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Brief Review

Systematic Review of the Association Between Physical Fitness and Musculoskeletal Injury Risk: Part 2—Muscular Endurance and Muscular Strength

de la Motte, Sarah J.1; Gribbin, Timothy C.1; Lisman, Peter1,2; Murphy, Kaitlin1; Deuster, Patricia A.1

Author Information
Journal of Strength and Conditioning Research: November 2017 - Volume 31 - Issue 11 - p 3218-3234
doi: 10.1519/JSC.0000000000002174

Abstract

Introduction

Musculoskeletal injuries (MSK-I) are a significant readiness problem in both military and athletic populations (18,22,27,58). Despite their prevention being identified as a top priority by many organizations (military, collegiate, and professional athletic entities), MSK-I remain the leading contributor to lost duty days and health care costs and a significant factor in the development of early onset osteoarthritis (29,54). Specific to military populations, in 2006, injuries accounted for 1.95 million medical encounters, and almost 1 million nondeployed U.S. active duty service members were affected (29). The prevalence of MSK-I during entry-level military training is especially high; roughly 25% of men and 55% of women require outpatient medical visits for MSK-I during U.S. Army basic training (25,30). In civilian populations, the Centers for Disease Control and Prevention reported 4.3 million nonfatal sports and recreation-related injuries were treated in U.S. hospital emergency departments over a 12-month span (11). Consequently, numerous epidemiological studies have evaluated factors associated with training-related MSK-I and identified several predictors of future injury risk, including age, sex, previous history of injury, lower frequency of previous physical activity or exercise, and anthropometrics (2,5,38,49,50,52,53,60), but modifiable risk factors—training load and physical fitness—may hold the greatest potential for reducing rates of MSK-I in all active populations (7).

Physical fitness encompasses several distinct components, including cardiorespiratory endurance, muscular strength (MS), muscular endurance (ME), flexibility, speed and agility, among others (17,57). Although not mutually exclusive, multiple aspects of fitness may contribute to MSK-I risk. Gaining an understanding of how specific fitness components are related to MSK-I risk will allow athletic trainers, coaches, clinicians, and researchers to develop targeted and focused injury prevention efforts for military and athletic populations.

To date, no published systematic literature reviews have examined the relationship between individual physical fitness components and MSK-I for both civilian and military populations. This is the second in a series of articles that review the evidence and assess the quality of relevant scientific literature published on physical fitness and MSK-I among military and civilian populations, aged 18–65 years. Our previous study focused on cardiorespiratory endurance. The objective of this study is to examine the evidence regarding the associations between MSK-I and both ME and MS using a systematic review process.

Methods

Experimental Approach to the Problem

Search Strategy

Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (see Checklist, Supplemental Digital Content 1, http://links.lww.com/JSCR/A52), we conducted a systematic review of the research literature to evaluate published, peer-reviewed studies, and military technical reports examining the association between components of physical fitness and the risk of sustaining a MSK-I in a military or civilian population between the ages of 18 and 65 years (see Figure, Supplemental Digital Content 2, http://links.lww.com/JSCR/A53). In consultation with a research librarian, the following databases were searched for relevant articles published from 1970 through December 2015: MEDLINE, EBSCO, EMBASE, and the Defense Technical Information Center. A combination of the following search items were used to capture all relevant studies: “injury,” “musculoskeletal diseases,” “musculoskeletal injury,” “sprain,” “strain,” “fracture,” “athletic injury,” “dislocation,” “tendonitis,” “bursitis,” “fasciitis,” “joints,” “risk assessment,” “injury prevention,” “attrition,” “physical fitness,” “exertion,” “physical endurance,” “physical education,” “training,” “exercise intervention,” “physical exercise,” “flexibility,” “muscular strength,” “muscular endurance,” “muscular power,” “aerobic fitness,” “aerobic capacity,” “aerobic power,” “anaerobic fitness,” “anaerobic capacity,” “anaerobic power,” “speed,” “balance,” “agility,” “maximal oxygen consumption,” “maximum oxygen consumption,” “V̇o2max,” “V̇o2peak,” “Military Personnel,” “Emergency Medical Technician,” “Athletes,” “dancer,” “emergency responder,” “police,” “firefighter,” “soldier,” “Army,” “Navy.” The search was last performed in February 2016.

Study Inclusion/Exclusion Criteria

Included articles met the following criteria: original research; full-text available; populations aged 18–65 years; reported measures of physical fitness; study outcomes included acute/traumatic (e.g., dislocation, fracture, sprain/strain, etc.) or overuse (e.g., stress fractures, tendonitis, bursitis, etc.) MSK-I (48); and a reported measure of association with injury (e.g., odds, hazard, risk or rate ratio). MSK-I was defined as an event that resulted in damage to the musculoskeletal system for which the subject visited a medical care provider or went to the emergency room (38). Publications were excluded if: (a) injury data reported were related to heat or cold injuries, animal bites, or other nonmusculoskeletal MSK-I; (b) injury data were self-report in nature; (c) study populations comprised children, the elderly, or adults with ill-health or physically/mentally disabled; or (d) studies were systematic reviews, literature reviews, case studies, or case series in design. The full-text articles meeting the inclusion criteria were obtained, and the full text of any article not clearly initially excluded, to determine the final inclusion or exclusion by the full study panel.

Data Abstraction

Data abstraction was split among the study investigators, with articles evenly divided among the authors. Relevant data were independently extracted using a standardized set of abbreviations and reporting methods and finally compiled in a single database. Data abstraction categories included population characteristics, MSK-I type and assessment, fitness components measured, fitness measurement tests, statistical analysis performed and the reported odds, risk, or hazard ratios of injury with and without adjustments.

Methodological Quality Assessment

In coordination with the US Army Public Health Command, study quality assessments were adapted from Bullock et al. (5) (see Quality Assessments, Supplemental Digital Content 3, http://links.lww.com/JSCR/A54). The assessment tools comprised 3 sections: (a) problem (1 item); (b) study design and methodology (6 items); and (c) data presentation and statistical analysis (4 items). Each question was scored from 0 to 2, with a maximum total score of 22. Each study was evaluated by 2 study investigators, who then met to compare scores and discuss any discrepancy until a consensus was reached. A third investigator acted as the tie breaker where the original investigators could not reach consensus.

As all studies received maximum points (2) for clearly identifying the research problem, scores were converted to a 20-point scale and expressed as percentage. Studies were then ranked according to their methodological quality based on percentage of total possible points, resulting in the following groups: (a) poor (below 70th percentile: ≤13.5 points); (b) fair (70–79th percentile: 14–15.5 points); (c) good (80–89th percentile: 16–17.5 points); or (d) excellent (90th percentile and above: >18 points). This methodological quality scoring rubric was modified from a previously published systematic review of physical training-related injury prevention strategies for military and athletic populations (5).

Assessment of Evidence Summary—Strength of the Evidence

Studies that found significant crude or adjusted associations between measures of ME and/or MS and MSK-I were distinguished from those that did not with symbols representing the level and direction of association. Significant associations between low levels of ME/MS (less fit) and elevated MSK-I risk with univariate and multivariate analyses were denoted with the symbols “+” and “++”, respectively, whereas associations between high levels of ME/MS (more fit) and elevated MSK-I risk were noted with “−” and “−−”. Next, the overall strength of the evidence was compiled and rated following criteria adapted from previous systematic reviews (5,6,23). The level of evidence for the relationship between ME/MS and MSK-I was ultimately determined to be: (a) Strong: 3 studies of at least good methodological quality, or at least 2 studies of excellent methodology with consistent multivariate findings; (b) Moderate: consistent results from 2 studies of good methodology, or 1 study of excellent methodology with multivariate findings: (c) Limited: one study of good or fair methodological quality with multivariate findings; multiple studies of good methodology with univariate findings: (d) Insufficient: results from studies of exclusively poor methodology; no evidence from multivariate analyses in excellent quality studies.

Results

Search Results

The database search yielded 4,229 articles and 5 additional articles were identified from a manual search and subject matter expert recommendations. After duplicates were removed, the 3,321 remaining articles were reviewed for inclusion or exclusion. Full-text articles were retrieved if exclusion could not be determined by title and abstract alone, or if the article passed the first eligibility screening. After preliminary review for inclusion, 234 full-text articles were reviewed for associations between MSK-I and one or more fitness components. Ultimately, 45 articles met the final inclusion criteria for ME/MS and MSK-I. Primary reasons for exclusion were the lack of an objective fitness assessment, the outcome data did not include injuries specific to the musculoskeletal system, and/or the statistical analyses did not include measures of association, such as odds, hazard, and/or risk ratios.

Characteristics of Included Studies

Most studies were focused on military samples (31 of 45; 68.9%), with 19 of these (61.3%) conducted in the basic training environment. The remaining studies assessed professional athletes (n = 6; 13.3%), collegiate athletes (n = 5; 11.1%), physical education majors (n = 2; 4.4%), and Federal Bureau of Investigation trainees (n = 1; 2.2%). Most studies (30 of 45; 66.7%) investigated multiple ME and/or MS assessments, whereas 12 (26.7%) focused on 1 assessment (e.g., isokinetic testing at a single joint). The remaining 3 studies used a combined score comprising results of several ME tests, which were impossible to separate out individually (40,45,46). Studies that assessed ME included timed tests or tests to fatigue/failure for push-ups, sit-ups, and pull-ups, whereas studies of MS evaluated performance on isometric, isokinetic, and/or isotonic strength assessments.

Musculoskeletal Definitions and Injury Ascertainment

Six studies (13.3%) specifically studied overuse injury outcomes, including bone stress injury/stress fracture (n = 4), overuse Achilles injury (n = 1), or nonspecific “overuse” injury (n = 1). All 6 studies were conducted in military populations, with follow-up times ranging from 6 weeks to 8 months of training and/or military service. Eight studies (17.8%) looked at acute injury outcomes, like ankle sprain (n = 3), hip/thigh muscle strain (n = 3), core or lower extremity sprain or strain (n = 1), or severe knee injury (n = 1). One study reported only contact injuries in professional rugby players, whereas another reported incidence of exertional medial tibial pain in female physical education students. One study from the Finnish military looked at all lower extremity injuries causing hospitalization. Finally, more than half of the studies (28 of 45; 62.2%) used a broad injury definition incorporating all variations of MSK-I, including those listed above.

Injury outcomes were obtained through a variety of mechanisms. Twelve studies (26.7%) used existing injury surveillance systems, including the Defense Medical Surveillance System (6 of 45; 13.3%), to obtain injury outcomes from ICD-9 or similar coding. A large proportion (16 of 45; 35.6%) of studies reviewed medical records to abstract injury data, with 3 of these also relying on advanced imaging to confirm diagnosis. Health care providers (such as team physicians, athletic trainers, physiotherapists, or base clinicians) directly diagnosed MSK-I in 15 studies (33.3%). Finally, 1 study obtained data directly from coaches, who filled out a standardized reporting form, whereas another used participant questionnaires in combination with an evaluation by a physician.

Methodological Quality

Nine studies were categorized as “excellent,” 20 were classified as “good,” 10 were categorized as “fair,” and 6 were considered to have “poor” methodological quality. Tables 1 and 2 provide a summary of the study characteristics, quality ratings, and outcomes and directions of association for all studies of “excellent” and “good” methodological quality which investigated the association between ME and MSK-I for men and women, respectively, whereas Table 3 presents these results for studies that examined the association between MS and MSK-I in men and/or women. Supplemental Digital Content 4 (See Table, http://links.lww.com/JSCR/A55) and Supplemental Digital Content 5 (See Table, http://links.lww.com/JSCR/A56) present results for all studies of “fair” and “poor” methodological quality that investigated the association between ME and MSK-I for men and women, respectively, whereas Supplemental Digital Content 6 (See Table, http://links.lww.com/JSCR/A57) presents these results for studies that examined the association between MS and MSK-I in men and/or women. Finally, Supplemental Digital Content 7 (See Table, http://links.lww.com/JSCR/A58) presents results for studies which used a combined score comprising results of several ME tests.

Table 1.
Table 1.:
Study methodological characteristics and results for the association between muscular endurance and MSK-I in men.*§
Table 1-A.
Table 1-A.:
Study methodological characteristics and results for the association between muscular endurance and MSK-I in men.*§
Table 1-B.
Table 1-B.:
Study methodological characteristics and results for the association between muscular endurance and MSK-I in men.*§
Table 1-C.
Table 1-C.:
Study methodological characteristics and results for the association between muscular endurance and MSK-I in men.*§
Table 2.
Table 2.:
Study methodological characteristics and results for the association between muscular endurance and MSK-I in women.*
Table 2-A.
Table 2-A.:
Study methodological characteristics and results for the association between muscular endurance and MSK-I in women.*
Table 3.
Table 3.:
Study methodological characteristics and results for the association between muscular strength and MSK-I in men and women.*†
Table 3-A.
Table 3-A.:
Study methodological characteristics and results for the association between muscular strength and MSK-I in men and women.*†

Evidence Summary—Strength of Evidence

Muscular Endurance

Push-Ups

Twenty-two of 45 studies (48.9%) investigated associations between MSK-I and push-ups (1,3,8,9,12,14,20,28,31,32,34–39,41,49,56,59,60,65), with only one of these being conducted in a nonmilitary environment (FBI trainees) (36). Push-up evaluations varied between 1- and 2-minute assessments, whereas 2 studies specifically investigated push-ups to failure (14,36). Most of the military studies (14 of 21; 66.7%) were focused on MSK-I in basic training: 5 studies included men only, whereas the remaining 16 included outcomes for both sexes.

For men, of the 22 studies investigating push-ups, 4 found it to be an independent risk factor for MSK-I in multivariate models (10,31,32,56). All 4 studies used 2-minute timed tests and consistently found that those who performed fewer push-ups were 1.2–3.1 times more likely to have an MSK-I over the course of the study. Study methodological quality was excellent in 1 study (32), good in 2 (10,56), and fair in the other (31). Two of these studies were on Army Basic Training (10,31), whereas the other 2 were conducted in the regular Army (32,56); follow-up times ranged from 8 weeks to 18 months. An additional 11 studies reported significant univariate associations between fewer push-ups and increasing MSK-I risk: 7 for 2-minute push-ups (8,12,20,28,38,39,59) and 4 for 1-minute push-ups (34,35,41,60). Methodological quality was primarily good (n = 6) (8,12,38,39,59,60), with an additional 3 studies of excellent quality (20,34,41). Overall, it seems that the level of evidence supporting fewer push-ups as associated with increasing MSK-I risk in men is strong.

In contrast to men, push-ups, regardless of test duration, were not found to be an independent predictor of MSK-I in any of the 16 studies with female participants (1,3,8,10,14,20,28,31,34–39,41,59). However, 7 studies reported significant univariate associations between lower push-up performance and risk of MSK-I (10,20,35,38,39,41,59), including 2 studies of excellent methodological quality (20,41) and 4 of good quality (8,38,39,59). Overall it seems that the level of evidence for push-up performance as a risk factor for MSK-I in women is limited.

Sit-Ups

Twenty-four studies investigated sit-up performance, either to failure (14,19), 1-minute (1,34–36,41,67), or 2-minute performance (3,8,9,20,28,31,32,37–39,43,49,56,59,61,65). For men, 2 studies, one each of excellent and good methodological quality, found that poor performance in a 2-minute sit-up test was significantly related to MSK-I risk in multivariate models, with the risk being 1.1–1.4 times higher compared with adequate or good sit-up performance (20,56). Three additional studies reported significant univariate associations between sit-ups and MSK-I (8,19,65). Specific to women, 4 of the 16 studies (2 good (8,39) and 1 each of excellent (41) and poor (35) methodological quality) reported significant univariate associations between a fewer number of sit-ups and MSK-I risk. Consequently, the level of evidence between sit-up performance and MSK-I risk was deemed moderate for men and limited for women.

Pull-Ups

A total of 4 studies investigated pull-ups or an equivalent (e.g., heaves or lifting) (4,36,43,60). Military physical fitness test guidelines state that pull-ups can be performed with either an overhand or underhand grip (47), whereas heaves are performed with an underhand grip only (4). In 1 study of excellent quality (43) and 2 studies of good methodological quality (36,60), no unadjusted or adjusted associations were noted between the number of pull-ups and MSK-I risk in men, regardless of test duration. One study of fair methodological quality investigated men and women together and reported a significant univariate association between better “heave” or lifting performance and lower MSK-I risk in British Army recruits (4). Overall, we concluded there is insufficient evidence to support pull-up performance as a risk factor for MSK-I.

Tests to Failure/Fatigue and Other

Eight studies included other measures of ME with each half comprising male-only military (4,51,60,67) and athlete populations (16,42,64,67). Measures included an “ammo can” hold to assess static arm endurance (4), a 60 (16) and 84.1 (19) kg bench press to failure, trunk ME tests such as a side bridge hold (42), prone bridge hold with (51) and without march (67), trunk flexion hold and wall-sit holds (64), the Beiring-Sorenson test (42), and a 1-minute back extension test (60). One study of good methodological quality found fewer back extension repetitions to be an independent risk factor for overuse MSK-I in Finnish conscripts (60), whereas another found a multivariate association with performance in both trunk-flexion and wall-sit hold tests, when used in combination with other factors (starter ≥1 game, Oswestry Disability Index ≥4), and incidence of core and lower extremity injury in collegiate athletes (64). Collectively, the level of evidence for this category was deemed moderate.

Muscular Strength

Isokinetic Testing

Eight studies investigated associations between MSK-I and isokinetic concentric and/or eccentric strength (15,21,24,33,44,63,66,68). Four studies examined strength at the ankle, including plantarflexion, dorsiflexion, inversion, and eversion (15,21,44,66), 3 examined knee extension and flexion strength (24,33,68), and 1 examined strength at the hip (abduction, adduction, external, and internal rotation) (63). Three studies examined women only (33,63,66), 4 men only (15,21,24,44), and 1 combined men and women in the analysis (68). Seven of 8 studies investigated the association between MS and the incidence of a particular lower xtremity injury, such as an ankle sprain (15,21,66), hamstring strain (24,68), or exertional medial tibial pain (63).

Three of 4 studies (2 good methodological quality, 1 poor quality) reported isokinetic ankle flexion strength as an independent risk factor for lower extremity injury (15,21,44). Of the 2 ankle sprain studies, 1 found asymmetrical eccentric plantarflexion/dorsiflexion strength to be independently associated with ankle sprain risk in male professional soccer players (15), whereas the other found that increased plantarflexion strength was an independent risk factor for injury in male volleyball players (21). Mahieu et al. (44) found that decreased plantarflexion strength was predictive of Achilles tendon overuse injury in Belgian Army basic trainees. The level of evidence for isokinetic ankle strength and MSK-I is moderate, although the specific direction of the overall association cannot be definitively determined.

Of the 3 studies evaluating isokinetic knee flexion and extension strength, 2 looked at hamstring injury as the outcome of interest (24,68) and the third any injury (33). In separate studies of good methodological quality, right-to-left knee flexion strength asymmetry (33) and knee flexion/extension asymmetries (33,68) were independent risk factors for injury in athletes. Taken together, there is moderate evidence that isokinetic knee flexion strength is associated with MSK-I.

Isotonic Testing

Seven studies investigated associations between MSK-I and performance on isotonic assessments of upper-body, lower-body, and/or whole-body strength (4,12,16,19,26,32,38). Only 2 studies included women (4,38) or were focused on a nonmilitary population (16,19). In 1 study of poor methodological quality, lower leg press strength was independently associated with increased stress fracture risk in soldiers completing Israeli Air Force basic training (26). Another study of fair methodological quality found that better performance on weighted chin-up at 1 repetition maximum was significantly associated with decreased contact injury risk in Australian rugby players (16). One study each of excellent (32) and fair (4) methodological quality reported a significant univariate association between dynamic lift strength and injury risk in military populations. Given the methodological quality of those studies reporting a multivariate association between performance on isotonic strength assessments and MSK-I, we concluded that the evidence supporting this association was limited.

Isometric Testing

Nine studies investigated associations between MSK-I and performance on isometric strength assessments (3,4,13,19,32,38,42,49,62). Five of the 9 studies were conducted in military populations (3,4,32,38,49), with 4 during basic training and including women (3,4,38,49). In 2 studies of fair methodological quality, decreased hip external rotation strength, and lower hip adduction and abduction strength were independent risk factors for injury in collegiate (42) and professional athletes (62), respectively. In 1 study of excellent methodological quality—albeit in Army Wheel Mechanics—moderate strength of the back extensors and lower isometric knee flexion strength were significantly associated with increased injury risk, whereas moderate elbow flexion strength was associated with decreased risk of MSK-I (32). Collectively, performance on assessments of isometric strength was determined to be moderately associated with MSK-I risk, although the exact direction of the association cannot be stated.

Muscular Strength and Endurance—Composite Fitness Scores

Four studies from the Finnish Defense Force reported composite scores derived from several ME assessments, including number of push-ups, sit-ups, pull-ups, and back lifts, in combination with other fitness measurements (e.g., horizontal jump distance), and their association with injury outcomes (40,45,46,60). Back lifts assess the back and hip extensor muscles and involve the participant lying prone hands behind their neck with their legs supported by a test administrator. Performance is measured by the number of back lifts (upper body lifted so that both scapulas are 30 cm higher than shoulder level) completed in 1 minute (55).

One study of good methodological quality found a multivariate association between higher composite scores and increased risk of hospitalization for any knee injury, new meniscal tears and anterior cruciate ligament (ACL)/posterior cruciate ligament (PCL) tears (40). In contrast, another study of good methodological quality found that lower scores were an independent risk factor for any MSK-I (60). Overall, the evidence supporting composite measures of strength and MSK-I is moderate, although the exact direction of association cannot be stated.

Discussion

Muscular strength and ME are important components of overall physical fitness and integral components of many military and athletic training regimens. In this article, we systematically reviewed and evaluated the level of evidence for associations between ME and/or MS and MSK-I risk. To our knowledge, this is the first study to systematically examine a broad definition of ME/MS and injury risk in civilian or military studies. Although results for some tests did vary by sex, taken together, our primary findings indicate there is (a) a strong evidence that poor performance in a push-up test is associated with risk of MSK-I; (b) moderate evidence that poor performance in sit-up test is associated with risk of MSK-I; (c) moderate evidence that isokinetic ankle and knee flexion strength, and isometric strength assessments at the back, elbow, or knee are associated with MSK-I risk; and (d) limited evidence that poor performance in a pull-up test and isotonic assessments of MS are associated with MSK-I.

Our examination of the research led to many interesting findings. First, our most straightforward finding is a strong level of evidence for poor push-up performance as risk factor for MSK-I in men. Push-ups are a standard component of fitness testing in military and other law enforcement environments, and evaluations varied between 1- and 2-minute assessments and push-ups to failure. Push-ups primarily assess upper-body ME, although in tests of maximum timed physical effort, a certain amount of mental determination is also required. Most studies with significant findings were conducted during basic training (8,9,12,28,31,34,35,38,39,41,59), where mental and physical limits are stretched, and included both sexes. Basic trainees enter training with a broad range of fitness levels as indicated by wide variability run times and push-up performance, whereas athletic populations tend to be more homogenous; these differences may help explain the strong level of evidence detected between poor performance and increased risk. However, the data for women suggested only limited evidence. Women typically have lower upper-body strength than men, and it could be that the female samples were actually more homogenous in upper-body strength, which limited finding significant results. Because of the sex-specific findings in this case, a timed push-up might be considered as part of a screen for injury risk for training men, but not women.

Similar to push-ups, we found moderate evidence for men and limited evidence for women, that fewer sit-ups, a measure of abdominal ME, were associated with greater MSK-I, almost exclusively in basic training. In the military, sit-ups are performed while the feet are held by a partner, which means the test is primarily isolated to the superficial abdominal muscles and hip flexors. Women with poor performance were up to 40% more likely to sustain an MSK-I during the study period, compared with those with adequate or good performance, which suggest that this movement could be considered as part of a battery for injury risk screening protocols in men. For women, although the evidence is limited, it could not be recommended at this point for risk stratifying for injury susceptibility, but this does not mean that poor sit-up performance is not related to injury risk in these training cohorts. Of the 3 studies with univariate findings (8,35,39), female samples were very large (n = 898, 717, and 516 basic trainees), whereas other study samples were much smaller. The largest female sample investigated discharge from basic training due to injury as an outcome (59), and therefore cannot be directly compared with other study outcomes. Future studies should continue to investigate sit-up performance in women as a risk factor for eventual injury.

Although injury outcomes for most studies were broad in scope (i.e., all acute/traumatic and overuse to any body part), several studies investigated narrow outcomes, primarily of the lower extremity (e.g., lateral ankle sprain) and their association with MS. Three of the 4 studies that evaluated ankle or Achilles outcomes with isokinetic strength testing at the ankle reported significant independent associations of varying directions (15,21,44). Hamstring injury was studied as an injury outcome in 2 studies (24,68), one of which found a multivariate association between hamstring to quadriceps strength ratio <0.6 and injury (68). Systematic reviews that investigate MS in association with specific injury outcomes (not pathology) should be considered as a future effort.

Knee injury was another outcome of interest. In a good methodological quality study of the Finnish military (N = 128,584), a higher composite ME score, which was computed in combination with an assessment of lower-body power, was an independent risk factor for increased incidence of knee injury (40). Specifically, composite scores were determined by the number of push-ups, sit-ups, pull-ups, and back lifts performed in 1-minute and horizontal jump distance. It is possible that those with greater composite scores were more athletic and/or performed riskier tasks during training maneuvers, which affected their exposure. Those in the highest quartile were 1.6 times more likely to have a knee injury requiring hospitalization, 3.5 times more likely to sustain a new meniscal tear, and 2.6 times more likely to tear their ACL and/or PCL over the course of 1 year. Most knee injuries (49%) occurred during the special and team training period, which is 57–180 days of service. Given this is a much longer time frame than many of the military studies investigating ME/MS, a longer exposure might explain some of the differences, as well as this being the only study to investigate knee injury outcomes. Both of these points make it difficult to compare across other military studies and those with a broader injury definition. Importantly, 2 other studies, both with the Finnish military, used the same composite score and did not find significant multivariate associations with MSK-I over 8 months of service (45,46). As such, given the level of evidence remains limited, these tests still could be considered as part of a screening panel for knee injury in the future.

Limitations

Our systematic review has several limitations. First, we examined only studies that reported significant risk ratios (RRs), odds ratios (ORs), or hazard ratios (HRs) in our final inclusion criteria, which limits the scope of our findings. Although significant univariate and multivariate findings were considered in our level of evidence determinations, the different types of analyses conducted to obtain the ratios make comparisons across studies difficult. Of the significant studies, 13 performed Cox regression by using survival modeling to obtain HR—or how the risk of an event per unit time (hazard) changed over time, and then ultimately how the hazard varies in response to covariates in multivariate modeling. As noted in part 1 of this series of systematic reviews, time to event analyses differs from traditional multivariate logistic regression modeling, so that HRs and RRs are not comparable to ORs. This compromises interpretation and comparisons of study findings with RRs, ORs, and HRs, which could be made simpler with an overall meta-analysis, and beyond the scope of this study.

A second limitation is that we modified previous methodological scoring rubrics and developed our own level of evidence determinations for this study. Both these metrics were modified from previously published systematic reviews and both were made specific to our questions of interest (5,6,23). However, as these are new measures, the measurement properties of each could be questioned.

Practical Applications

This manuscript systematically reviews the evidence and assesses the quality of relevant scientific literature published on ME and MS components of physical fitness, and their association with the risk of MSK-I among military and civilian populations. Results indicate that several measures of ME/MS are moderately or strongly associated with increased risk of MSK-I in civilian athletic and/or military populations. Importantly, these findings have practical applications for strength and conditioning coaches and other personnel responsible for the development and implementation of injury prevention programs in military and civilian athletic populations. Given the current evidence, practitioners should include an assessment of ME/MS as a measure for MSK-I risk stratification in these populations, in particular military basic trainees. However, additional research is needed to identify and recommend specific assessments of ME/MS that are predictive of MSK-I in both men and women. Future studies should also consider measures of MS and ME as a function of upper body, lower body, and core strength, and their likely association with specific, rather than general, MSK-I.

Acknowledgments

The authors have no conflicts of interest to disclose. The opinions or assertions contained herein are the private ones of the authors and are not to be construed as official or reflecting the views of the Uniformed Services University, Department of the Army, Department of the Air Force, Department of the Navy or the United States Department of Defense.

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Keywords:

muscular strength and endurance; military; injury; risk factors

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