The hip adductor muscles play an important role in movement and stability at the hip. Injury to this muscle group at times has been classified as a groin strain and reported in a number of activities such as ice hockey (5), soccer (19), Australian football (14), and swimming (7). The identification of muscle properties that are associated with an increased injury risk is paramount in the development of conditioning programs that aim to reduce injury incidence. The objective of this review was to examine the relationship between hip adductors' strength, flexibility, and injury risk. The review was based on articles identified by computerized searches using MEDLINE (January 1966 to September 2007) and SPORTDISCUS (January 1966 to September 2007) databases. The following search terms were used in various combinations: hip, adductors, strength, flexibility, groin, and injury. Other studies were identified from the reference list of the articles located by the electronic searches. Articles were included if they reported measurements of hip adductor strength or flexibility and prospectively monitored adductor injuries in sport.
The hip adductor muscles are located on the anteromedial aspect of the thigh and comprise the following muscles: gracilis, pectineus, adductor magnus, adductor longus, and adductor brevis. Quadratus femoris also produces adduction at the hip (15). Activation of the hip adductors occurs in more movements than just hip adduction. This muscle group has the secondary joint actions of hip rotation (10) and flexion or extension depending on initial joint position (11). In addition to providing movement, the hip adductors may generate significant tension while stabilizing the hip and controlling the alignment of the lower limb.
The proposed mechanism for adductor muscle strains is the overstretching and eccentric force of the adductors attempting to decelerate the limb during rapid abduction and external rotation as occurs in ice-skating or sudden change in direction (12,13,17). Based on this proposed mechanism, muscle strength or flexibility may be related to injury risk.
Flexibility and Injury Risk
Eight studies were identified that investigated the relationship between flexibility and injury of the hip adductors (Table 1). The flexibility of the hip adductors has mostly been assessed by using a goniometer to measure the maximal range of hip abduction. Hip rotation has also been used as an indication of the flexibility of the hip adductors. The flexibility testing may be active where the subject moves his/her own limb or passive where the tester moves the limb. The hip motion during the test may be unilateral or bilateral. It has been found that low hip adductor flexibility was associated with an increased risk of injury in 3 of the 4 soccer studies (2,3,8,18) but not in ice hockey (4,12,17) or rugby league (13). The earliest study (12), which involves high school and college ice hockey players, was limited because of the small sample size, provided minimal details of the flexibility test, and did not report statistical analysis of the flexibility results. Another study with ice hockey player (17) also utilized a small sample size and thus would have had reduced statistical power to determine a significant relationship. There has been a larger scale study of 1,292 professional ice hockey players (4), but no significant association was detected between flexibility and injury. This study was the only one to incorporate an active flexibility test, and it is unclear as to whether a passive test would have yielded a significant relationship. In addition, this study monitored groin and abdominal strains as a group and did not report isolated hip adductor injury. A confounding issue is the definition of the injury; some studies have specific hip adductor injuries, whereas others have grouped adductor injuries along with other injuries within the “groin strain” category.
The investigation of professional rugby league players (13) involved the longest duration and measured hip abduction as well as hip rotation flexibility but did not find a significant association with injury. The studies involving the soccer players had the largest sample sizes, apart from the 1 ice hockey study. The first soccer study (3) did not report the level of competition but the other 3 soccer studies (2,8,18) used elite athletes. Most of the soccer studies (3,8,18) reported the injury as a specific adductor injury, not groin injury. It can only be speculated why low flexibility is more likely to be a risk factor for adductor injury in soccer compared with ice hockey or rugby league. Possible factors may include body type, movement patterns, and speed of movement.
Strength and Injury Risk
Four of the 8 studies that assessed hip abduction flexibility and injury risk also measured hip adduction strength (Table 1). Isokinetic dynamometry has been considered to be the gold standard for assessing dynamic muscle strength. There are a number of considerations when using an isokinetic device for measuring hip adduction or abduction: the position of the subject during testing (lying vs. standing), type of muscle action (concentric, eccentric, isometric), isolated vs. continuous test repetitions, and hip position (neutral vs. rotated). The strength values may be reported as bilateral limb differences or as adduction to abduction strength ratios. In addition to strength, muscular endurance may be evaluated, e.g., total work for 20 continuous repetitions. The disadvantages of isokinetic dynamometry include the high cost and lack of portability.
Another option to measure strength is the use of a hand-held dynamometer (HHD). These are portable load cell devices used to measure force. The tester holds the device in his/her hand and presses it against the limb of the subject as he/she exerts a maximal effort. A “make test” is when the subject exerts a maximal isometric force against the HHD. A “break test” is when the tester applies the HHD to subject's limb and exerts enough force to overcome the subject's isometric position. The HHD displays a force value, and this should be expressed as a torque value by determining the moment arm length for the subject and multiplying it by the force value. Testing hip adduction or abduction may involve placement of the HHD near the knee (short lever) or near the ankle (long lever). As with isokinetic testing, the values for HHD testing may be expressed as right-left limb differences or joint action differences.
Three of the 4 studies evaluating strength and injury risk involved ice hockey players (4,12,17). The earliest study (12) used an isokinetic dynamometer and found that a bilateral difference in concentric adductor peak force and power of 25% or more was significantly related to adductor strain injury. The other 2 ice hockey studies (4,17) used HHD and produced conflicting results. The study (17) that found a significant relationship between strength and injury employed a break test with long lever, whereas the other study (4) that did not detect a significant association used a make test with short lever. The variation in methodology may partially explain the different findings. The break test may be considered the detection of the force at the changeover point from isometric to the beginning of the eccentric muscle action. This test may be more appealing than the make test because the eccentric component may be a capability related to muscle strain injury risk (6). It has recently been reported that the long-lever position produced greater torque and reliability values than the short-lever position (9). The most recent investigation between hip adduction strength and injury involved rugby league players, used isokinetic dynamometry (13), and generated a multivariate predictive model that included a number of variables.
Because hip adductor strength imbalances were found to be associated with an increased risk of hip adductor strain injury in 2 of the 3 ice hockey studies (12,17), an intervention study was undertaken to investigate the influence of increasing adductor strength on subsequent injury risk. Tyler et al. (16) assessed the adduction to abduction strength ratio of 58 professional ice hockey players using a HHD break test with long lever. Thirty-three players who had strength ratios less than 80% were considered at risk and undertook an intervention program. It involved an adductor strengthening program for 6 weeks preseason, 3 times per week with the goal of attaining an adduction to abduction strength ratio greater than 80%. The program comprised various exercises: ball squeezes, resisted hip adduction, seated adduction machine, sliding board, unilateral lunges, and ice-skating-specific adductor drills. Historic injury data from the preceding 2 seasons were used as the control. For the 2 seasons after the intervention, there was a 4.5-fold reduction in adductor strain incidence. It was concluded that addressing adductor strength deficits reduced injury risk. Although this study was not a randomized controlled trial with a large sample size, it does provide some evidence that the detection and correction of strength weakness may reduce injury risk. These findings need to be duplicated in larger studies and other sports for the results to be universally accepted. It is worth noting that eccentric conditioning may be important in preventing muscle strain injuries (1).
There is some low- to moderate-level evidence from cohort studies to show that low hip adductor flexibility or strength is associated with an increased risk for muscle strain injuries in certain activities. Strengthening the hip adductor muscles has assisted in reducing the injury risk. Although more research is required to support this finding, it may be advisable to include hip adductor exercises in the training programs of athletes who are predisposed to hip adductor injuries.
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