REVIEW ARTICLE. Factors Influencing Running-Related Musculoskeletal Injury Risk Among U.S. Military Recruits. COL Joseph M. Molloy, SP USA (Ret.

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1 REVIEW ARTICLE MILITARY MEDICINE, 181, 6:512, 2016 Factors Influencing Running-Related Musculoskeletal Injury Risk Among U.S. Military Recruits COL Joseph M. Molloy, SP USA (Ret.) ABSTRACT Running-related musculoskeletal injuries among U.S. military recruits negatively impact military readiness. Low aerobic fitness, prior injury, and weekly running distance are known risk factors. Physical fitness screening and remedial physical training (or discharging the most poorly fit recruits) before entry-level military training have tended to reduce injury rates while decreasing attrition, training, and medical costs. Incorporating anaerobic running sessions into training programs can offset decreased weekly running distance and decrease injury risk. Varying lower extremity loading patterns, stride length or cadence manipulation, and hip stability/strengthening programming may further decrease injury risk. No footstrike pattern is ideal for all runners; transitioning to forefoot striking may reduce risk for hip, knee, or tibial injuries, but increase risk for calf, Achilles, foot or ankle injuries. Minimal evidence associates running surfaces with injury risk. Footwear interventions should focus on proper fit and comfort; the evidence does not support running shoe prescription per foot type to reduce injury risk among recruits. Primary injury mitigation efforts should focus on physical fitness screening, remedial physical training (or discharge for unfit recruits), and continued inclusion of anaerobic running sessions to offset decreased weekly running distance. INTRODUCTION Initial entry training (IET) injuries across the U.S. Armed Forces may be the single most significant medical impediment to military readiness. 1 Approximately 25% of male and 50% of female U.S. military recruits sustain at least one physical training-related injury. 2 Between 60% and 80% of IET injuries are lower extremity overuse injuries; they are often associated with excessive running relative to initial fitness level and individual running capability. 2 This article will review potential influences of running-related variables on injury risk, addresse current military interventions, and provide evidence-based recommendations for running-related musculoskeletal injury (RRMI) mitigation among recruits. RRMI OVERVIEW Although popular for improving aerobic fitness, running presents risk for injury. Unfortunately, lack of standardized or consensus definition for RRMI results in a wide range of reported RRMI incidences and prevalence rates and limits comparisons among studies. 3 Per systematic review of 30 studies including male and female civilian runners worldwide, Nielsen et al 4 reported incidences ranging from 11 to 85% or 2.5 to 38 injuries per 1,000 hours of running. Rehabilitation and Reintegration Division, Office of the Surgeon General (Army), 7700 Arlington Boulevard, Defense Health Headquarters, Falls Church, VA doi: /MILMED-D Yamato et al 5 recently proposed a consensus definition for RRMI among recreational runners as running-related (training or competition) musculoskeletal pain in the lower limbs that causes a restriction on or stoppage of running (distance, speed, duration, or training) for at least 7 days or 3 consecutive scheduled training sessions, or that requires the runner to consult a physician or other health professional. Prior injury is a known risk factor for future RRMI, likely due in part to incomplete healing or rehabilitation, residual biomechanical or structural deficits, or development of compensatory gait patterns after injury. 6,7 Grier et al reported low aerobic fitness and elevated body mass index as risk factors for RRMI per analysis of survey responses from 1,320 male soldiers in a deploying U.S. Army unit. 8 Low levels of aerobic fitness, muscular endurance and muscular strength and pre-enlistment physical inactivity are associated with overuse musculoskeletal injuries in general during IET. 9 RUNNING PROGRAM PARAMETERS AND INJURY RISK Multiple researchers have demonstrated a dose response relationship between weekly running distance and increasing risk of injury. Researchers have alternately reported increased injury risk when running greater than 32 km/20 miles per week or greater than 64 km/40 miles per week. 7,10,11 These reports rely in part upon prospective cohort, survey-based studies. Van Middelkoop et al 12 used a prospective cohort, survey-based design when reporting running less than 512 MILITARY MEDICINE, Vol. 181, June 2016

2 40 km/25 miles per week to be a strong protective factor against calf injury among 694 male recreational marathoners. The weekly running distance threshold for U.S. military members may be less than the previously cited reports from the civilian literature. Per survey responses from 950 participants, Grier et al 13 reported that U.S. Army male soldiers running more than 16 miles per week had a 2.24 times greater risk of injury than soldiers running 7 or fewer miles per week. In contrast, Nielsen et al 4 noted a higher relative injury threshold among runners reporting greater weekly running distances. He suggested that experienced runners (who presumably run greater distances) better understand the injury threshold than novice runners. Researchers have also associated running frequency and duration with RRMI risk. Per two randomized controlled trials, Pollock et al 14 reported in 1977 that running more than 3 days weekly or more than 30 minutes per session increased risk disproportionately to potential fitness benefits for the general population. Rapid increases in weekly running distance may increase risk. Following an explorative, 1-year prospective cohort study involving 873 healthy novice runners, Nielsen et al 15 reported increased risk for certain injuries among runners who increased weekly distance by greater than 30% across 2 weeks versus those who increased distance by less than 10%. There are conflicting reports concerning the effect of intensity (running speed) on injury risk. Per Fields et al s 16 review, most researchers report no association between running speed and injury risk. However, Nielsen et al 4 noted that researchers reliance upon self-reported running speed and failure to account for varying running intensities during and among running sessions could influence potential associations between intensity and injury risk. Nielsen et al 17 suggested that sudden changes in running volume might increase risk for knee (patellofemoral and patellar tendinopathy) and iliotibial band pain, whereas changes in running speed or pace might be associated with risk for Achilles tendinopathy, calf (gastrocnemius), and plantar fascial pain. Weekly running distance is the product of three factors: running frequency, duration, and intensity. Nielsen et al 4 noted that researchers have generally failed to account for pertinent interactions among these factors. However, Jones et al 18 hypothesized that each factor contributes to injury risk proportionate to its contribution to weekly distance. Greatest variability generally occurs in running frequency or duration. For example, running twice weekly versus twice daily (14 times weekly) would account for sevenfold variability in frequency. Similarly, running for 15 minutes per session versus 90 minutes per session would account for sixfold variability in duration. However, running faster than 5 minutes per mile versus running slower than 10 minutes per mile would only account for slightly more than twofold variability. Jones et al suggested that running intensity s relatively minimal variability made it difficult to demonstrate as a risk factor. They hypothesized that the considerably greater variabilities in running frequency and duration accounted for their comparatively greater contributions to injury risk. Jones et al additionally hypothesized that modifications of running frequency and duration were more important than intensity-based modifications for injury mitigation. The U.S. Department of Defense s Joint Services Physical Training Injury Prevention Work Group recommended interval training as an effective means of improving cardiovascular fitness while minimizing repetitive stresses on the lower extremities. 2 Physical fitness programming such as the U.S. Army s Physical Readiness Training (PRT) and the U.S. Navy Operational Fitness and Fueling System emphasize anaerobic/interval running sessions to offset lesser running distance, duration, and/or frequency while developing aerobic and anaerobic fitness. 19,20 Across three military field studies, PRT reduced injury risk among Army recruits by 33 to 45% and maintained or improved fitness when compared with traditional physical training programs. 21 The 8- to 12-week duration of IET across the four U.S. military services may be too brief to fully realize initial aerobic fitness and muscular strength gains among previously sedentary recruits. Previously inactive individuals may achieve 5 to 20% aerobic fitness gains as assessed by maximal oxygen uptake (VO 2 max) after regularly performing aerobic exercise for 2 to 3 months; individuals typically attain their highest achievable VO 2 max levels within 8 to 18 months of continued training. 22,23 Initial strength and/or lean muscle mass gains generally plateau after 2 to 6 months of consistent strength training This emphasizes the need for regular physical activity or exercise throughout childhood and adolescence to fully develop anaerobic and aerobic fitness, muscular strength, and muscular endurance among recruits. RUNNING SURFACE AND INJURY RISK Clinicians frequently recommend running on soft surfaces to reduce injury risk. Little (if any) evidence supports this recommendation (with the possible exception of increased risk among women running on concrete per a prospective, surveybased study including 98 habitual female runners). 16,27,28 Recent findings of prospective cohort and retrospective studies are conflicting concerning associations between running surface hardness, hilly or irregular terrain running and injury risk or prior injury among high school runners. 29,30 Runners may adjust gait patterns or lower extremity stiffness/leg compliance in response to changes in running surface hardness. 31,32 Adjustments appear to be unique to each runner. 31 INDIVIDUAL FOOT TYPE AND INJURY RISK Disagreement exists whether extremes of arch height or foot mobility increase injury risk. This is due in part to varied criteria for categorizing arch height. Tong and Kong 33 reported associations between foot type extremes and injury risk upon systematic review, but reported the strength of the relationships as low (odds ratio [OR] = 1.23). Barnes et al 34 could not conclude whether a relationship existed between MILITARY MEDICINE, Vol. 181, June

3 foot structure or function and tibial stress injury risk upon systematic review, but stated that foot type extremes likely increased risk for tibial stress injury. Per a nonrandomized, two-group injury survey of 20 high-arched and 20 low-arched runners, Williams et al 35 associated the low-arched foot with increased risk for medial, soft tissue, and knee injuries. However, Hreljac 36 reported conflicting findings addressing magnitude and rate of pronation and injury risk. Similarly, Ferber et al 37 reported minimal and conflicting evidence associating excessive pronation with injury risk. Williams et al 35 associated the high-arched foot with increased risk for lateral, bony, and ankle injuries, whereas Fields et al 16 reported moderate evidence associating the high-arched foot with injury risk. However, Nielsen et al 38 reported that the vast majority of foot types (as assessed per Foot Posture Index) will experience similar injury survival after 250 km of running per an observational prospective cohort study of 927 novice runners (each foot assessed separately). Nielsen et al 38 also reported significantly fewer injuries per 1,000 km of running among pronated versus neutral feet. The small number of highly pronated and highly supinated feet in Nielsen et al s 38 study limited comparisons with neutral feet; Nielsen et al noted a trend toward an almost similar risk among highly supinated and neutral feet, while highly pronated feet seem to face an increased risk versus neutral feet after 50 and 100 km of running. HIP MUSCULAR STRENGTH AND INJURY RISK Per clinical review, Ferber et al 37 noted that hip musculature (particularly abductor and external rotator) weakness or sideto-side strength asymmetries may increase injury risk because of inadequate dynamic hip stabilization while running. Research addressing potential associations between hip musculature strength and RRMI risk among novice runners is inconsistent. Thijs et al 39 reported no association between isometric hip muscle strength and patellofemoral pain among 77 novice female runners across 10 weeks of training. Conversely, Ramskov et al 40 associated greater peak eccentric hip abduction strength with decreased patellofemoral pain risk among 629 novice runners across (but not beyond) the first 50 km of a self-controlled running program. However, Ramskov et al s analysis did not adjust for potential confounding variables such as running history or prior injury. Given the conflicting findings, hip stability or strengthening exercises such as those included in the Army s PRT and U.S. Navy Operational Fitness and Fueling System programs could potentially benefit recruits with underlying hip musculature weakness or strength asymmetries. FATIGUE AND INJURY RISK General or local muscular fatigue may increase injury risk. Local muscular fatigue reduces the muscle s ability to absorb loads; this may increase loading of, and strain upon the adjacent skeleton per controlled laboratory findings. 41 Tibial stress injury risk may increase as a runner fatigues because of gradually increasing impact loading and a concurrent imbalance in ankle dorsi/plantarflexor muscle activity resulting from tibialis anterior muscular fatigue. 42 Differences in fatigue adaptation may disproportionately increase external loads on female runners with a history of tibial stress fracture. 43 The potential association between fatigue and injury risk is relevant to the military recruit population given recruits prolonged bouts of standing and increased loading and physical activity requirements during IET. Per a controlled laboratory study including 20 high-arched and 20 low-arched recreational runners, Butler et al 44 reported that running shoes may increasingly influence running mechanics as a runner progressively fatigues. Running shoes may also influence local muscular fatigue during prolonged running. Per a controlled laboratory study including 20 female recreational runners, Cheung and Ng 45 reported that motion control shoe wear may enhance fatigue resistance of the tibialis anterior and peroneus longus muscles, and potentially decrease injury risk among runners with excessive rearfoot pronation during prolonged running. TRADITIONAL RUNNING SHOE TYPE AND INJURY RISK Butler et al 44 reported that running in a shoe specific for foot type can influence rearfoot motion or shock absorption. However, others question whether cushioning, rearfoot pronation control, or midsole hardness influence risk per literature review and randomized controlled trial findings. 46,47 Despite conflicting reports, clinicians frequently prescribe traditional running shoe types per individual foot type to address injury risk (Table I). Minimal (if any) evidence supports traditional running shoe prescription per foot type to reduce injury risk per systematic review, observational prospective cohort, retrospective cohort, and randomized controlled trial findings Grier et al 50 found no difference in injury rates among 1,332 male U.S. Army Brigade Combat Team soldiers wearing any of the three traditional shoe types (motion control, stability, cushioned/neutral) or minimalist shoes. Nielsen et al 38 found runners with pronated, supinated, and neutral foot postures to be at similar risk for injury after running 250 km in cushioned/neutral shoes. Per three randomized controlled trials, Knapik et al assessed effect of running shoe prescription on injury risk among 4,963 male and 2,240 female U.S. Air Force, Army, and Marine Corps recruits. Even when limiting comparisons to high- and low-arched recruits, prescription per static plantar foot shape had little effect on injury risk versus providing recruits with new stability shoes regardless of foot type. Knapik et al 51 reported modestly elevated, but statistically nonsignificant risk for low-arched trainees wearing motion control shoes and high-arched trainees wearing cushioned shoes versus low- and high-arched counterparts wearing stability 514 MILITARY MEDICINE, Vol. 181, June 2016

4 TABLE I. Traditional Running Shoe Prescription per Arch Height/Foot Type Traditional Shoe Type Characteristics Recommended Arch Height/Foot Type Cushioned or Neutral Soft, Single Density Medial Midsole High-Arched Foot No Medial Posting Normal-Arched Foot With Decreased Mobility Focus: Shock Absorption Stability Moderately Firm, Multidensity Medial Midsole Normal-Arched Foot Moderately Firm Heel Counter Relatively Rigid, Low-Arched Foot Medial Posting Relatively Mobile, High-Arched Foot Focus: Medial Support and Shock Absorption Motion Control Firm, Multidensity Medial Midsole Low-Arched Foot Medial Posting Rigid Heel Counter Focus: Rearfoot Pronation Control Normal-Arched Foot With Increased Mobility (Illustrations courtesy of Jennifer Donnelly, Directorate of Communications, U.S. Army Medical Command.) shoes. They also reported similar injury rates based upon particular brands and models of shoes. 52 Knapik et al 52,53 recommended that the U.S. Army stop prescribing shoe type per static plantar foot shape, but continue providing new shoes while enabling recruits to select from a variety of shoes accommodating a wide range of foot lengths and widths. They noted that a single running shoe is not likely to accommodate the variety of foot types encountered in basic training. 52 As noted, several researchers reported that runners quickly adapt to changes in running surface hardness by altering leg compliance when contacting the ground. 31,32 This might account in part for researchers and clinicians inabilities to demonstrate changes in injury risk per shoe prescription if a runner similarly adapts to differences in shoe midsole hardness by altering leg compliance when contacting the ground. Not yet studied is the effect of shoe prescription on injury risk among moderate-to-high distance runners. Knapik et al s 53 Army recruits ran 0.5 to 3.0 miles or performed sprints two to three times weekly. Marine recruits ran approximately 40 miles across 12 weeks of training. 51 These training volumes fall below the previously discussed thresholds associated with increased injury risk. Also unknown is the effect of shoe prescription for secondary injury prevention among currently or previously injured runners. Knapik et al 52 noted that gait differences associated with foot type may be more applicable to symptomatic and previously injured individuals than to those who are not experiencing symptoms or who have not been previously injured. Further unknown is the effect of shoe prescription per foot mobility rather than static foot type; varying degrees of mobility exist for each static foot type. 44 FOOTSTRIKE PATTERN AND INJURY RISK No footstrike pattern is ideal for all runners. Per retrospective cohort findings, Warr et al 54 reported no association between footstrike pattern and history of overuse injury across the previous year among 795 male and 232 female U.S. Army soldiers. Other findings are conflicting concerning associations between footstrike patterns and injury risk. Rearfoot striking produces a rapid, high-impact peak force at contact and subsequently a high loading rate during early stance phase of running. Several researchers associated rearfoot striking, high-impact peak forces or dynamic loading variables with increased risk for tibial stress injuries/ stress fractures, hip, knee, or low back pain, and plantar MILITARY MEDICINE, Vol. 181, June

5 fasciitis However, others reported no association or conflicting associations between injury risk and impact/ground reaction forces or loading variables Forefoot striking decreases impact peak force at contact in comparison with rearfoot striking, although impact forces are present. 63 Midfoot striking can produce a range of impact peak forces based upon knee and ankle compliance. 57 Although minimizing impact peak forces, forefoot striking eccentrically loads the ankle plantar flexors and increases demand at the foot and ankle because of a shift of power absorption from knee to ankle. 64 Researchers have suggested that forefoot striking may increase risk for ankle plantar flexor, Achilles tendon, and foot and ankle injuries in general. 57,65 As noted, Nielsen et al 17 reported a potential association between change in running speed and risk for Achilles tendinopathy, calf, and plantar fascial pain. Future research should assess for potential relationships among running speed, footstrike pattern and risk for Achilles, calf and foot pain. TRANSITIONING TO FOREFOOT RUNNING Researchers and clinicians question whether rearfoot strikers who are injury-free should transition to forefoot or barefoot striking merely to prevent injury. 66,67 Heiderscheit 68 noted, there is too much heterogeneity among runners to believe that one running pattern is universally ideal. General consensus supports a gradual transition should one adopt a forefoot striking pattern. Formal transition programs generally include a gradual increase in walking duration, followed by a gradual increase in running duration while wearing minimalist shoes. Typical programs include calf stretching and strengthening and intrinsic foot muscle strengthening. 69 The minimal recommended duration for transition programming appears to be runner-specific; the time required to elicit a change in running mechanics while adapting to minimalist footwear remains unknown. 69 RUNNING CADENCE, STRIDE LENGTH, AND INJURY RISK Forefoot striking generally produces a shorter stride length and faster cadence per given running speed. Increasing cadence by 10% above one s preferred rate while maintaining running speed decreases stride length and subsequent hip and knee joint loading (regardless of footstrike pattern). 70 Decreased loading may result from change in footstrike pattern, increased knee flexion at footstrike, or combination thereof. 71 The Joint Services Work Group reported insufficient evidence to recommend stride length or cadence manipulation to reduce injury risk. 2 However, runners may respond more readily to slight changes in cadence and stride length versus full transition to forefoot striking; load redistribution to the ankle musculature per cadence or stride length manipulation is typically much more gradual or less pronounced (B. Heiderscheit, personal communication, 2014). Per controlled laboratory study of 10 moderately active adult males, Hobara et al 71 reported that increasing cadence by 15% might decrease tibial stress fracture risk by reducing lower extremity loading variables, even when accounting for increased number of steps or loading cycles to travel a given distance. Per probabilistic modeling addressing loading variables, Edwards et al 72 reported that shortening stride length by 10% might decrease tibial stress fracture risk, even when accounting for potential metabolic costs (and thus fatigue onset) associated with a 10% decrease in stride length. Per TABLE II. Shoe Type Traditional Comparison of Traditional, Transitional Minimalist, Minimalist, and Barefoot Running Shoes Transition Minimalist or Partial Minimal Minimalist Barefoot Comparison Greater (9 14 mm) Heel Drop Thicker, More Stable Midsole More Substantial Arch Support More Substantial Heel Counter Slight (4 8 mm) Heel Drop Some Midsole Cushioning Partial Arch Support More Flexible Heel Counter Minimal (0 3 mm) Heel Drop No Midsole/Midsole Cushioning No Arch Support No (or Flexible) Heel Counter No Heel Drop No Midsole/Midsole Cushioning No Arch Support No Heel Counter (Illustrations courtesy of Elizabeth Holder, Web Strategy, Defense Health Agency and Mark Fischer, Visual Information Division, U.S. Army Public Health Center.) 516 MILITARY MEDICINE, Vol. 181, June 2016

6 systematic review, observable changes likely require a 10% or greater increase in cadence, although a 5% increase in cadence may elicit some changes. 73 MINIMALIST RUNNING SHOES AND INJURY RISK Minimalist shoes are characterized by minimal or no height differential (heel drop) between the heel and forefoot, no arch support or midsole and either a very flexible or no heel counter (Table II). 69 They are designed to promote forefoot striking or simulate barefoot running while protecting the foot. However, questions remain on both counts. Altman and Davis 67 noted that minimal footwear is being used to mimic barefoot running, but it is not clear whether it truly does. Per laboratory observation, approximately half of 16 habitually rearfoot striking, female recreational joggers continued to rearfoot strike in minimalist shoes despite two weeks of minimalist shoe wear. 74 In light of runners variable responses to minimalist shoe or forefoot striking transitions, manufacturers introduced a minimalist variant known as the transition minimalist or partial minimal shoe. Transition minimalist shoes provide a slight heel to toe height differential/drop, some midsole cushioning, a partial arch support and some degree of heel counter, but to a lesser extent than a traditional shoe. Transition minimalist shoes could theoretically bridge the transition from traditional to minimalist shoes. Grier et al 50 reported similar injury rates between U.S. Army Brigade Combat Team soldiers wearing traditional versus minimalist running shoes. Davis 69 suggested that transition minimalist shoe wear might increase injury risk; she suggested that these shoes provide just enough cushioning to encourage rearfoot striking, yet not enough cushioning to protect against impact forces. Following a prospective, randomized clinical trial including 99 runners, Ryan et al 75 reported greater overall injury rates among transition minimalist shoe wearers versus runners wearing traditional (cushioned/ neutral) or minimalist shoes. However, minimalist shoe wearers in Ryan et al s 75 study reported more shin and calf pain than runners wearing transition minimalist or cushioned/ neutral shoes. Currently, minimalist shoe wear does not appear to be associated with greater or lesser injury risk when compared with traditional running shoe wear. Shoe selection for research purposes may influence minimalist shoe findings. Bonacci et al 76 reported that different shoes have little impact on highly trained runners gait and that prescription of minimalist shoes as a mechanism to change running mechanics may not be justified. Similarly, Willy and Davis 77 questioned whether minimalist shoes provide enough feedback to induce an alteration that is similar to barefoot running. Bonacci and Willy used the Nike Free 3.0 model (Nike, Beaverton, Oregon) for a minimalist shoe in their measures of kinematic and kinetic variables; the Free 3.0 could be considered as a transition minimalist shoe because of its cushioning and 4-mm elevated heel. Runners maybemorelikelytomaintainrearfootstrikingpatternsin transition minimalist shoes versus true minimalist shoes. RUNNING SHOE ROTATION, VARIED LOWER BODY LOADING PATTERNS, AND INJURY RISK Per prospective 22-week follow-up, questionnaire responses from 195 male and 69 female runners, Malisoux et al 78 reported decreased risk among the 115 male and 33 female runners who alternated wear of more than one pair of running shoes across 22 weeks. They could not determine whether all runners alternated wear of different shoe models; some might have alternated wear of different pairs of the same shoe model. Malisoux et al hypothesized that alternating shoe wear might have decreased risk by varying external and internal loads on the musculoskeletal system. Although alternating shoe wear may ultimately prove to be an effective intervention, runners in Malisoux et al s study ran greater distances per running session than those typically performed in IET. Further, shoe alternation may be impractical during IET where three of the individual Services provide each recruit with a monetary stipend to purchase one pair of running shoes. Malisoux et al 78 also reported decreased risk among runners who participated in other sports across the 22 weeks, potentially because of the varied loading patterns of the different activities. Per retrospective cohort study with questionnaire responses from 156 elite female and 118 elite male adult distance runners, Fredericson et al 79 similarly reported decreased stress fracture incidence among those who had previously played basketball and/or soccer as children or adolescents. Fredericson et al hypothesized that the intermittent, varied, high-intensity loading sustained during soccer or basketball resulted in greater and more symmetrically distributed bone mass with greater bone stiffness. Fredericson et al s hypotheses are supported by Turner and Robling s 80 report that bone development responds better to short, intense exercise sessions (e.g., sprinting) versus longer, lesser intense activities such as distance running. Turner and Robling further noted that bone tissue quickly becomes desensitized to lengthy exercise bouts. The varied weight bearing loads sustained during lateral, pivoting, jumping, and landing movements associated with PRT and other IET activities could potentially reduce injury risk among recruits. That said, IET is of insufficient duration to increase bone mass (a surrogate measure for bone strength). A minimum of 6 to 8 months of exercise programming must occur to achieve a measureable increase in steady-state bone mass. 81 Further, greatest bone mass gains occur just before and during puberty. 82 As much as 90% of peak bone mass is attained by ages 18 and 20 among females and males, respectively. 83 This highlights the need for varied, high-intensity loading physical activity or exercise throughout childhood and adolescence to optimize bone health among recruits. MILITARY MEDICINE, Vol. 181, June

7 FIGURE 1. Shoe fitting techniques. (Illustrations courtesy of Elizabeth Holder, Web Strategy, Defense Health Agency. Reprinted with permission of Lower Extremity Review.) RUNNING SHOE FITTING AND REPLACEMENT GUIDELINES Lack of evidence supporting traditional running shoe prescription for primary injury prevention has led to the recommendation that runners focus on comfort, fit (length and width), and shoe condition when selecting and replacing shoes. Findings are conflicting concerning age of running shoes and RRMI risk. Per prospective 12-week follow-up, questionnaire responses from 205 male and 635 female, primarily recreational runners, Taunton et al 28 reported inconclusive results when associating older shoe wear with increased risk among females, but decreased risk among males. The injury rate for runners wearing shoes more than 2 years old was less than the mean injury rate in Taunton et al s study. Per a prospective controlled trial including 3,007 U.S. Marine recruits, Gardner et al 84 identified a modest trend (albeit statistically nonsignificant) associating running shoe age with stress fracture incidence. Twenty recruits were diagnosed as having stress fractures while wearing shoes that were less than 1 week old, whereas none of the 44 recruits wearing shoes greater than 1 year old sustained a stress fracture. Taunton et al did not match participants for aerobic fitness level, previous running history, or weekly running distance. Similarly, Gardner et al s Marine recruits were not matched for fitness or running history. These variables affect injury risk; individuals with older running shoes might have been fitter or had more extensive running histories than counterparts with new shoes. Others might have purchased new shoes in response to injury; new shoes might be a response to, rather than risk factor for injury. Given the conflicting findings concerning shoe age and RRMI risk, it is also possible that some runners adapt to gradual changes in midsole hardness as a shoe ages by altering leg compliance when contacting the ground. Clinicians frequently recommend replacing running shoes after 350 to 600 miles of wear. 85,86 Per personal opinion of Dr. Robert Wilder, Asplund and Brown noted that midsole shock absorption capability naturally degrades within 2 years, even if the shoe has never been worn. They subsequently recommended replacing shoes at least every 6 months. 86 The Joint Services Work Group noted that reports from shoe manufacturers and biomechanical studies on running shoes show that shoes should provide satisfactory support and cushion for 400 to 600 miles of use and, therefore, should be replaced accordingly to prevent injury. 87 However, the Work Group found insufficient evidence to recommend replacing shoes at standard distance intervals to reduce injury risk. 2 Molloy and Teyhen 88 recommended that heavier runners and those with prior injuries or extremes of arch height/foot mobility replace shoes closer to 400 miles of wear. They provided shoe fitting guidelines (Fig. 1), and recommended FIGURE 2. Shoe replacement consideration (midsole compression). Medial (A) or lateral (B) midsole compression evident when viewed from rear. (Illustrations courtesy of Elizabeth Holder, Web Strategy, Defense Health Agency. Reprinted with permission of Lower Extremity Review.) 518 MILITARY MEDICINE, Vol. 181, June 2016

8 TABLE III. Summary of Findings and Recommendations Issue Findings Recommendations RRMI classification Consensus or standardized RRMI definition recently proposed. Has not been validated or translated to multiple languages. 5 Attempt to validate proposed consensus definition and translate to multiple languages as appropriate. Future RRMI-based studies should adhere to proposed consensus definition to reduce variability in reported RRMI incidence and prevalence rates and enable comparisons among studies. 5 Physical fitness and recruit musculoskeletal injury risk Pre-enlistment physical activity levels and recruit musculoskeletal injury risk Hip musculature strength Pre-enlistment physical fitness screening Remedial physical training for poorly fit recruits Weekly running distance and RRMI risk Running speed/pace and RRMI risk Running surface type and RRMI risk Static foot type and RRMI risk Effect of traditional running shoe prescription per static foot type on RRMI risk Comparison of traditional versus transition minimalist versus minimalist running shoes on RRMI risk Low physical fitness and pre-enlistment physical inactivity are associated with overuse musculoskeletal injury risk during military IET. 9 Eight to 12 week duration of IET may be too brief to fully realize initial aerobic fitness and muscular strength gains among previously sedentary recruits Hip musculature (particularly abductor and external rotator) weakness and/or side-to-side strength asymmetries may increase RRMI risk due to inadequate dynamic hip stabilization while running. 37 However, research findings addressing potential association between hip musculature strength and RRMI are inconsistent. 39,40 Screening recruits for physical fitness and providing remedial physical training before beginning IET (or discharging the most poorly fit recruits) tend to reduce injury rates while decreasing attrition rates, training and medical costs. 90 Dose response relationship exists between weekly running distance and increasing RRMI risk. 18 Running frequency and duration likely play greatest roles in RRMI risk. 18 Thresholds for increased RRMI risk vary, potentially due in part to varied definitions for RRMI. 7,10 14 Findings are conflicting concerning the effect of running speed on RRMI risk; most researchers report no association between running speed and RRMI risk. 4,16,17 Sudden change in running speed might increase risk for Achilles, calf and plantar fascial pain. 17 Minimal (if any) association between running surface and RRMI risk. 16,27 30 No consensus concerning association between static foot type and RRMI risk. 16,33 38 Minimal (if any) evidence associating traditional running shoe prescription per static foot type to reduce RRMI risk Traditional versus minimalist running shoe wear does not appear to affect RRMI risk. 50 Transition minimalist shoe might increase risk by providing enough cushioning to encourage rearfoot striking, but not enough cushioning to protect against impact forces. 69 Emphasize regular physical activity/exercise programming for American children and adolescents (the military s future recruiting pool) that stresses development of all components of physical fitness. Runners and recruits with underlying hip musculature weakness and/or strength asymmetries should perform targeted hip strengthening exercises to correct weaknesses or asymmetries. Given inconsistent research findings to date, perform additional research addressing potential associations between hip musculature weakness or asymmetries and RRMI risk. Re-evaluate the U.S. Military Entrance Processing Command waiver process to more effectively screen out recruits with prior musculoskeletal injuries that are likely to recur or exacerbate during IET. Enforce minimum physical fitness standards before IET. Continue emphasis on standardized physical training programs that use anaerobic/interval running sessions to offset lesser running distance, duration and/or frequency while developing recruits aerobic and anaerobic fitness. Perform research to assess for relative relationships among running speed, footstrike pattern and risk for Achilles, calf and foot pain. De-emphasize running surfaces during RRMI risk interventions. Ensure each recruit has a new, properly fitting (length/ width), comfortable pair of athletic shoes during IET De-emphasize running shoe prescription per static foot type when mitigating RRMI risk for individuals without history of RRMI Perform research addressing (1) effect of running shoe prescription on RRMI risk among moderate to high distance runners (greater than 40 miles/week) (2) effect of running shoe prescription for secondary RRMI prevention among currently or previously injured runners (3) effect of running shoe prescription per foot mobility and (4) RRMI risk among runners wearing transitional minimalist shoes versus traditional or minimalist running shoes. (continued) MILITARY MEDICINE, Vol. 181, June

9 TABLE III. Continued Issue Findings Recommendations Effect of alternating or rotating different running shoe models (or different pairs of the same shoe model) on RRMI risk Effect of running shoe age on RRMI risk Recent questionnaire-based findings suggest that alternating or rotating shoes may decrease RRMI risk. 78 Findings are conflicting among the few studies that compared age of running shoes with RRMI risk. 28,84 immediately replacing shoes with obvious medial or lateral midsole compression (Fig. 2) regardless of distance run in the shoes. Per literature review, Nigg 59 proposed that comfortable running shoes might lessen a runner s energy expenditure by minimizing muscular activity related to shoe wear. Shoe comfort might thus decrease injury risk by delaying onset of fatigue when running. 88 Conversely, uncomfortable shoes might increase injury risk by eliciting compensatory gait patterns. 88 Asplund and Brown 86 noted that running shoes should be comfortable upon initial wear; there is no break in period for running shoes. MULTIFACTORIAL NATURE OF RRMI RRMI are multifactorial in nature. Hamill 89 noted that it s very simplistic to say that (running) injuries result from one or Perform research addressing the effect of alternating or rotating wear of different running shoe models (or different pairs of the same shoe model) on RRMI risk. Adhere to clinical recommendations to replace running shoes after 350 to 600 miles of wear. 85,86 Heavier runners and those with prior RRMI or extremes of arch height/foot mobility may benefit from shoe replacement at the lesser end of the 350 to 600 mile range. 88 Immediately replace shoes with obvious medial or lateral midsole compression. 88 Immediately replace shoes that are no longer comfortable. Joint Services Work Group reports insufficient evidence exists to recommend replacing shoes at standard distance intervals to reduce RRMI risk. 2 Footstrike pattern and RRMI risk No footstrike pattern is ideal for all runners. 68 Do not transition injury-free rearfoot strikers to forefoot striking merely to prevent injury. 66,67 Running cadence, stride length and RRMI risk Effect of varied, intermittent high intensity loading on RRMI risk Rearfoot striking may increase risk for tibial stress injuries/stress fractures, hip, knee or low back pain and plantar fasciitis Forefoot striking may increase risk for ankle plantar flexors/calf muscles, Achilles tendon and foot and ankle injuries in general. 57,65 Stride length or cadence manipulation is a more gradual approach to altering lower extremity loading versus transitioning to forefoot striking. However, Joint Services Work Group reports insufficient evidence exists to recommend stride or cadence manipulation for injury prevention. 2 Prior or concurrent participation in activities requiring varied, intermittent high intensity loading appears to decrease RRMI (to include stress fracture) risk. 78,79 Military entry-level training is of insufficient duration to increase bone mass; minimum of six to eight months of exercise programming must occur to achieve measureable increase in steady-state bone mass. 81 Greatest bone mass gains occur by the end of puberty. 82,83 Ensure a gradual transition when adopting a forefoot striking pattern. Transition program should include calf stretching and strengthening and intrinsic foot muscle strengthening. Minimum duration of transition programming will vary among runners. 69 Perform research addressing effect of stride and cadence manipulation on RRMI risk among runners with and without RRMI history. Emphasize regular physical activity/exercise programming that stresses varied, high intensity loading for children and adolescents to optimize bone health among recruits. Continue emphasis on standardized military physical training programs incorporating lateral, pivoting, sprinting, jumping and landing movements. two factors. Given the multifactorial nature of RRMI,the Services have proactively addressed recruit physical fitness levels to minimize injury risk and attrition. The Joint Services Work Group did not recommend physical fitness assessment or developmental physical fitness programming for physically unfit recruits before basic training, citing the intervention s incomplete systematic review. 2 Despite the lack of a recommendation, the Work Group cited studies reporting that screening for physical fitness and instituting remedial physical training for poorly fit recruits before IET decreased attrition andtendedtoreduceinjuryrisk. 90 Similarly, the introduction of a physical fitness accession standard at the beginning of Air Force Basic Military Training led to a 54% decrease in diagnosed stress fractures and $14.3M annual savings in training and medical costs between 2010 and 2012 (N. Baumgartner, F. Littlebird, T. Cropper, unpublished data, 2014). 520 MILITARY MEDICINE, Vol. 181, June 2016

10 CONCLUSION RRMI are multifactorial in nature; low physical fitness, prior injury, and weekly running distance are associated with RRMI risk in civilian or military recruit populations. Establishing a standardized, consensus RRMI definition will better enable clinicians and researchers to evaluate potential risk factors and related interventions. Table III provides a summary of the author s findings and recommendations to reduce RRMI risk. ACKNOWLEDGMENTS The author acknowledges the following individuals for their manuscript reviews and subsequent recommendations: Bruce H. Jones, MD, MPH, MAJ Tanja C. Roy, PhD, DPT, Keith G. Hauret, MSPH, MPT, Tyson L. Grier, MS, U.S. Army Public Health Center and LTC(P) Robert Oh, MD, Office of the Surgeon General (U.S. Army). These individuals received no compensation for their contributions. The author acknowledges the following individuals for their illustrations: Mark Fischer, U.S. Army Public Health Center Jennifer Donnelly, U.S. Army Medical Command; and Elizabeth Holder, Defense Health Agency. These individuals received no compensation for their contributions. REFERENCES 1. National Research Council: Physical fitness and musculoskeletal injury. In: Assessing Fitness for Military Enlistment: Physical, Medical, and Mental Health Standards, pp Edited by Sackett P, Mavor A. Washington, DC, National Academies Press, Bullock S, Jones B, Gilchrist J, Marshall S: Prevention of physical training-related injuries: recommendations for the military and other active populations based on expedited systematic reviews. Am J Prev Med 2010; 38(1 Suppl): S Yamato T, Saragiotto B, Hespanhol L Jr, Yeung S, Lopes A: Descriptors used to define running-related musculoskeletal injury: a systematic review. J Orthop Sports Phys Ther 2015; 45(5): Nielsen R, Buist I, Sørensen H, Lind M, Rasmussen S: Training errors and running related injuries: a systematic review. Int J Sports Phys Ther 2012; 7(1): Yamato T, Saragiotto B, Lopes A: A consensus definition of runningrelated injury in recreational runners: a modified Delphi approach. J Orthop Sports Phys Ther 2015; 45(5): Saragiotto B, Yamato T, Hespanhol L Jr, Rainbow M, Davis I, Lopes A: What are the main risk factors for running-related injuries? Sports Med 2014; 44(8): Hootman J, Macera C, Ainsworth B, Martin M, Addy C, Blair S: Predictors of lower extremity injury among recreationally active adults. Clin J Sport Med 2002; 12(2): Grier T, Canham-Chervak M, McNulty V, Jones B: Risk factors associated with running injuries in the United States Army. Med Sci Sports Exerc 2012; 44(5): S Molloy J, Feltwell D, Scott S, Niebuhr D: Physical training injuries and interventions for military recruits. Mil Med 2012; 177(5): Macera C: Lower extremity injuries in runners: advances in prediction. Sports Med 1992; 13(1): Walter S, Hart L, McIntosh J, Sutton J: The Ontario cohort study of running-related injuries. Arch Intern Med 1989; 149(11): Van Middelkoop M, Kolkman J, Van Ochten J, Bierma-Zeinstra S, Koes B: Risk factors for lower extremity injuries among male marathon runners. Scand J Med Sci Sports 2008; 18(6): Grier T, Canham-Chervak M, McNulty V, Jones B: Extreme conditioning programs and injury risk in a U.S. Army brigade combat team. US Army Med Dep J 2013; October December: Pollock M, Gettman L, Milesis C, Bah M, Durstine L, Johnson R: Effects of frequency and duration of training on attrition and incidence of injury. Med Sci Sports 1977; 9(1): Nielsen R, Parner E, Nohr E, Sørensen H, Lind M, Rasmussen S: Excessive progression in weekly running distance and risk of runningrelated injuries: an association which varies according to type of injury. J Orthop Sports Phys Ther 2014; 44(10): Fields K, Sykes J, Walker K, Jackson J: Prevention of running injuries. Curr Sports Med Rep. 2010; 9(3): Nielsen R, Nohr E, Rasmussen S, Sørensen H: Classifying runningrelated injuries based upon etiology, with emphasis on volume and pace. Int J Sports Phys Ther 2013; 8(2): Jones B, Cowan D, Knapik J: Exercise, training and injuries. Sports Med 1994; 18(3): U.S. Department of the Army: Army Physical Readiness Training, Field Manual Washington, D.C., Available at accessed June 7, U.S. Navy and Marine Corps Public Health Center. Navy Operational Fitness and Fueling System (NOFFS): Cardiovascular Fitness. Available at Accessed October 4, Knapik J, Rieger W, Palkoska F, Van Camp S, Darakjy S: United States Army Physical Readiness Training: rationale and evaluation of the physical training doctrine. J Strength Cond Res 2009; 23(4): Costill D: Inside Running: Basics of Sports Physiology. Indianapolis, IN, Brown and Benchmark Press, Wilmore J, Costill D: Physiology of Sport and Exercise. Ed 2. Champaign, IL, Human Kinetics Publishers, Kraemer W, Adams K, Cafarelli E, et al: American College of Sports Medicine position stand. Progression models in resistance training for healthy adults. Med Sci Sports Exerc 2002; 34(2): Baechle T, Westcott W: Fitness Professional s Guide to Strength Training Older Adults. Ed 2. Champaign, IL, Human Kinetics Publishers, Fry A, Häkkinen K, Kraemer W: Chapter 6: Special Considerations in Strength Training. Handbook of Sports Medicine and Science, Strength Training for Sport, pp Ames, IA, Blackwell Science, Macera C, Pate R, Powell K, Jackson K, Kendrick J, Craven T: Predicting lower-extremity injuries among habitual runners. Arch Intern Med 1989; 149(11): Taunton J, Ryan M, Clement D, McKenzie D, Lloyd-Smith D, Zumbo B: A prospective study of running injuries: the Vancouver Sun Run In Training clinics. Br J Sports Med 2003; 37(3): Tenforde A, Sayres L, McCurdy M, Collado H, Sainani K, Fredericson M: Overuse injuries in high school runners: lifetime prevalence and prevention strategies. PM R 2011; 3(2): Rauh M: Summer training factors and risk of musculoskeletal injury among high school cross-country runners. J Orthop Sports Phys Ther 2014; 44(10): Dixon S, Collop A, Batt M: Surface effects on ground reaction forces and lower extremity kinematics in running. Med Sci Sports Exerc 2000; 32(11): Ferris D, Liang K, Farley C: Runners adjust leg stiffness for their first step on a new running surface. J Biomech 1999; 32(8): Tong J, Kong P: Association between foot type and lower extremity injuries: systematic literature review with meta-analysis. J Orthop Sports Phys Ther 2013; 43(10): Barnes A, Wheat J, Milner C: Association between foot type and tibial stress injuries: a systematic review. Br J Sports Med 2008; 42(2): Williams D, McClay I, Hamill J: Arch structure and injury patterns in runners. Clin Biomech 2001; 16(4): Hreljac A: Etiology, prevention, and early intervention of overuse injuries in runners: a biomechanical perspective. Phys Med Rehabil Clin N Am 2005; 16(3): MILITARY MEDICINE, Vol. 181, June