Adductor:Abductor Strength Ratio and Groin Injury
A systematic review of the risk factors for groin and hip injuries in field-based sport athletes (Ryan, DeBurca and Mc Creesh, 2014) published in the British Journal of Sports Medicine, found the leading risk factors for groin/hip injury to include a history of groin/hip injury, older age, adductor strength and adductor:abductor strength ratio. With the two most prominent risk factors (a history of groin/hip injury, and older age) being non-modifiable, the focus of the athlete, Physiotherapist and Strength and Conditioning Coach alike should turn to the modifiable risk factors of adductor strength and adductor:abductor strength ratio. The research on the relationship between lower adductor strength and an increased risk of groin injury is well documented. Read more on that here. However, research on the association between adductor:abductor strength ratio and groin injury risk is much less documented. A potential reason for this may be the feasibility of assessing large cohorts in a sporting environment, as the testing procedure would typically be much longer than a more commonly used adductor squeeze test. Another reason may be the availability of accurate and cost-effective testing equipment to assess abductor strength in various positions of hip flexion.
Adductor:Abductor Strength Ratios
It has been reported that an adductor:abductor ratio of 1:1 and adductor strength equal to that of the un-injured limb are acceptable clinical milestones for return to sport after a groin injury. However, with greater adductor:abductor ratios regularly reported in the literature, a ratio of 1:1 should be seen as the minimal acceptable strength ratio with which an athlete could return to sport. Normative data for adductor:abductor ratios in elite-level athletes have been reported across a range of field sports. A 2015 study by (Prendergast et al.) reported hip adductor:abductor ratios of approximately 1.13, 1.07 and 1.03 for elite, sub-elite and amateur Australian football players respectively. Not surprisingly, the same study reported higher adductor strength and higher abductor strength values in elite players to those seen in sub-elite and amateur players. A similar study (Lonie et al. 2020), which also assessed elite Australian football players, found that adductor:abductor strength ratios differ across the season, with increased values seen in mid-season compared to those seen at the start of pre-season. This study reported adductor:abductor values of 1.11 and 1.08 (for dominant leg and non-dominant leg respectively) at the start of pre-season, 1.09/1.12 at the end of pre-season, 1.11/1.21 at mid-season, and 1.01/1.11 at the end of the season. These findings provide valuable insight into how adductor:abductor strength ratios can fluctuate throughout the season, likely in response to strength training, training load and game load. Furthermore, these alterations should be considered if you are using an athletes pre-season adductor:abductor strength scores as a clinical marker to either return them to sport after a groin injury, or as an indicator of readiness to train.
Similar adductor:abductor ratios have been reported in elite soccer players. (Thorborg et al. 2011) assessed 100 male soccer players and found adductor:abductor strength ratios of 1.04 and 1.06 for dominant and non-dominant limbs respectively. The authors noted that the difference between the dominant and non-dominant limbs was within the measurement variation of the test procedure, and so concluded that there was no statistically significant difference between adductor:abductor strength ratios between dominant and non-dominant legs.
In terms of prospective studies investigating the association between adductor:abductor strength ratios and incidence of groin injury, the literature is a little scarce! In addition, most of the studies on this topic are quite old, with only a handful of studies on field-based-sport athletes fitting the criteria. One such study (O'Connor, 2004) on professional rugby league players, vaguely links adductor:abductor strength ratios to a higher incidence of groin injuries over a two-year period. However, the author did not show that adductor:abductor strength ratio alone was associated with a higher risk of groin injury, and so, very little significance should be drawn from this study.
Two other prospective studies of note are those of (Tyler et al. 2001) and (Tyler et al. 2002). Both assessed the adductor:abductor strength ratios of professional ice hockey players and monitored the incidence of groin injury over a two year period. (Tyler et al. 2001) found an adductor:abductor strength ratio of 0.95 in players who did not sustain a groin injury in the subsequent two seasons, compared to 0.78 in the players who did sustain a groin injury in the subsequent two seasons. These adductor:abductor strength ratios are much lower to those reported in soccer, rugby league and Australian football players, which is likely due to the playing surface of ice hockey in comparison to field-based sports. This study also reported that adductor:abductor strength ratio was the best predictor of future groin injury in ice hockey players, when compared with other hip strength and flexibility markers. In fact, these findings reported that a player with a pre-season adductor:abductor strength ratio lower than 0.8:1 was 17 times more likely to sustain an adductor injury in the following competitive season. The 2002 study by the same authors assessed the effectiveness of a pre-season exercise programme to prevent adductor muscle strains in professional ice hockey players. Their results found that players with an adductor:abductor strength ratio of less than 0.80, lowered their incidence of adductor muscle strains by following a pre-season hip-strengthening exercise programme. Of course, it should be noted again that these finding are only relevant to ice hockey players, and similar studies on field-based sport athletes are needed.
When we analyse the body of research on adductor:abductor strength ratios (be it normative data or prospective studies), one of the constant variables across studies is the type of testing equipment used to assess the athletes. With various types of testing equipment being used, the difficulty in comparing scores from different testing protocols across different sports increases. For example, a recent study (Welsh et al. 2020) assessing the influence of various hip joint angles on adductor:abductor strength ratios in ice hockey players, measured forces via a uniaxial load cell. In contrast, (O, Connor, 2004) used a Cybex machine (isokinetic dynamometry) whilst testing adductor:abductor strength with their rugby league cohort. Numerous other studies used a hand-held dynamometer to assess strength ratios (Tyler et al. 2001) (Thorborg et al. 2011) (Prendergast et al. 2015) (Wollin et al. 2018). Whilst hand-held dynamometry has been shown as a reliable method of testing hip strength, other research has shown it to be susceptible to between tester bias, with the strength and experience of the tester influencing test scores (Kemp et al. 2013). Isokinetic dynamometry on the other hand, eliminates the influence of a tester bias. However it requires considerable expertise to operate, and it isn't portable, which limits how useful it can be in a team sport environment where large numbers need to be tested regularly.
Recently, a novel and portable testing device, which can assess both hip adduction and hip abduction, has been developed - the GroinBar, Vald Performance, Australia. The GroinBar allows for the testing of numerous positions of hip and knee flexion, does not require tester expertise, and has been shown to have excellent test re-test reliability for adductor strength testing (Ryan et al. 2018).
A number of studies have been conducted in the past two years using the GroinBar to measure adductor and abductor isometric strength ratios. (O'Brien et al. 2019) collected normative values for both soccer players and Australian football players. Their findings show slightly lower ratios than those seen in previous studies. In the Australian football cohort (36 players), adductor:abductor ratios of 0.9 and 0.9 were found for dominant and non-dominant limbs respectively, in the short-lever, 45-degree hip flexion position. Meanwhile, ratios of 1.1 and 1.2 were found for the long-lever, 0-degree hip flexion position (tested at the ankle), in the same group. With regard to the soccer cohort (31 players) in this study, adductor:abductor strength ratios of 0.9 and 1.0 were found in the short lever, 45-degree hip flexion position, whereas ratios of 1.1 and 1.1 were found in the long-lever, 0-degree hip flexion position. Reference values for ice hockey players are also available. (Oliveras et al. 2020) collected normative adductor:abductor strength values using the GroinBar, in 187 professional ice hockey players. Their study reported adductor:abductor strength ratios of 1.07 (goalkeepers), 1.05 (defenders) and 1.02 (forwards), as tested in the 45-degree hip flexion position. This study did not specify between dominant and non-dominant leg - presumably due to the nature of ice-hockey.
Whilst these studies provide valuable normative data across three sports, a prospective study associating the adductor:abductor strength ratio with rate or risk of groin injury is typically much more informative from an injury prevention perspective. Fortunately, a prospective study (Bourne et al. 2020), has recently been published, which used the GroinBar to assess adductor:abductor strength ratios in professional soccer players. The authors tested 204 soccer players and obtained a complete prospective follow-up with 152 of these. Two test positions were used - long-lever 0-degree hip flexion tested at the ankle, and short-lever 60-degree hip flexion tested at the knee. Of the 152 follow-up athletes, 24 of them sustained a groin injury over the course of the playing season. This injured group had pre-season adductor:abductor strength ratios of 1.03 (dominant) and 0.99 (non-dominant) in the short-lever position, and 1.15 (dominant) and 1.10 (non-dominant) in the long-lever position. In comparison, the players who did not go on to sustain a groin injury, had pre-season adductor:abductor ratios of 0.99 (dominant) and 1.02 (non-dominant) in the short-lever position, and 1.14 (dominant) and 1.15 (non-dominant) in the long-lever position. While the strength ratios seen here between the injured and un-injured groups are similar, both the adductor and abductor scores produced by both groups were markedly different. The un-injured group produced higher scores on every single test, leading the authors to conclude that greater adductor and abductor strength is associated with a reduced risk of groin injury.
Overall, more research is needed to assess the association between adductor:abductor strength ratios and incidence of groin injury. Standardised, repeatable testing protocols should be used and test equipment should be constant across studies. Future studies should look to use the GroinBar, and should test in numerous positions of hip flexion, replicating the test positions commonly used in adductor strength studies. Normative data for other sports and at various levels should also be collected, as reference values play a vital role in rehabilitation and return to play protocols.