Background The identification of risk factors for groin injury in sport is important to develop and implement injury prevention strategies.
Objective To identify and evaluate the evidence examining risk factors for groin injury in sport.
Material and methods Nine electronic databases were systematically searched to June 2014. Studies selected met the following criteria: original data; analytic design; investigated a risk factor(s); included outcomes for groin injury sustained during sport participation. The Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines were followed and two independent authors assessed the quality and level of evidence with the Downs and Black (DB) criteria and Oxford Centre of Evidence-Based Medicine model, respectively.
Results Of 2521 potentially relevant studies, 29 were included and scored. Heterogeneity in methodology and injury definition precluded meta-analyses. The most common risk factors investigated included age, hip range of motion, hip adductor strength and height. The median DB score across studies was 11/33 (range 6–20). The majority of studies represented level 2 evidence (cohort studies) however few considered the inter-relationships between risk factors. There is level 1 and 2 evidence that previous groin injury, higher-level of play, reduced hip adductor (absolute and relative to the hip abductors) strength and lower levels of sport-specific training are associated with increased risk of groin injury in sport.
Conclusions We recommended that investigators focus on developing and evaluating preparticipation screening and groin injury prevention programmes through high-quality randomised controlled trials targeting athletes at greater risk of injury.
- Risk factor
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Groin injuries are common in many sports that involve rapid acceleration and deceleration, sudden changes in direction and kicking such as soccer,1–12 rugby,13 Australian rules football,14 ice hockey,15 Gaelic football and cricket.15 ,16 In addition to frequent occurrence, prospective collection of injury data over consecutive soccer seasons has demonstrated that those with a previous groin injury are at a 2.4 (hazard ratio; 95% CI 1.2 to 4.6) times greater risk of groin injury than payers with no previous history.10 This vicious cycle of injury and re-injury may result not only in reduced performance and missed training/competition but chronicity, the end of an athletic career and future mobility disability.
According to van Mechelen,17 the prevention of sport injuries occurs through a four-step process beginning with establishing the extent of the specific injury through a validated injury surveillance system. This is followed by identifying injury risk factors and causal mechanisms through prospective analysis of specific injury patterns, development and introduction of preventative strategies and evaluation of these strategies by determining their impact on injury incidence. Throughout this process it is important to acknowledge that a sport injury is unlikely to result from a single risk factor but rather as a consequence of complex interactions of multiple risk factors and inciting events.18 Thus, studies aimed at identifying risk factors for groin injuries in sport should utilise a prospective design and ensure an adequate sample size to facilitate biostatistical methods that consider the interrelationships between various risk factors.19
Consensus regarding the risk factors for groin injury is lacking and this may be due, in part, to methodological limitations and heterogeneity of previous studies. Our 2007 systematic review of risk factors for groin strain injury in sport reported a deficiency in prospective studies.20 Based on the studies available at that time (n=11; 2 cross-sectional and 9 prospective), there was support for an association of previous injury and greater hip adductor to abductor strength ratio, sport specificity of training and amount of preseason sport-specific training as individual risk factors in groin strain injury. Although this review did not include a formal assessment of the quality or level of evidence of the included studies, it reported significant concerns regarding the internal validity of the included studies. Further, it recommended that any future studies examining risk factors for groin strain injury in sport employ consistent injury definitions, use validated and reliable injury reporting systems to quantify outcome measures and consider the inter-relationships between risk factors by controlling for potential confounding variables (eg, player exposure and previous injury).
As identification of risk factors and their causal mechanisms is a precursor to the development of effective prevention strategies, the lack of consensus related to risk factors for groin injury in sport has likely hindered the process of developing and evaluating groin injury prevention strategies in sport. The objective of this review was to update this previous systematic review and summarise the evidence related to risk factors for groin injury in sport, including critical appraisal of the literature.
This review was conducted according to the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines.21
Data sources and search
Relevant studies were identified by searching nine online databases, selected based on their relevance to the research topics, from inception to June 2014. These databases included: MEDLINE (1966-present), CINAHL (Cumulative Index to Nursing and Allied Health Literature; 1982–present), Cochrane database for Systematic and Complete Reviews (1975–present), Cochrane Controlled Trials Registry (1975–present), Cochrane Injuries Group Trials Register, Sport Discus (1980–present), EMBASE (Excerpta medical databases; 1974–present), PubMed (public Medline) and SCOPUS. A combination of medical subject headings (MeSH) and text words were used to execute each search. Table 1 outlines the search terms used by injury, anatomical region or tissue type and risk concept along with the combinations of search terms that formed each search strategy. The only limits set were that studies be published in a peer-reviewed journal. The Cochrane database for Systematic and Complete Reviews was included to identify any systematic reviews and/or meta-analyses such that their reference lists could be manually searched alongside those of all selected studies to identify relevant articles not identified by the search strategies. Manuscripts were organised using the reference management software package, EndNotes V.7.1 (Thomson Reuters, 2013). The number of references obtained from each search strategy for each database was recorded and a running total constructed. After accounting for duplication, the titles and corresponding abstracts of all returned records were reviewed by (JLW) to identify potentially relevant studies. Finally, the full text of all potentially relevant studies was reviewed to determine final study selection by (JLW, CAE).
Studies were included if they investigated the association between any potential injury risk factor (defined as any factor that may increase the potential for injury) or injury prevention strategy with groin injury (defined as any or all of the following; groin or hip adductor injury or muscle strain, tenderness on palpation of the hip adductor or flexor muscles, adductor bone-tendon junction or pubic symphysis and/or pain on resisted hip adduction). Additional inclusion criteria included: primary research of original data, analytic design (eg, experimental, cohort, case–control or cross-sectional), an outcome measure of groin injury sustained during sport participation, an objective exposure measure of one or more potential risk factor or injury prevention strategy for groin injury in sport and study participants who were involved in any sport that involved rapid acceleration and deceleration, sudden changes in direction and kicking. The definition of groin injury was modified slightly (omitted lower abdominal muscles) from the original systematic review to be consistent with clinical entity of adductor-related pain proposed by Holmich et al22 and, as a greater number of studies focusing on the hip adductors and adductor bone-tendon junction were available than at the time of the original review.
Studies were excluded if the injury outcome was only described in general terms such as thigh or hip injury, were not written in English or involved animal models or cadavers. Further, conference proceedings/abstracts, review articles (systematic and narrative), case series or case studies, editorials, commentaries and opinion-based papers were excluded.
Data extraction and study rating process
Data extracted from each study included; study design, study location and population (sport, level, age, sample size), injury outcome (definition), injury estimates (incidence proportion, incidence rate, prevalence), measures of risk (difference in means, correlations, OR, incidence rate ratios; IRR and risk ratio; RR), risk factors and results (significant and non-significant). If available, injury estimates (injury rates) were used to calculate point estimates of IRR (IR exposed/injury rate in unexposed). Two authors (lead author and one of three coauthors) independently assessed the quality and level of evidence of each study. Quality of evidence was evaluated based on criteria for internal validity (study design, quality of reporting, presence of selection and misclassification bias, potential confounding) and external validity (generalisability) using the Downs and Black (DB) quality assessment tool which assigns an individual score calculated out of 33 total points for each study (10 points for reporting, 3 points for external validity, 7 points for bias, 3 points for confounding and 1 for power: see online supplementary appendix 1).23 The level of evidence represented by each study was categorised based on the Oxford Centre of Evidence Based Medicine (OCEBM) model (see online supplementary appendix 2).24 As per study exclusion criteria, levels 1a, 2a, 3a (systematic reviews), 4 (case series) and 5 (opinion-based papers) were not included. Discrepancies in DB scoring or OCEBM categorisation were resolved first by consensus between the two reviewers who rated the study and if required, by the senior author (CAE).
Extracted data, quality and level of evidence were summarised for each study. The quantity, quality and level of evidence for the most commonly investigated modifiable and non-modifiable risk factors for groin injury in sport were collated.
Identification of studies
An overview of the study identification process is provided in figure 1. The initial search yielded 7760 articles (including eight identified through reference list search), 5239 duplicates were removed leaving 2512 potentially relevant articles. Following the removal of studies not meeting inclusion criteria based on abstract review (eg, injury and injury risk were not investigated, population or dance form did not match criteria) this was narrowed to 70. Subsequent to further manuscript evaluation by the two independent reviewers (JLW and CAE), 41 were excluded leaving 29 studies deemed appropriate for inclusion to the systematic review. Electronic or hard copies of two potentially relevant articles were not available for review and as they had not met similar inclusion criteria for the previous review, published in 2007, they were excluded.25 ,26 One study27 included in the previous systematic review was excluded as it did not provide an independent estimate of groin injury (eg, combined low back, groin and hamstring injuries), while another study included in the previous review,15 that included abdominal muscle strain in the injury definition, was included as 83% of the reported injuries were related to the adductor muscles. Owing to inconsistent methodology and injury definition as well as the heterogeneity of the risk factors examined meta-analysis was precluded (see online supplementary table S1).
Characteristics of the 29 included studies are summarised in online supplementary table S1. These consisted of 2 intervention studies (1 randomised controlled trial, 1 quasi-experimental), 21 cohort (19 prospective, 1 historical, 1 pilot), 5 case–control and 1 cross-sectional study representing approximately 14 different countries. The median number of participants per study was 219 (range 18–2299) and the combined total number of athletes investigated across studies was 12 131 (9925 males and 2206 females). Twenty-eight of the studies are believed to have included male athletes (11 of these did not specify the sex of their participants however based on the sport investigated it is likely the participants were male) spanning the ages of 12–38 years, while five studies included female athletes (age range 15–41 years). Among the 23 follow-up studies 13 had a follow-up time greater than one season (range 9 weeks—9 seasons), 7 had at least 50 injury cases (range 4–672) and 9 utilised a multivariate statistical approach to identify risk factors for groin injury in sport. Of the 19 studies published since 2007, 1 was a randomised controlled trial, 14 were cohort (12 prospective, 1 historical and 1 pilot) and 4 were case–control. Six of these 19 studies utilised multivariate statistical approaches and 5 had at least 50 injury cases.
Descriptions of injury estimates (incidence proportion, incidence rate, prevalence), effect estimates (IRR, RR, OR) and significant and non-significant groin injury risk factors are presented in online supplementary table S1.
Quality and level of evidence
The highest level of evidence demonstrated by all reviewed studies was level 1b (Individual randomised controlled trial). The majority (21/28) of studies were classified as level 2b which corresponds to cohort studies.
The median methodological quality for all 29 studies, based on the DB criteria, was 11/33 (range 6–20) with an initial moderate between rater agreement of 65.5% (κ=0.62).43 The aim of the DB criteria is to assess scientific study methodological quality (inclusive of randomised and non-randomised intervention as well as observational studies). Owing to the majority of included studies being observational in nature, seven items (4, 8, 14, 19, 23, 24 and 27; totalling 10 points) on the DB checklist were not applicable. Therefore, 27 of the 29 articles did not have the opportunity to achieve a full score due to their study design. Areas in which the included studies were consistently limited included: incomplete description of how the sample was representative of the population of interest (eg, insufficient description of participant characteristics such as sex, history of previous groin injury, training exposure), limited description of the characteristics of those lost to follow-up, use of invalid or unreliable measures, insufficient reporting of how participants lost to follow-up and differing length of follow-up were accounted for in statistical analyses, inadequate sample size and lack of adjustment for potential modification and confounding by factors such as exposure and previous injury. Further, several of the case–control studies that report a matched design did not account for matching in their analyses (eg, independent t tests vs paired t tests).
Synthesis of results
The quantity, quality and level of evidence for the most commonly investigated modifiable and non-modifiable risk factors for groin injury in sport are summarised in table 2. The most common risk factors investigated included age, hip range of motion, hip adductor strength, height and weight. There is level 1 and 2 evidence that previous groin injury, higher level of play, reduced hip adductor strength (isolated and relative to hip abductor strength) and lower levels of sport-specific training are associated with increased risk of groin injury in sport. Further, there is consistent evidence to suggest that older age, higher weight or body mass index (BMI), height, reduced hip range of motion (ROM) and performance on fitness tests such as jump height, leg power (squat), 40 m sprint, sidestepping, kicking and VO2max estimated from a shuttle run are not associated with groin injury in sport.
To our knowledge, this is the first systematic review examining risk factors for groin injury in sport that considers both a formal evaluation of study quality and level of evidence. Overall the quality and level of evidence investigating risk factors for groin injury in sport has improved in the past 7 years since our systematic review in 2007.20 Specifically, there are a greater number of prospective studies with larger sample sizes employing multivariate statistical techniques.
Key findings—risk factors
Consistency across the literature support previous groin injury, higher level of play, reduced hip adductor strength (absolute and relative to the hip abductors) and lower levels of sport-specific training as risk factors for groin injury in sport. To date, many authors have speculated on the mechanisms underlying these risk factors. The general consensus regarding the mechanism by which previous injury is a risk factor is inadequate rehabilitation following the initial injury and/or inherent physiological risk in certain individuals that puts them at greater risk of both the initial and subsequent injuries.2 ,10 ,15 ,31 The risk associated with higher level of play may result from a higher intensity in training and game play as well as a greater number of training hours.32 Decreased levels of hip adductor strength (both absolute and in comparison to the hip abductors) may result in decreased muscle capacity, imbalances between the synergistic functions of hip adductor and abductor muscles, and increased risk of muscle injury during movements involving side-to-side cutting, striding, quick acceleration/deceleration and sudden direction changes.9 ,15 Sport-specific training (specifically, pre-season) may address muscle weakness and imbalance as well as promote function specific recruitment resulting in more effective utilisation and less muscle fatigue.20 Consequently reduced sport-specific training may place an athlete at higher injury risk when faced with an increase in training load as the playing season begins.
Although there have been valuable contributions made to the evidence base related to identifying risk factors for groin injury in sport in the past 7 years the conclusions of this systematic review and that of the previous20 are surprisingly similar. Specifically:
previous groin injury,
reduced relative hip adductor strength and
reduced sport-specific training
were all identified as risk factors for groin injury in sport previously. Previous groin injury, and reduced hip adductor strength have also been identified as risk factors for groin/hip injury in field-based sports in a recent systematic review of seven studies.44
In addition, Ryan et al44 reported that older age, higher BMI and reduced hip abductor ROM are risk factors for groin/hip injury in field-based sport. The discrepancies between these findings and those of the current review are likely due to the limited scope of sports considered and the inclusion of studies investigating both hip and groin injuries in the field-based sport review. Of the 29 studies included in the current review, 12 investigated older age as a risk factor for groin injury in sport (see online supplementary table S1). Of these, all but two studies (including one randomised controlled trial (RCT), eight cohort) found no association between older age (both as a dichotomous and continuous variable) and groin injury in sport (see table 2). Similarly, five of six included studies investigating BMI and six of nine investigating hip ROM found no association between the exposure variables and groin injury.
What can we learn from other injuries?
Looking beyond the groin injury literature, the current findings are relatively consistent with a recent systematic review and meta-analysis (including 34 studies) of risk factors for hamstring injury in sport45 which identified previous hamstring injury, quadriceps peak torque and older age as the exposure variables most consistently associated with hamstring muscle strain-type injury. The discrepancy in findings regarding increasing age as a risk factor for groin and hamstring injury may be related to the relatively narrow age range (mean age ≤25.8 years with SDs ranging between 0.8 and 4.6 years) represented in the 12 studies that have investigated age as a risk factor for groin injury. Further, the conclusion that increasing age is a risk factor for hamstring injury45 is potentially influenced by the findings of one study by Arnason et al2 Accordingly additional consideration of the prospective relationship across between age and injury risk across a wider age span for both muscle groups is recommended.
Meta-analyses were not possible due to inconsistent methodology and heterogeneity of the definition of groin injury in the included studies. Further, despite a comprehensive search strategy and rigorous approach to study selection it is important to acknowledge the possibility of omitting a relevant study and inclusion of only English language manuscripts.
As the conclusions and recommendations contained within this review are based on a synthesis and evaluation of existing literature they are limited by its inadequacies. In several instances (eg, game play, fitness tests) there was a lack of consistent high-quality evidence to support nominating a particular exposure variable as a risk factor due to inadequate reporting of concepts essential to establishing internal and external validity. The biggest threats to internal validity were related to the possibility of selection bias and potential confounding. Specifically, due to the lack of reporting of participant characteristics it was often difficult to determine if the athletes selected for a study differed systematically from those in the source population (selection bias). Equally important was the consistent omission of the characteristics of those lost to follow-up, which made it impossible to determine if those lost to follow-up were systematically different from those retained in the study. The inability to assess for selection bias not only questions the internal validity of several studies, it impacts the degree to which the findings of these studies can be generalised to the larger athletic population from which the sample was drawn (external validity).
As stated earlier, it is highly unlikely that a groin injury is a result of a single risk factor, but rather the consequence of complex interactions between multiple risk factors and inciting events.18 Multivariate biostatistical techniques can be used to explore these complex interactions given an adequate sample size. Bahr and Holme19 estimated that 50 injury cases are needed to detect a moderate to strong association between a risk factor and sport injury. Of the 29 studies included in this review only nine employed these techniques, of which only three had 50 or more injury cases and were able to assess these interactions.9 ,15 ,31 As a result, the association between the potential risk factor and groin injury reported in the studies that did not employ these techniques may be biased as they failed to consider any potential confounder (eg,. extraneous variables that may have distorted the relationship between the exposure variable and groin injury).
The last point of consideration is that studies to date may not have considered all possible risk factors for groin injury in sport. For example, a recent systematic review and position statement released by the American Medical Society for Sports Medicine highlights that although there is a lack of clinical data a high ratio of workload-to-recovery time may lead to overuse injuries and burnout in youth sport.46 To the best of our knowledge the relationship between measures of over training or physiological fatigue and groin injury and sport have yet to be investigated.
Both prospective cohort and intervention study designs are important for identifying potential risk factors for injury in sport.19 While prospective cohort studies are critical for establishing temporality between a risk factor and subsequent injury, RCTs provide the strongest evidence for the causal nature of a risk factor (eg, hip abductor and adductor strength, decreased levels of sport-specific training) and the effectiveness of modifying that factor on injury outcomes. Based on the additional prospective studies undertaken in the past 7 years (involving larger samples and employing multivariate statistical techniques), consistency of the finding of the current review with those of the previous review20 and the challenges and high cost of undertaking high quality prospective cohort studies, it is recommended that investigators shift their focus from prospective cohort to high quality RCTs. Specifically, future research should include RCTs that target athletes at greater risk of groin injury during sport (eg, high levels of play, previous injury) with prevention programmes that include interventions targeting the hip abductor and adductor muscles in conjunction with off and preseason sport specific training.
To date two separate intervention studies aimed at addressing modifiable risk factors (eg, dynamic balance, muscle strength and agility), for sport-related lower extremity and groin injuries have been undertaken.32 ,47 Engebretsen et al47 investigated the effectiveness of an injury prevention programme on high-risk (eg, previous injury and/or reduced function) soccer players, while Holmich et al32 selected a cluster (soccer team) design to facilitate implementation. Unfortunately both studies lacked sufficient statistical power to demonstrate a significant effect of the proposed intervention on the occurrence of sport-related groin injuries and Engebretsen et al47 report that player compliance to the training programmes was poor with only 19.4% of the groin injury high-risk group carrying out the minimum recommended training volume. Other reasons for null findings in these studies may be that the intervention did not sufficiently address the risk factors present. For example, there is a body of evidence suggesting that persistence of neuromuscular changes post-injury may have detrimental long-term consequences that contribute to re-injury through increased joint load, decreased movement, and decreased loading variability.48–53 Consequently, prevention programmes focused on purely building strength without restoring coordinated motor control (eg, eliminating protective cocontraction) may not prove as effective. Regardless of the lack of effect detected in these two landmark intervention studies, valuable lessons can be learned from both, the least of which is the importance of developing an implementation strategy and then tracking and accounting for adherence to the prevention programmes in the analysis.
The quality of studies investigating risk factors for groin injury has improved in the past 7 years.
There is relatively consistent level 1 and 2 evidence to suggest that previous groin injury, higher level of play, reduced hip abductor and adductor strength and lower levels of sport-specific training are associated with increased risk of groin injury in sport.
Further, there is consistent level 2 evidence suggesting that higher weight, BMI or height, reduced hip ROM and performance on fitness test such as jump height, leg power (squat), 40 metre sprint, sidestepping, kicking and VO2max estimated from a shuttle run are not associated with groin injury in sport.
Based on the work performed in the field in the past 7 years and the challenges and high cost of undertaking high-quality prospective cohort studies aimed at identifying risk factors for groin injury in sport it is recommended that investigators turn their focus to high-quality randomised controlled trials targeting athletes at greater risk of injury (those at a high level of play with a previous injury) with prevention programmes targeting the hip abductor and adductor muscles in conjunction with off and preseason sport-specific training.
What are the new findings
There has been an improvement in the quality (eg, larger sample size, employing multivariate statistical techniques) and level of evidence of studies investigating risk factors for groin injury in sport in the past 7 years.
There is level 1 and 2 evidence that previous groin injury, higher level of play, reduced hip adductor strength and lower levels of sport-specific training are associated with an increased risk of groin injury in sport.
Based on the work performed in the field, it is recommended that investigators turn their focus to high-quality randomised controlled trials targeting athletes at greater risk of groin injury in sport with prevention programmes targeting the hip abductor and adductor muscles in conjunction with off and preseason sport-specific training.
The authors would like to acknowledge the assistance of the University of Calgary, Faculty of Kinesiology librarian Alex Hayden as well as research assistants Lisa Loos, Leticia Janzen and Rhys Johnson.
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Contributors JLW and CAE were responsible for the conception and design of the study. JLW and CAE independently reviewed the literature. JLW extracted data from the included studies, while all four authors were involved in rating the literature. JLW was the primary author in preparing the manuscript however all authors contributed to the interpretation of the findings, critical revision of the manuscript and reviewed the document prior to submission.
Funding The Sport Injury Prevention Research Centres is supported by an International Olympic Committee Research Centre Award. JLW is funded through an Alberta Innovates Health Solutions Postdoctoral Clinician Fellowship. CAE holds a Professorship in Pediatric Rehabilitation Alberta Children's Hospital Foundation.
Competing interests None.
Provenance and peer review This paper was commissioned by the 1st World Conference on Groin Pain in Athletes, Doha, Qatar, November 2014; externally peer reviewed.