the FS pattern identified during filming was the FS utilized throughout the participants history of reported injuries. Because injuries were looked at over the past 5 years, we could not account for effects of terrain, footwear, or other confounding variables. The small number of NHS Soldiers, specifically Soldiers using a forefoot strike prevented us from statistically assessing injury types across FS patterns. However, the forefoot strikers did not report a preponderance of any stereotypical forefoot strike injury to the foot, ankle, or calf region, which indicates that in this small cohort, those types of injuries were not common. Only one battalion and male Soldiers were included in this study, and it is unknown how FS patterns from other Army occupations or units may or may not be related to performance and injury. Although our results lend preliminary insight into the relationship between FS patterns, performance, and injury in a combat arms battalion, our results should be interpreted with caution as they represent a limited number of Soldiers (all male) within a single MOS. It is unknown how FS patterns may or may not be related to performance and injury across the Army, specifically within other MOS or in female Soldiers. CONCLUSION Of 341 male Combat Arms Soldiers, 87% were characterized as running with an HS pattern, which was similar to previous research on civilian runners. We observed no difference between runners utilizing HS and NHS patterns in 2-mile run time or retrospective injury incidence. On the basis of this analysis of male Combat Arms Soldiers, neither FS pattern was advantageous for decreasing running-related injury or improving performance. ACKNOWLEDGMENTS Research supported in part by appointments (R.E.F., S.G.S.) to the Postgraduate Research Participation program funded by U.S. Army Research Institute of Environmental Medicine & administered by Oak Ridge Institute 833

5 for Science and Engineering. The authors thank the 1-66 Armor Battalion from Fort Carson, CO, for their participation, particularly the coordination efforts of CPT Anthony Spalloni. The authors also thank Ms. Amanda Winkler and Ms. Caitlin Dillon for their assistance in data management. REFERENCES 1. NY Daily News: Running more popular than ever in US; survey finds Americans running in record numbers. Available at accessed October 1, Wang BW, Ramey DR, Schettler JD, Hubert HB, Fries JF: Postponed development of disability in elderly runners: a 13-year longitudinal study. Arch Intern Med 2002; 162: Fries JF, Singh G, Morfeld D, Hubert HB, Lane NE, Brown BW Jr: Running and the development of disability with age. Ann Intern Med 1994; 121: Kaufman KR, Brodine S, Shaffer R: Military training-related injuries: surveillance, research, and prevention. Am J Prev Med 2000; 18: van Gent RN, Siem D, van Middelkoop M, van Os AG, Bierma- Zeinstra SM, Koes BW: Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review. Br J Sports Med 2007; 41: Hill OT, Bulathsinhala L, Scofield DE, Haley TF, Bernasek TL: Risk factors for soft tissue knee injuries in active duty U.S. Army soldiers, Mil Med 2013; 178: McDougall C: Born To Run: A Hidden Tribe, Superathletes, and the Greatest Race the World Has Never Seen. New York, Knopf, Perl DP, Daoud AI, Lieberman DE: Effects of footwear and strike type on running economy. Med Sci Sports Exerc 2012; 44: Diebal AR, Gregory R, Alitz C, Gerber JP: Forefoot running improves pain and disability associated with chronic exertional compartment syndrome. Am J Sports Med 2012; 40: Daoud AI, Geissler GJ, Wang F, Saretsky J, Daoud YA, Lieberman DE: Foot strike and injury rates in endurance runners: a retrospective study. Med Sci Sports Exerc 2012; 44: Goss DL, Gross MT: A review of mechanics and injury trends among various running styles. US Army Med Dep J 2012; July-September: Altman AR, Davis IS: Barefoot running: biomechanics and implications for running injuries. Curr Sports Med Rep 2012; 11: Lieberman DE, Venkadesan M, Werbel WA, et al: Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature 2010; 463: Lorenz DS, Pontillo M: Is there evidence to support a forefoot strike pattern in barefoot runners? A review. Sports Health 2012; 4: Tam N, Astephen Wilson JL, Noakes TD, Tucker R: Barefoot running: an evaluation of current hypothesis, future research and clinical applications. Br J Sports Med 2014; 48: Murphy K, Curry EJ, Matzkin EG: Barefoot running: does it prevent injuries? Sports Med 2013; 43: Goss DL, Gross MT: Relationships among self-reported shoe type, footstrike pattern, and injury incidence. US Army Med Dep J 2012: Taunton JE, Ryan MB, Clement DB, McKenzie DC, Lloyd-Smith DR, Zumbo BD: A prospective study of running injuries: the Vancouver Sun Run In Training clinics. Br J Sports Med 2003; 37: Jones SB, Knapik JJ, Sharp MA, Darakjy S, Jones BH: The validity of self-reported physical fitness test scores. Mil Med 2007; 172: Hasegawa H, Yamauchi T, Kraemer WJ: Foot strike patterns of runners at the 15-km point during an elite-level half marathon. J Strength Cond Res 2007; 21: Larson P, Higgins E, Kaminski J, et al: Foot strike patterns of recreational and sub-elite runners in a long-distance road race. J Sports Sci 2011; 29: Kasmer ME, Liu XC, Roberts KG, Valadao JM: Foot-strike pattern and performance in a marathon. Int J Sports Physiol Perform 2013; 8: