Non-modifiable risk factors

The most prominent risk factor for groin/hip injuries identified in this review, and across the literature, was player history of a previous injury.2 ,3 ,5 ,21 ,22 ,26 The largest cohort study to date of male soccer players found previously injured players were at seven times greater risk of sustaining new groin injury than their non-injured counterparts.3 ARF players have six times greater risk of sustaining a groin/hip injury if they had a history of a previous groin/hip injury than those without.5 In ARF 32% of adductor strain injuries were reported as recurrent injury27 while this figure was as high as 44% in the ice hockey28 population. Causative factors for groin/hip injuries in FBS population may include both intrinsic (ie, muscle strength21) and extrinsic (field conditions1) risk factors. There may also be clinical reasons for player's history of previous injury making them more at risk of another groin/hip injury. These include: remaining deficits in physical conditioning, scar tissue formation, inadequate rehabilitation, reduced proprioception, altered movement patterns or premature early return to play after the initial injury and the number of times the player has sustained this type of injury.22 ,29 ,30 Previous history of groin/hip injury has not yet been identified as a risk factor in an RU population study, nor has the OR risk been calculated for those with such a risk factor.

The second most prominent, non-modifiable risk factor was older age.3 ,12 As the players age their body's collagen tissue changes in nature becoming less elastic and less able to absorb forces.31 ,32 This then results in the tissues being less adaptable to respond to quick force changes (characteristic of the biomechanics involved in FBS) or recover from fatigue.31 ,32 This hypothesis was supported by a study by Johnson et al33 which stated that abduction to adduction torque significantly decreases with advancing age. The physiological evidence for increasing age as a risk factor for injuries cannot be disputed.31–33 Of the seven studies examined in this systematic review two3 ,12 support the argument that increasing age was a risk factor for players in sustaining a groin/hip injury. Four studies5 ,21 ,23 ,24 did not examine increasing age as a risk factor for groin/hip injury. One study22 did not support the argument, as significant results were not found in their study. The authors22 did acknowledge that analytical methods differed to the study by Arnason et al3 which may explain the discrepancy in results.

An atypical groin/hip injury risk factor identified in this systematic review was that early maturing football players have a greater probability of sustaining a groin/hip injury.23 This risk factor was identified in only one study23 which examined chronological and skeletal age in relation to injury incidence and severity in soccer over a 10-season study period. This finding was also supported by the work of Backous et al34 and Linder et al35 These studies found that a higher rate of sports injury was established in the more mature soccer players, who were also found to be muscularly weak.34 ,35 Furthermore there was a direct correlation observed between advanced sexual maturity and injury incidence in ARF players.23 The hypothesis behind this risk factor was that, although players may be chronologically the same age as their playing counterparts, skeletally they were ‘older’ and thus at increased risk of a groin/hip injury, as discussed previously.

The final non-modifiable injury risk factor identified was that players with a smaller dominant femur diameter were at increased risk of injury.12 The author speculated that a smaller dominant femur diameter may alter the origin and insertion point of the adductor longus, which may alter the muscle efficiency. Furthermore this anatomical change may alter the muscle's ability to generate force and cope with stress.12 This explanation was currently a hypothesis and warrants further investigation for confirmation.

Modifiable risk factors

The most prominent modifiable risk factors were BM and WHAM (table 5), both identified in 2/7 studies.

WHAM was identified as a risk factor for groin/hip injuries in FBS in 2/7 studies in this review.12 ,21 Engebretsen et al21 stated that soccer players with WHAM have a four times greater risk of sustaining a groin injury compared to those without weakened musculature. This was supported by O'Connor12 whose study demonstrated that non-injured players had higher peak torques in hip adduction compared to those that sustained an injury. While the evidence for WHAM as an injury risk factor in FBS is limited it is well established within ice hockey literature. Emery26 stated that reduced muscle strength was a risk factor for acute muscle strains in ice hockey players. Tyler et al28 evaluated hip adductor strength in 47 ice hockey players and found that muscle strength was 18% lower in the players who sustained an adductor injury. The authors hypothesised that the cause of injury was related to eccentric loading of the adductors when decelerating during strid.28 While FBS place differing demands on players, they share high levels of acceleration and deceleration to ice hockey, suggesting a similar need for balanced hip muscle strength to resist injury.

Within the literature there was conflicting evidence regarding increased or decreased BM as a risk factor for groin/hip injuries in FBS players. O'Connor12 argued that a higher BM in conjunction with smaller dominant femur diameter places Rugby League players at a statistically significant risk for sustaining a groin injury. In contrast Verrall et al24 stated that ARF players with a lower BM have a greater risk of injury than their team-mates. Examining these studies individually highlights some methodological differences. First, there were 100 participants in O'Connor's study, which was a good representation of the population group. The results demonstrate that those with lower BM remain uninjured (p<0.05) and those of higher BM are at greater risk of injury (p=0.058). Increased BM was also supported as a risk factor for injuries in RU players in the New Zealand rugby injury and performance project.36 In comparison to O'Connor,12 only 29 ARF players participated in the study by Verrall et al.24 This sample size was a low representation of this population and lowers the external validity of the study. Additionally, authors based their results and analysis on only four players developing a chronic groin injury as result of low BM. The small sample size may reduce our confidence in the findings of this study.24 Authors express that increased risk occurs in athletes who were lighter as they were unable to meet the extensive aerobic demands of ARF.24 However they stated it was difficult to account for the finding of decreased BM being associated with development of chronic groin injury.24 Thus, on the weight of the current literature,12 ,24 the more robust argument was that increased BM places FBS players at risk of a groin/hip injury during the playing season.

Hip abduction ROM and total hip rotation ROM have both been identified in individual studies as a significant risk factor for groin/hip injury in soccer and ARF players, respectively.3 ,24 Arnason et al3 found that players with reduced hip abduction ROM had a significant risk of sustaining a groin injury. These results were supported by earlier work of Ekstrand and Gillquist37 who demonstrated that soccer players, who had less hip abduction than the control group, were more at risk of a groin injury. The biomechanics and intensity of the sport place a high demand on the hip musculature and could result in muscle tightness and injury.5 Gabbe et al5 stated that synergistic and opposing muscle length or ROM may cause strain on adductors leading to injury. Decreased abduction ROM was supported as a groin/hip injury risk factor in soccer with further studies warranted to confirmation it as a risk factor across FBS.

Total hip rotation (internal and external) ROM has been identified as a risk factor for ARF players in sustaining a groin/hip injury throughout playing season. Authors stated that players with lower total hip rotation ROM at the start of the season were more likely to subsequently develop a chronic groin injury.24 It has been highlighted already that this study had a small population size and subsequently the results may not be completely reliable. These results were somewhat supported by Verrall et al38 who propose that players activity in ARF could lead to a capsular response, with subsequent restriction of hip ROM, biomechanically disadvantaging the athlete and making them more vulnerable to injury. To conclude that decreased total hip ROM was a significant risk factor for groin/hip injuries across FBS, more high-quality studies were required.

The final intrinsic modifiable variable identified was the ratio of strength forces of muscles (more specifically hip adductors vs abductors) acting on and around the hip joint.12 O'Connor12 found peak torque differences in non-dominant limb adduction and abduction with rotation and bilateral difference in extension peak torque between injured and non-injured rugby league players. These results became part of the author's variable list in determining whether players were predisposed to groin/hip injury. Merrifield and Cowan39 support the argument that strength ratio differences acting in and around the hip joint increases a players risk of injury. Authors stated that a strength imbalance of greater than 25% was identified in players prone to adductor strains. The importance of these strength balances has been a long-standing theory, with research published in 1987 highlighting that the failure of these stabilising and propulsive muscles was resultant in adductor strain injuries.40 This theory is also supported by a study which identified that once high-performance athletes sustained an adductor muscle strain, the player may be at increased risk of rectus strain or athletic pubalgia due to an imbalance of forces across the anterior hip musculature.41 Furthermore, an investigation of the ice hockey population found that adduction strength was 95% of abduction strength in the uninjured players versus injured athletes adduction strength was only 78% of abduction strength.28 Authors concluded that those hockey players with an adductor-to-abductor strength ratio of less than 80% were 17 times more likely to sustain a groin injury.28 Although these figures were in reference to ice hockey players (the biomechanics of which are different to FBS) this information has transferability across populations with regard to injury prevention. As strength ratio differences was only identified as part of a list of groin/hip injury risk factors in one FBS study,12 confirmation is needed as to whether it is a stand-alone groin/hip injury risk factor for players participating in FBS.

Limitations of this review

Despite the excellent quality of the literature examined (table 2) several limitations must be identified. First, due to the heterogeneity of the studies included (especially the methodology employed) it was not possible to perform statistical or meta-analysis of the results. The different definitions of injury, risk factors examined and how they were examined across the studies resulted in risk factors being pooled and discussed together.

A second limitation was that any studies outside the English language were excluded from this systematic review, which may have limited the risk factors identified. Third, all the included studies were lacking clearly defined inclusion/exclusion criteria. Although the population in each study was clearly defined using playing experience and anthropometric data, the absence of inclusion and exclusion criteria inhibits study reproducibility. While all the studies provided height and weight means of the investigated population group, none examined BM index in terms of injury occurrence. This extra variable may have been identified as a significant as a risk factor for groin/hip injuries in FBS players.

Three of the seven studies analysed did not have a control group.5 ,22 ,23 This was not a requirement for quality assessment as authors felt studies investigating injury risk factors (based on variables such as injury incidence and duration) should not require comparison to a control group. For example; Le Gall et al23 examined the effects of early versus late maturers and their risk of injury. A control group for such a study was not warranted unless the authors wanted to compare the injury incidence to another soccer group or FBS. A final methodological limitation was that, although studies examined in this review followed players through the season with regard injury status and training/match exposure, they have only tested player's objective outcome measures once during the playing season.3 ,12 ,21 It would be incorrect to assume that player's strength, ROM, flexibility and so forth do not change as injury occurrence, fatigue and extrinsic risk factors ensue during the playing season.

Additionally it should be acknowledged that the CASP quality appraisal tool is limited in its specificity in scoring studies. For example, it does not give account for specific methodological factors such as sample size therefore studies may get an inflated quality score. Lastly, a notable absence in this systematic review was the investigation of external/extrinsic factors, such as weather conditions, playing and training surfaces, lengths of training and boot styles on FBS player's risk of injury. A previous review also observed this fact and suggested that future studies investigating extrinsic risk factors were essential.2