M ILITARY M EDICINE, 180, 3:321, 2015
Physical Training in Boots and Running Shoes: A Historical Comparison of Injury Incidence in Basic Combat Training Joseph J. Knapik, ScD*f; Bruce H. Jones, MD, MPH*; Ryan A. Steelman, M PH*f
ABSTRACT For many years, U.S, Army soldiers performed physical training (PT) in a modified duty uniform and combat boots. The belief that PT in combat boots was associated with injuries lead to the introduction of running shoes for PT in 1982. A historical comparison was conducted examining injuries before and after the change to running shoes in Basic Combat Training (BCT). Searches in literature databases and other sources identified 16 studies with quantita tive data on injury incidence during 8-week BCT cycles. Employing studies with similar injury definitions (n = 12), injury incidence was compared in the boot and running shoe periods using meta-analyses, statistics, and risk ratios (RRs) with 95% confidence intervals (95% CIs). The boot and shoe periods demonstrated little difference in overall injury incidence (men: RR[boot/shoes] = 1.04, 95% Cl = 0.91-1.18, p = 0.50; women: RR = 0.94, 95% Cl = 0.85-1.05, P = 0.27) or in lower extremity injury incidence (men: RRfboot/shoes] = 0.91, 95% Cl = 0.64-1.30, p = 0.66; women: RR = 1.06, 95% Cl = 0.89-1.27, p = 0.51). These analyses provided little support for a reduction in injury risk after the switch from boots to running shoes for PT in BCT. A large randomized, prospective cohort study should be conducted to determine if injury rates are different when PT is conducted in running shoes versus boots.
INTRODUCTION For many years, the U.S. Army performed all physical train ing (PT) and physical fitness testing in a modified duty uni form (i.e., “fatigues”) and combat boots.1,2 In September 1982, General Glenn Otis, Commander of the Army Training and Doctrine Command, issued a communique to com manders permitting running shoes for all PT and testing.3 Subsequently, the use of running shoes was codified into Army doctrine with the publication of the 1985 edition of Army Field Manual 21-20.4 The reason for the change was the belief of many that running in boots was the cause of many of the injuries that were seen in the military.5-7 How ever, when the doctrine was changed, there had been no randomized, prospective trial comparing injury rates among groups performing PT in shoes versus boots, and to date no such study has been conducted. Both before and after the change in footwear, injuries had been a common problem among those serving in the military services.8-11 If the belief that injuries were associated with the use of boots for PT were true, it was reasonable to expect a decrease in injuries after Army doctrine was changed to mandate the use of running shoes. An appropriate place to test this hypothesis may be Basic Combat Training (BCT). In BCT, all recruits perform PT and testing together. They eat in the same dining facilities and sleep in the same barracks areas. The BCT environment is more controlled and stan
*US Army Institute of Public Health, 5158 Blackhawk Road, Aberdeen Proving Ground, MD 21010. fOak Ridge Institute for Science and Education, 4692 Millennium Drive, Suite 101, Belcamp, MD 21017. The views, opinions, and/or findings contained in this report are those of the authors and should not be construed as official Department of the Army position, policy, or decision, unless so designated by other official documen tation. Approved for public release; distribution is unlimited. doi: 10.7205/MILMED-D-14-00337
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dardized than the operational Army where soldiers have more freedom and likely perform different amounts of physical activity that can influence injury risk. Studies of injuries in BCT have been conducted for a number of years. In this article, we performed a systematic review of the literature to find studies that had examined injury rates in BCT before and after the change to running shoes. The aim of the investigation was to use historical data to determine if BCT injury rates were reduced after the adop tion of running shoes in place of boots for PT.
METHODS Literature searches were conducted in PubMed (MEDLINE) and the Defense Technical Information Center. Keywords for the search included military personnel, recruit, soldier, and basic training with injury, overuse, wound, and trauma. To find additional studies, the reference lists of the articles obtained were searched as were the literature databases of the investigators. Additional information was obtained from the authors that were not included in the articles. Articles were selected for the review if they provided quantitative data on injuries among recruits in U.S. Army BCT, or if such data could be calculated from the information provided. Titles were first examined and abstracts were reviewed if the article appeared to involve basic training and injuries. The full text of the article was retrieved if there was evidence of a quantitative assessment of injuries in U.S. Army BCT. Although all studies meeting the review criteria were initially selected, only studies conducted when BCT was 8 weeks in length were considered in the final analysis. This was because of the closer proximity between the boot and shoe periods (BCT was extended to 9 weeks after Octo ber 1998), to control for time at risk, and because injury risk is not uniform across the weeks of BCT.12'13 Studies involv ing One-Station Unit Training were excluded because of the
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variable lengths and types of training compared to BCT. Also excluded were studies involving basic training in countries other than the United States, studies in Advanced Individual Training, studies in military services other than the U.S. Army (i.e., Navy, Air Force, and Marine Corps), and studies that did not provide quantitative assessments of injury rates. Contacts with the authors indicated that data from a single collection episode was often used in more than one publica tion, so these studies were only considered once for the pur pose of data extraction to avoid biasing the analysis toward data collection episodes with a larger number of published studies. The search was completed in February 2014. The methodological quality of the investigations was assessed using the technique of Loney et al,14 which was developed specifically for rating studies reporting incidences. Studies were graded on an 8-point scale, which included assessments for sampling methods, sampling frame, sample size, measurement tools, measurement bias, response rate, statistical presentation, and description of subject sample. The 8 items were rated as either “yes” (1 point) or “no” (no point), based on specific criteria. Thus, the maximum possi ble score was 8. One modification to the “statistical presen tation” was to allow 0.5 points for studies that provided only incidence and 1 point for studies that provided both inci dence and confidence intervals. Two authors independently rated each of the selected articles. Following the indepen dent evaluations, the reviewers met to examine the scores and to reconcile major differences. The average score of the reviewers served as the methodological quality score. Scores were converted to a percent of the total score by dividing the average score for each study by 8 and multiply ing by 100%. The summary statistic derived from each study was the injury incidence. This was the X of recruits with one or more injuries/X of all recmits in the investigation x 100 (% injured). Several different injury incidences could be derived from one study if several different injury definitions were provided within the study. Men and women were analyzed separately because of the large difference in injury incidence. The Comprehensive Meta-Analysis Statistical Package, Version 2 (Biostat, Englewood, New Jersey) was used to perform a meta-analysis among studies with similar injury definitions. A random model was used that involved provid ing the number of injured recruits and the number not injured to produce a summary injury incidence (SII) with summary 95% confidence intervals (S95% CIs) that represented the pooled results from the individual investigations. Besides meta-analysis, injury incidence was compared directly in studies with similar injury definitions. The number of recruits injured and those not injured were grouped into “periods” (i.e., boot versus running shoe). Two x two tables (boot period/shoe period x injured/not injured) were constructed and analyzed using the online Open Epidemiology Calcu lator.1^ For each comparison, a risk ratio (RR), 95% Cl, and x2 p-value was calculated.
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RESULTS
There were 8,257 publications that were examined, 24 that had investigated injuries in BCT, and 16 that fully met the review criteria (i.e., had an 8-week BCT cycle). Examination of the publications and communications with the authors indicated that the 16 studies were from 12 unique data col lection episodes. In studies that used only a portion of the data collected in a single episode,16'17 the study with the larger number of recmits was considered. If a particular injury defi nition was included in only one study with a smaller number of recmits, injury incidence with that definition was included and was appropriately footnoted. Table I shows the publications separated into the two periods (boot and mnning shoes) and the 12 data collection episodes. Three of the data collection episodes were in the boot period and 9 were in the running shoe period. Studies spanned a 22-year period from November 1976 to Decem ber 1998. All three of the data collection episodes in the boot period were at Fort Jackson, South Carolina. Six of the 9 data collection episodes in the shoe period were at Fort Jackson, two at Fort Leonard Wood, Missouri, and one at Fort Bliss, Texas. Data collection methods differed. In the boot period, Reinker and Ozbume10 did not clearly specify how their data were collected. Kowal18 used a self-report questionnaire with con firmation from medical records. Bensel and Kish16 reviewed hard copy medical records. All studies in the shoe period obtained injuries from hard copy medical records. A number of different injury definitions were used, as shown in Table I. The study by Reinker and Ozbume10 provided only inju ries seen in specialty clinics (estimated at about 20% of all injuries by the authors) and those occurring over several BCT cycles. All other investigations provided data on injuries seen by any medical care provider and in single BCT cycles. Because of these differences, the Reinker and Ozbume study10 was not considered further in the analysis. Methodological quality scores ranged from 31% to 75% with a mean ± standard deviation score of 62 ± 13%. The mean ± standard deviation score in the boot period was 66 ± 4% (exclusive of Reinker and Ozbume10) and 64 ± 11% in the running shoe period. Despite multiple injury definitions, two injury definitions were similar in the boot and shoe periods. The definition used by Kowal18 in the boot period was comparable to that of a number of other studies in the mnning shoe period.1213'20~28 This injury definition involved examining physical damage to the body of any type that was seen by a medical care pro vider. Injuries in the mnning shoe period were subjected to meta-analysis to obtain a pooled SII and S95% Cl and these data are in Table II. Table II indicates that injury incidence was similar in the boot and running shoe periods for both men and women. Besides the meta-analysis, data on injured versus noninjured recruits from the Kowal study18 were directly compared to the data grouped from the other studies with similar injury definitions.12'13'20-28 Table III shows that for
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TABLE I.
Summary of Studies Examining Cumulative Incidence of Injury in U.S. Army Basic Combat Training
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324 Continued
ND, no data collected on this gender; NS, not stated. "From Jones et al21 (n = 1349 men, 921 women). *From Bell et al16 in - 509 men, 352 women). 'From Altarac et al17 (n = 1087, 915 women). ';Only the two companies with completed 8-week cycles were considered. T his was the very last 8-week BCT cycle at Fort Jackson and after this, BCT cycles were 9 weeks. The article examined four BCT cycles and only the final one is unique and shown in this row. The other three BCT cycles were reported in prior publications.
TABLE i.
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TABLE II.
Comparison of Overall Injury Incidence Before and After Army BCT Recruits Performed Physical Training in Running Shoes
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both men and women, there was little difference in the injury risk in the two periods. The second injury definition that was similar in the boot and running shoe periods was for lower extremity injuries. Bensel and K ish19 looked at this type of injury in the boot period, as did Jones et al20 22 in the running shoe period. Table III directly compares injury incidence in the two stud ies. There was little difference in the injury incidence in the two periods for either men or women.
DISCUSSION The present analysis suggested that there was little difference in injury incidence in the period when BCT recruits per formed PT in boots compared to the period when they trained in running shoes. Two separate injury definitions (overall injuries and lower extremity injuries) could be analyzed and they produced similar results. For overall injuries, both a meta-analysis and a %2 analysis produced virtually the same results denoting little difference in injury incidence in the 2 periods. Both male and female recruits had similar findings for all comparisons. Some authors29'10 have hypothesized that boots have inad equate cushioning and transmitted large impact forces to the bones and soft tissue of the lower limbs and spine that, over time, might increase injury rates. deMoya11 examined ground reaction forces (GRFs) while military officers ran at 12 km/h across a force platform in an Etonic KM running shoe and a military boot. It was reported that there were significant dif ferences in peak forces and time to peak forces, but quantita tive data were not presented. Mechanical testing indicated that “the shock absorption and impact cushioning character istics were significantly better for all variables measured in the running shoe than in the boot,” although again, no quan titative data were presented.11 Studies involving participants running across force plates may be influenced by the fact that few trials are collected and subjects must target the force plate area instead of running in a natural, rhythmic manner. Another study32 had subjects run at 10 km/h on a treadmill instrumented with a force platform under the belt. For each participant, between 70 and 90 force plate samples were taken and averaged while running in running shoes and then again while running in a standard Greek military boot. There was little difference in the GRF profile (impact peak forces, push-off rate, loading rate) between the running shoes and the boots.12 GRFs represent external loading, and following Isaac Newton’s third axiom or law of motion,31 the same forces should be experienced internally. However, internal body structures are complex consisting of bones, muscles, ligaments, tendons, and other tissues that likely experience forces differently and with distinct injury risk. A study with strain gauges implanted in the mid-tibial area showed no difference in tension and compression microstrains between an Israeli Army boot and a Nike® Air Max running shoes when participants ran on a force-plate-instrumented treadmill at 13 km/h.34
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If high forces or high rates of loading experienced during running were associated with injuries as some suspect,15’36 then one intervention that might reduce injury risk would be to increase the amount of cushioning in footwear. Windle et al37 tested four different types of cushioning insoles includ ing a Saran insole (commonly used in U.S. military boots in the boot period) and a Sorbothane insole. The Sorbothane insole provided the highest shock attenuation and the Saran insole the least. A large, controlled trial of a Sorbothane insole in U.S. Marine Corps recruits showed that injury rates were similar whether the recmits were wearing the Sorbothane insoles or a standard mesh insole in their boots.13 Other studies examining injury risk using different types of shock-absorbing insoles or midsoles have produced mixed results.29'39^ 12 Factors other than shock absorption may be important in the association between footwear and injuries. It is well established that when running speed is held constant, running in boots results in disproportionately higher energy expendi ture than running in shoes.43 44 Energy expenditure is about 1% greater for every 100 g added to the foot,41’45 and thus it might be expected that running in heavier boots would decrease running speeds. Faster running speeds have been associated with higher injury incidence in runners.46-49 Run ning shoes have been designed for different foot characteris tics50 and these characteristics can influence gait mechanics.51-54 For example, individuals with low arches have disproportion ate foot flexibility that causes excessive pronation during initi ation of the stance phase of running and “motion control” running shoes have been developed by running shoe com panies to reduce this pronation.5152 However, running shoes designed for different foot types do not influence injury rates in basic training.55-57 Like running shoes, boot construction can differ considerably and this can have variable effects on running mechanics,53 but how these factors might influence injury rates is not known at this time. This historical analysis has limitations. First, the shoe period studies were conducted over a long period of time (14.9 years) and BCT administrative policies, training tech niques, and medical care may have differed over the period. However, examination of the studies that were in the closest proximity for overall injury rates18'21 and lower extremity injury rates19'21 suggested similar injury rates in the boot and running shoe periods. Second, an attempt was made to select studies with similar injury definitions but these definitions differed slightly. Nonetheless, examination of the “types” of injuries captured in boot and running shoe periods suggested that these were very similar. Third, data collection methods differed somewhat in the Kowal study18 versus the other studies used for comparison,1213'2()-28 although both studies used medical records. On the other hand, the data collection methods in the Bensel and K ish19 study was almost identical to that of the Jones et al21 study. In summary, this investigation suggested that there was little difference in injury incidence after the U.S. Army switched from conducting BCT PT in combat boots to
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conducting BCT PT in running shoes. A randomized pro spective cohort study should be conducted to specifically test the hypothesis of no difference in injury rates when per forming PT in boots versus running shoes. A finding of little difference in injury rates would justify performing PT in boots in consonance with other training activities performed in the U.S. Army. A finding of higher boot injury rates would justify the current practice of using running shoes.
14.
15.
16.
ACKNOWLEDGMENTS This research was supported in part by an appointment to the Knowledge Preservation Program at the U.S. Army Public Health Command (USAPHC) administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USAPHC.
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Physical Training in Boots and Running Shoes: A Historical Comparison of Injury Incidence in Basic Combat Training Joseph J. Knapik, ScD*f; Bruce H. Jones, MD, MPH*; Ryan A. Steelman, M PH*f
ABSTRACT For many years, U.S, Army soldiers performed physical training (PT) in a modified duty uniform and combat boots. The belief that PT in combat boots was associated with injuries lead to the introduction of running shoes for PT in 1982. A historical comparison was conducted examining injuries before and after the change to running shoes in Basic Combat Training (BCT). Searches in literature databases and other sources identified 16 studies with quantita tive data on injury incidence during 8-week BCT cycles. Employing studies with similar injury definitions (n = 12), injury incidence was compared in the boot and running shoe periods using meta-analyses, statistics, and risk ratios (RRs) with 95% confidence intervals (95% CIs). The boot and shoe periods demonstrated little difference in overall injury incidence (men: RR[boot/shoes] = 1.04, 95% Cl = 0.91-1.18, p = 0.50; women: RR = 0.94, 95% Cl = 0.85-1.05, P = 0.27) or in lower extremity injury incidence (men: RRfboot/shoes] = 0.91, 95% Cl = 0.64-1.30, p = 0.66; women: RR = 1.06, 95% Cl = 0.89-1.27, p = 0.51). These analyses provided little support for a reduction in injury risk after the switch from boots to running shoes for PT in BCT. A large randomized, prospective cohort study should be conducted to determine if injury rates are different when PT is conducted in running shoes versus boots.
INTRODUCTION For many years, the U.S. Army performed all physical train ing (PT) and physical fitness testing in a modified duty uni form (i.e., “fatigues”) and combat boots.1,2 In September 1982, General Glenn Otis, Commander of the Army Training and Doctrine Command, issued a communique to com manders permitting running shoes for all PT and testing.3 Subsequently, the use of running shoes was codified into Army doctrine with the publication of the 1985 edition of Army Field Manual 21-20.4 The reason for the change was the belief of many that running in boots was the cause of many of the injuries that were seen in the military.5-7 How ever, when the doctrine was changed, there had been no randomized, prospective trial comparing injury rates among groups performing PT in shoes versus boots, and to date no such study has been conducted. Both before and after the change in footwear, injuries had been a common problem among those serving in the military services.8-11 If the belief that injuries were associated with the use of boots for PT were true, it was reasonable to expect a decrease in injuries after Army doctrine was changed to mandate the use of running shoes. An appropriate place to test this hypothesis may be Basic Combat Training (BCT). In BCT, all recruits perform PT and testing together. They eat in the same dining facilities and sleep in the same barracks areas. The BCT environment is more controlled and stan
*US Army Institute of Public Health, 5158 Blackhawk Road, Aberdeen Proving Ground, MD 21010. fOak Ridge Institute for Science and Education, 4692 Millennium Drive, Suite 101, Belcamp, MD 21017. The views, opinions, and/or findings contained in this report are those of the authors and should not be construed as official Department of the Army position, policy, or decision, unless so designated by other official documen tation. Approved for public release; distribution is unlimited. doi: 10.7205/MILMED-D-14-00337
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dardized than the operational Army where soldiers have more freedom and likely perform different amounts of physical activity that can influence injury risk. Studies of injuries in BCT have been conducted for a number of years. In this article, we performed a systematic review of the literature to find studies that had examined injury rates in BCT before and after the change to running shoes. The aim of the investigation was to use historical data to determine if BCT injury rates were reduced after the adop tion of running shoes in place of boots for PT.
METHODS Literature searches were conducted in PubMed (MEDLINE) and the Defense Technical Information Center. Keywords for the search included military personnel, recruit, soldier, and basic training with injury, overuse, wound, and trauma. To find additional studies, the reference lists of the articles obtained were searched as were the literature databases of the investigators. Additional information was obtained from the authors that were not included in the articles. Articles were selected for the review if they provided quantitative data on injuries among recruits in U.S. Army BCT, or if such data could be calculated from the information provided. Titles were first examined and abstracts were reviewed if the article appeared to involve basic training and injuries. The full text of the article was retrieved if there was evidence of a quantitative assessment of injuries in U.S. Army BCT. Although all studies meeting the review criteria were initially selected, only studies conducted when BCT was 8 weeks in length were considered in the final analysis. This was because of the closer proximity between the boot and shoe periods (BCT was extended to 9 weeks after Octo ber 1998), to control for time at risk, and because injury risk is not uniform across the weeks of BCT.12'13 Studies involv ing One-Station Unit Training were excluded because of the
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variable lengths and types of training compared to BCT. Also excluded were studies involving basic training in countries other than the United States, studies in Advanced Individual Training, studies in military services other than the U.S. Army (i.e., Navy, Air Force, and Marine Corps), and studies that did not provide quantitative assessments of injury rates. Contacts with the authors indicated that data from a single collection episode was often used in more than one publica tion, so these studies were only considered once for the pur pose of data extraction to avoid biasing the analysis toward data collection episodes with a larger number of published studies. The search was completed in February 2014. The methodological quality of the investigations was assessed using the technique of Loney et al,14 which was developed specifically for rating studies reporting incidences. Studies were graded on an 8-point scale, which included assessments for sampling methods, sampling frame, sample size, measurement tools, measurement bias, response rate, statistical presentation, and description of subject sample. The 8 items were rated as either “yes” (1 point) or “no” (no point), based on specific criteria. Thus, the maximum possi ble score was 8. One modification to the “statistical presen tation” was to allow 0.5 points for studies that provided only incidence and 1 point for studies that provided both inci dence and confidence intervals. Two authors independently rated each of the selected articles. Following the indepen dent evaluations, the reviewers met to examine the scores and to reconcile major differences. The average score of the reviewers served as the methodological quality score. Scores were converted to a percent of the total score by dividing the average score for each study by 8 and multiply ing by 100%. The summary statistic derived from each study was the injury incidence. This was the X of recruits with one or more injuries/X of all recmits in the investigation x 100 (% injured). Several different injury incidences could be derived from one study if several different injury definitions were provided within the study. Men and women were analyzed separately because of the large difference in injury incidence. The Comprehensive Meta-Analysis Statistical Package, Version 2 (Biostat, Englewood, New Jersey) was used to perform a meta-analysis among studies with similar injury definitions. A random model was used that involved provid ing the number of injured recruits and the number not injured to produce a summary injury incidence (SII) with summary 95% confidence intervals (S95% CIs) that represented the pooled results from the individual investigations. Besides meta-analysis, injury incidence was compared directly in studies with similar injury definitions. The number of recruits injured and those not injured were grouped into “periods” (i.e., boot versus running shoe). Two x two tables (boot period/shoe period x injured/not injured) were constructed and analyzed using the online Open Epidemiology Calcu lator.1^ For each comparison, a risk ratio (RR), 95% Cl, and x2 p-value was calculated.
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RESULTS
There were 8,257 publications that were examined, 24 that had investigated injuries in BCT, and 16 that fully met the review criteria (i.e., had an 8-week BCT cycle). Examination of the publications and communications with the authors indicated that the 16 studies were from 12 unique data col lection episodes. In studies that used only a portion of the data collected in a single episode,16'17 the study with the larger number of recmits was considered. If a particular injury defi nition was included in only one study with a smaller number of recmits, injury incidence with that definition was included and was appropriately footnoted. Table I shows the publications separated into the two periods (boot and mnning shoes) and the 12 data collection episodes. Three of the data collection episodes were in the boot period and 9 were in the running shoe period. Studies spanned a 22-year period from November 1976 to Decem ber 1998. All three of the data collection episodes in the boot period were at Fort Jackson, South Carolina. Six of the 9 data collection episodes in the shoe period were at Fort Jackson, two at Fort Leonard Wood, Missouri, and one at Fort Bliss, Texas. Data collection methods differed. In the boot period, Reinker and Ozbume10 did not clearly specify how their data were collected. Kowal18 used a self-report questionnaire with con firmation from medical records. Bensel and Kish16 reviewed hard copy medical records. All studies in the shoe period obtained injuries from hard copy medical records. A number of different injury definitions were used, as shown in Table I. The study by Reinker and Ozbume10 provided only inju ries seen in specialty clinics (estimated at about 20% of all injuries by the authors) and those occurring over several BCT cycles. All other investigations provided data on injuries seen by any medical care provider and in single BCT cycles. Because of these differences, the Reinker and Ozbume study10 was not considered further in the analysis. Methodological quality scores ranged from 31% to 75% with a mean ± standard deviation score of 62 ± 13%. The mean ± standard deviation score in the boot period was 66 ± 4% (exclusive of Reinker and Ozbume10) and 64 ± 11% in the running shoe period. Despite multiple injury definitions, two injury definitions were similar in the boot and shoe periods. The definition used by Kowal18 in the boot period was comparable to that of a number of other studies in the mnning shoe period.1213'20~28 This injury definition involved examining physical damage to the body of any type that was seen by a medical care pro vider. Injuries in the mnning shoe period were subjected to meta-analysis to obtain a pooled SII and S95% Cl and these data are in Table II. Table II indicates that injury incidence was similar in the boot and running shoe periods for both men and women. Besides the meta-analysis, data on injured versus noninjured recruits from the Kowal study18 were directly compared to the data grouped from the other studies with similar injury definitions.12'13'20-28 Table III shows that for
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TABLE I.
Summary of Studies Examining Cumulative Incidence of Injury in U.S. Army Basic Combat Training
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324 Continued
ND, no data collected on this gender; NS, not stated. "From Jones et al21 (n = 1349 men, 921 women). *From Bell et al16 in - 509 men, 352 women). 'From Altarac et al17 (n = 1087, 915 women). ';Only the two companies with completed 8-week cycles were considered. T his was the very last 8-week BCT cycle at Fort Jackson and after this, BCT cycles were 9 weeks. The article examined four BCT cycles and only the final one is unique and shown in this row. The other three BCT cycles were reported in prior publications.
TABLE i.
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TABLE II.
Comparison of Overall Injury Incidence Before and After Army BCT Recruits Performed Physical Training in Running Shoes
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both men and women, there was little difference in the injury risk in the two periods. The second injury definition that was similar in the boot and running shoe periods was for lower extremity injuries. Bensel and K ish19 looked at this type of injury in the boot period, as did Jones et al20 22 in the running shoe period. Table III directly compares injury incidence in the two stud ies. There was little difference in the injury incidence in the two periods for either men or women.
DISCUSSION The present analysis suggested that there was little difference in injury incidence in the period when BCT recruits per formed PT in boots compared to the period when they trained in running shoes. Two separate injury definitions (overall injuries and lower extremity injuries) could be analyzed and they produced similar results. For overall injuries, both a meta-analysis and a %2 analysis produced virtually the same results denoting little difference in injury incidence in the 2 periods. Both male and female recruits had similar findings for all comparisons. Some authors29'10 have hypothesized that boots have inad equate cushioning and transmitted large impact forces to the bones and soft tissue of the lower limbs and spine that, over time, might increase injury rates. deMoya11 examined ground reaction forces (GRFs) while military officers ran at 12 km/h across a force platform in an Etonic KM running shoe and a military boot. It was reported that there were significant dif ferences in peak forces and time to peak forces, but quantita tive data were not presented. Mechanical testing indicated that “the shock absorption and impact cushioning character istics were significantly better for all variables measured in the running shoe than in the boot,” although again, no quan titative data were presented.11 Studies involving participants running across force plates may be influenced by the fact that few trials are collected and subjects must target the force plate area instead of running in a natural, rhythmic manner. Another study32 had subjects run at 10 km/h on a treadmill instrumented with a force platform under the belt. For each participant, between 70 and 90 force plate samples were taken and averaged while running in running shoes and then again while running in a standard Greek military boot. There was little difference in the GRF profile (impact peak forces, push-off rate, loading rate) between the running shoes and the boots.12 GRFs represent external loading, and following Isaac Newton’s third axiom or law of motion,31 the same forces should be experienced internally. However, internal body structures are complex consisting of bones, muscles, ligaments, tendons, and other tissues that likely experience forces differently and with distinct injury risk. A study with strain gauges implanted in the mid-tibial area showed no difference in tension and compression microstrains between an Israeli Army boot and a Nike® Air Max running shoes when participants ran on a force-plate-instrumented treadmill at 13 km/h.34
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If high forces or high rates of loading experienced during running were associated with injuries as some suspect,15’36 then one intervention that might reduce injury risk would be to increase the amount of cushioning in footwear. Windle et al37 tested four different types of cushioning insoles includ ing a Saran insole (commonly used in U.S. military boots in the boot period) and a Sorbothane insole. The Sorbothane insole provided the highest shock attenuation and the Saran insole the least. A large, controlled trial of a Sorbothane insole in U.S. Marine Corps recruits showed that injury rates were similar whether the recmits were wearing the Sorbothane insoles or a standard mesh insole in their boots.13 Other studies examining injury risk using different types of shock-absorbing insoles or midsoles have produced mixed results.29'39^ 12 Factors other than shock absorption may be important in the association between footwear and injuries. It is well established that when running speed is held constant, running in boots results in disproportionately higher energy expendi ture than running in shoes.43 44 Energy expenditure is about 1% greater for every 100 g added to the foot,41’45 and thus it might be expected that running in heavier boots would decrease running speeds. Faster running speeds have been associated with higher injury incidence in runners.46-49 Run ning shoes have been designed for different foot characteris tics50 and these characteristics can influence gait mechanics.51-54 For example, individuals with low arches have disproportion ate foot flexibility that causes excessive pronation during initi ation of the stance phase of running and “motion control” running shoes have been developed by running shoe com panies to reduce this pronation.5152 However, running shoes designed for different foot types do not influence injury rates in basic training.55-57 Like running shoes, boot construction can differ considerably and this can have variable effects on running mechanics,53 but how these factors might influence injury rates is not known at this time. This historical analysis has limitations. First, the shoe period studies were conducted over a long period of time (14.9 years) and BCT administrative policies, training tech niques, and medical care may have differed over the period. However, examination of the studies that were in the closest proximity for overall injury rates18'21 and lower extremity injury rates19'21 suggested similar injury rates in the boot and running shoe periods. Second, an attempt was made to select studies with similar injury definitions but these definitions differed slightly. Nonetheless, examination of the “types” of injuries captured in boot and running shoe periods suggested that these were very similar. Third, data collection methods differed somewhat in the Kowal study18 versus the other studies used for comparison,1213'2()-28 although both studies used medical records. On the other hand, the data collection methods in the Bensel and K ish19 study was almost identical to that of the Jones et al21 study. In summary, this investigation suggested that there was little difference in injury incidence after the U.S. Army switched from conducting BCT PT in combat boots to
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conducting BCT PT in running shoes. A randomized pro spective cohort study should be conducted to specifically test the hypothesis of no difference in injury rates when per forming PT in boots versus running shoes. A finding of little difference in injury rates would justify performing PT in boots in consonance with other training activities performed in the U.S. Army. A finding of higher boot injury rates would justify the current practice of using running shoes.
14.
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ACKNOWLEDGMENTS This research was supported in part by an appointment to the Knowledge Preservation Program at the U.S. Army Public Health Command (USAPHC) administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USAPHC.
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