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Hip flexibility and strength measures: reliability and association with athletic groin pain
  1. P Malliaras1,
  2. A Hogan2,
  3. A Nawrocki3,
  4. K Crossley4,
  5. A Schache5
  1. 1
    Brunel University, Uxbridge, UK
  2. 2
    SPORTSMED SA, Stepney, South Australia, Australia
  3. 3
    Alphington Sports Medicine Clinic, Melbourne, Victoria, Australia
  4. 4
    Department of Mechanical Engineering and School of Physiotherapy, University of Melbourne, Victoria, Australia
  5. 5
    Department of Mechanical Engineering, University of Melbourne, Victoria, Australia
  1. Correspondence to Anthony Schache, Senior Research Fellow, School of Mechanical Engineering, University of Melbourne, Melbourne, Australia 3010; anthonys{at}unimelb.edu.au

Abstract

Objective: Groin pain commonly affects football players and can be associated with prolonged recovery periods. Understanding the relationship between groin pain and reliable measures of hip flexibility and strength may facilitate the development of optimal rehabilitation and prevention strategies. In this study, the reliability and association with athletic groin pain of hip flexibility and strength measures were investigated.

Methods: A cohort of 29 football players (15–21 years) participating in junior elite competitions (Australian Rules football and soccer) were recruited. The intra-rater reliability (n  =  13) and inter-rater reliability (n  =  12) of various hip flexibility (bent knee fall out test, hip internal rotation, hip external rotation) and strength (hip abduction, hip internal rotation, hip external rotation, hip adduction (squeeze test)) measures were investigated using intraclass correlation coefficients (ICC). Reliable hip flexibility and strength measures were compared between football players with (n  =  10) and without (n  =  19) groin pain.

Results: The bent knee fall out test, hip internal rotation flexibility and the squeeze test demonstrated acceptable (ICC>0.75) intra-rater and inter-rater reliability, while hip external rotation flexibility and hip abduction strength demonstrated acceptable intra-rater but not inter-rater reliability. Hip internal and external rotation strength tests were not found to be reliable. Football players with groin pain had significantly reduced force production on the squeeze test (p>0.05).

Conclusion: Several hip flexibility and strength measures were found to be reliable. Only the squeeze test discriminated between football players with and without groin pain.

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Groin pain commonly affects athletes participating in football codes that require a combination of running, rapid agility movements and repetitive kicking.1 2 3 It has been reported that groin pain may account for up to 13% of all football injuries.1 2 4 One of the main problems encountered with groin pain is that it is often associated with longstanding symptoms, limiting function and performance for months or even years.5 Groin pain represents a significant clinical challenge due to the difficulty experienced in identifying the injured structure(s), which is due in part to the complex anatomy of the region. To assist in classifying groin pain and guide management, Holmich et al6 proposed three groin pain “clinical entities” (adductor-, iliopsoas- and rectus abdominus-related). These clinical entities were developed on the basis of a series of reliable clinical measures that examined the reproduction of groin pain via palpation, passive stretch and active muscle contraction.7

In addition to the clinical measures evaluated by Holmich et al,7 there are other measures frequently used to quantify hip flexibility (eg, hip axial rotation, combined hip flexion–abduction–external rotation) and strength (eg, adductors, abductor, internal/external rotators) among athletes. Little is known about the reliability and discriminate ability of these measures among football players with and without groin pain. Reduced hip internal and external rotation flexibility has been shown to discriminate between Australian Rules football players with and without longstanding groin pain8 and may indicate an increased risk of developing longstanding groin pain.9 In these studies by Verrall and co-workers,8 9 hip axial rotation flexibility was recorded with the hip flexed to 90°, whereas among football players, rotation closer to neutral hip flexion–extension may better reflect function. Combined hip flexion–abduction–external rotation (bent knee fall out test) is likely to reflect hip and/or adductor muscle flexibility. No study has thus far investigated the relationship between the bent knee fall out test and groin pain.

While hip muscle weakness has been suggested to be a risk factor for groin pain,10 11 and strength exercises have been recommended as a suitable intervention,12 as with hip flexibility, little is actually known about the reliability and discriminative ability of measures that quantify hip strength among athletes with groin pain. Holmich et al7 reported moderate reliability (k = 0.65) in manually assessing the strength (“strong”, “intermediate” or “weak”) of a maximal bilateral hip adduction contraction (static) among primarily young adult male soccer players. Other methods, such as dynamometry, may offer greater sensitivity in measuring hip strength; however, their reliability and discriminative ability among athletes with groin pain is yet to be established. It is clear that further work is required in evaluating the reliability and discriminate validity of hip flexibility and strength measures for examining athletes with groin pain.

The first aim of this study was to identify reliable hip flexibility measures (bent knee fall out, axial rotation) and strength measures (adductors, abductor, internal/external rotators) among football players with and without groin pain. The second aim was to determine if hip flexibility and strength measures, found to be reliable, were capable of discriminating between football players with and without groin pain.

Methods

Participants

Elite junior (15–21 years old) male football players (Australian Rules football and soccer) were invited to participate in this study. Elite junior football players were recruited as they suffer a high incidence of groin pain.3 Football players with groin pain were included if they reported groin pain while running or performing rapid agility movements and had at least one of the following symptoms: (1) groin pain standing on one leg, (2) groin pain or stiffness in the morning, (3) groin pain at night or (4) groin pain when coughing or sneezing. Asymptomatic football players did not have any of these signs and symptoms. One of the investigators (PM) assessed the inclusion and exclusion criteria for each potential participant via direct interview. This study was granted ethics approval by the ethics committee at La Trobe University, Melbourne, Australia and participants provided signed informed consent.

Procedures

Demographic and anthropometric data (age, weight, height, years of football competition, hours of weekly football playing, hours of training (before injury) and dominant kicking leg) were recorded via direct interview. The site of groin pain (pubic, adductor, hip, abdominal) and the side of groin pain (left, right, bilateral) was recorded for symptomatic football players. Clinical signs and symptoms were assessed by one of the authors (PM) and used to determine the primary and, when applicable, secondary clinical entities using the classification system described by Holmich et al.7

One examiner (PM) assessed the intra-rater reliability of hip flexibility and strength measures (subset of 13 football players: 5 soccer and 8 Australian Rules players). Measures were taken after a standardised warm-up (consisting of stretching and stationary cycling) and a practice trial for familiarisation. The examiner undertook an episodic memory distraction task during the 30 min between the first and second measurements. Two raters (PM and AN) assessed the inter-rater reliability of the same measures (subset of 12 Australian Rules football players). The order of testing was randomised for each footballer.

Hip flexibility

Bent knee fall out test

The player was positioned in a crook lying position with their knees flexed to 90° (established using a template) and their feet together. They were instructed to allow their knees to fall outward while keeping their feet together, and the examiner used gentle overpressure to check that the player had relaxed at the limit of movement. The distance between the most distal point on the head of the fibula and the surface of the plinth was measured using an inflexible tape measure (to the nearest 0.5 cm) and recorded (fig 1).

Figure 1

Bent knee fall out test.

Prone hip internal rotation

The player was positioned in a prone position with their knees flexed to 90°. They were instructed to allow both feet to fall outwards (ie, hip internal rotation) while keeping their knees together. A spirit level, placed across the greatest convexity of both gluteus maximus muscles, was used to monitor for unwanted rotation of the pelvis during the hip internal rotation. The examiner used gentle overpressure to check that the player had relaxed at the limit of movement. A fluid-filled gravity inclinometer with a 360° dial (Plurimeter CE, Dr Rippstein, La Conversion, Switzerland) was placed 10 cm proximal to the inferior tip of each lateral malleolus. The angle between the lateral border of the lower leg and the vertical was measured using the inclinometer (to the nearest degree) and recorded (fig 2). In addition, a set-square was used to align the long axis of the lower leg with the vertical axis. The inclinometer was used to measure the angle between the lateral aspect of the lower leg and the vertical axis (to the nearest degree).

Figure 2

Prone hip internal rotation.

Supine hip external rotation

The player was positioned in supine with their non-test hip flexed (and that foot resting on the end of the plinth) and their test leg hanging free over the end of the plinth. The inclinometer was placed 10 cm proximal to the lateral aspect of the lateral malleolus. The examiner passively moved the lower leg into hip external rotation until he perceived the end of range, and/or movement of the pelvis was observed (fig 3). No overpressure was applied. The angle between the lateral aspect of the lower leg and the vertical was measured using the inclinometer (to the nearest degree).

Figure 3

Supine hip external rotation.

Hip strength measures

Hip abduction

Hip abduction strength was assessed in supine with an electronic dynamometer (Lafayette Instrument Company, Lafayette, Indiana, USA) placed 5 cm proximal to the lateral joint line of the knee. The players were instructed to push into abduction as hard as they could for 3 s, with the same degree of verbal encouragement provided for each effort (fig 4). The maximum force displayed on the dynamometer (to the nearest 0.1 kg) was recorded, and the distance between the greater trochanter and the dynamometer (ie, the moment arm) was measured using a flexible tape measure (to the nearest 0.5 cm).

Figure 4

Hip abduction strength.

Hip internal and external rotation strength

Hip internal and external rotation strength was assessed in the same position as that used for the supine hip external rotation test. The dynamometer was placed 10 cm proximal to the medial malleolus to measure external rotation and 10 cm proximal to the lateral malleolus to measure internal rotation. Each player was instructed to push into internal or external rotation as hard as he could for 5 s, with the same degree of verbal encouragement provided for each effort (fig 5). The maximum force displayed on the dynamometer (to the nearest 0.1 kg) was recorded, and the distance between the medial or lateral knee joint line and the dynamometer (ie, the moment arm) was measured using a flexible tape measure (nearest 0.5 cm).

Figure 5

Hip internal and external rotation strength.

Hip adduction strength (squeeze test)

With the player lying in supine, templates were used to position the leg in 0°, 30° and 45° of hip flexion. The squeeze test was quantified using a sphygmomanometer (Welch Allyn Disytest, Skaneateles, New York, USA) that was pre-inflated to 10 mm Hg. The cuff of the sphygmomanometer was placed between the knees, and the player was instructed to squeeze the cuff as hard as he could. The highest pressure displayed on the sphygmomanometer dial (to the nearest 5 mm Hg) and the site and side of any groin pain experienced during the test was recorded (fig 6).

Statistical methods

An average of the two measurements recorded for each test was used in the statistical analyses. The angle between the distal tibia and the long axis of the lower leg was subtracted from the external rotation range and added to the internal rotation range. Torque (τ) was calculated (τ = moment arm (m) × force (kg)) for each dynamometry measure. Group differences in demographic data were investigated using independent t tests (age, height, weight, training, years of football) or the χ2 test (favoured kicking leg, playing position). Intraclass correlation coefficients (ICC) and 95% CIs were calculated to determine intra-rater reliability (model 2,1) and inter-rater reliability (model 3,1).13 Reliability was judged based on established criteria: good reliability ⩾0.75, moderate reliability <0.75 and ⩾0.50, and poor reliability <0.50.15 To express measurement error in the original units of measurement, the SEM and 95% CI were calculated (SEM = SDav(√1−ICC), where SDav = average standard deviation). For hip flexibility and strength measures with acceptable intra-rater reliability (ICC>0.75), group differences were assessed using independent t tests. In addition, total flexibility (sum of left and right measures) for prone hip internal rotation and supine hip external rotation was investigated. The alpha level was set at 0.05 for all analyses. Post hoc power was calculated for non-significant findings.15

Results

Nineteen asymptomatic players and 10 players with groin pain were recruited. The groups were homogeneous with regards to all demographic and anthropometic characteristics (table 1). Eight players with groin pain had an insidious onset of pain, and two players had an acute onset. Five players with groin pain had been showing symptoms for between 4 and 5 weeks, while five players with groin pain had been showing symptoms for 6 weeks or longer (median = 5.5 weeks, IQR = 4–11 weeks). Nine players had symptoms consistent with the adductor-related clinical entity (pubic bone tenderness, pain with resisted adduction) and six of these players had secondary iliopsoas-related symptoms (tenderness lower lateral abdomen and/or distal to inguinal ligament, pain with Thomas test extension stretching).6 One player had symptoms consistent with the abdominal-related clinical entity (tenderness of the distal rectus abdominis tendon or insertion into pubic bone, pain with resisted rectus abdominis contraction).6 Eight players had bilateral symptoms and two had left-sided pain.

Table 1

Demographic data for players in the control and groin pain groups

Reliability of hip flexibility and strength measures

The bent knee fall out test, prone hip internal rotation and the squeeze test demonstrated good intra-rater and inter-rater reliability (table 2). Supine hip external rotation and hip abduction strength demonstrated good intra-rater reliability only (table 2). Measures of hip internal and external rotation strength demonstrated poor to moderate reliability (table 2).

Table 2

Test–retest and inter-rater reliability for each hip flexibility and strength measure

Discriminate validity of reliable hip flexibility and strength measures

Players with groin pain had a significantly lower measure on the squeeze test compared with their asymptomatic counterparts when tested at 0° and 30° of hip flexion (table 3). There was no significant difference evident between the groups for any of the other tests (p>0.05) (table 3).

Table 3

Hip flexibility and strength among players in the control and groin pain groups

Discussion

This study investigated the reliability and discriminant validity of selected hip flexibility and strength measures among junior football players with and without groin pain. While this study investigated a small cohort of symptomatic players (n  =  10), all football players in the study were young (15–21 years) and had a similar training load (table 1). The diagnosis of players with groin pain was not known and included players with symptoms for 4–5 weeks (50%) and ⩾6 weeks (50%). However, 9 of the 10 symptomatic players had adductor-related pain, and 6 of these players also had secondary iliopsoas-related pain.

Reliability

The bent knee fall out test, prone hip internal rotation and squeeze tests demonstrated acceptable intra-rater and inter-rater reliability. The findings for the Prone hip internal rotation test are consistent with previous reliability studies.16 17 To our knowledge, this is the first study to report the reliability of the bent knee fall out test. The squeeze test has previously been shown to be reliable in measuring the presence of pain7 and strength using a handheld dynamometer.18 This study introduces the traditional sphygmomanometer as a cost-effective alternative to the handheld dynamometer.

The supine hip external rotation flexibility and the hip abduction strength tests demonstrated acceptable intra-rater reliability, but not inter-rater reliability. This indicates that these measures are suited to clinical and research situations that rely on repeat measures by the same examiner. Given the difficulty in determining the end range of motion, it is possible that the supine hip external rotation test may be more reliable if performed as an active movement.16 The strength measures of hip IR and hip ER using a handheld dynamometer did not demonstrate acceptable reliability in the current cohort. This may be partly related to the small cohort in this study. Additionally, the hip internal and external rotator muscles were not found to display high torques. Hence, for these two measures, the magnitude of the measurement error was large in relation the size of the measure.

Discriminative validity

An important finding of this study was that football players with groin pain produced significantly less force on the squeeze test (0° and 30° of hip flexion) when compared with asymptomatic players. This result is consistent with the difference (ie, athletes with symptoms compared with asymptomatic athletes with adductor-related groin pain for >4 weeks) evident in the descriptive statistics provided by Mens et al.19 It cannot be determined whether the significantly reduced maximal force production on the squeeze test is because of muscle weakness or pain inhibition (or both). Likewise, the cause or effect relationship between reduced strength on the squeeze test and the presence of longstanding groin pain cannot be determined. A previous study investigating Australian Rules football players reported that assessing the presence or absence of pain on the squeeze test is specific (88%) in identifying longstanding groin pain (tenderness at the pubic symphysis and superior pubic rami for >6 weeks) and therefore may be useful in diagnosis, although the sensitivity of this test was low (40%).20 Taken together with the findings from the current study, the squeeze test would appear to be capable of discriminating between athletes with and without groin pain.

Hip internal and external rotation flexibility and total hip rotation flexibility did not differ among football players with and without groin pain, despite these measures demonstrating acceptable reliability. Williams21 first proposed that restricted hip internal rotation flexibility may result in greater load being placed on the groin region, leading to injury. Recently, Verrall et al8 reported an association between reduced hip internal and external rotation flexibility and longstanding groin pain (with MRI evidence of bone marrow oedema) among elite adult Australian Rules football players. The differences between the results of Verrall et al8 and those of the current study might be explained by different test positions for measuring hip rotation flexibility (eg, greater chance of bony impingement in flexion), different age of the cohorts with groin pain and different groin pain classifications. It must also be noted that post hoc power calculations revealed that the current study was inadequately powered to detect a significant difference for the hip rotation flexibility tests (table 3).

While this study is limited by a cross-sectional design and a small cohort, the results do provide further insight into the characteristics of elite junior football players with groin pain. Prospective studies are required to serially investigate the association between groin pain and reliable measures of hip flexibility and strength. Furthermore, the inter-rater reliability of hip strength and flexibility measures needs to be investigated among players with groin pain, as performance on some measures may show greater variability.

Conclusion

The bent knee fall out, prone hip internal rotation flexibility and the squeeze tests all demonstrated acceptable reliability, while supine hip external rotation flexibility and hip abduction strength tests may be limited to repeat measurement by a single examiner. Football players with groin pain were found to have significantly reduced strength on the squeeze test when compared with their asymptomatic counterparts.

What is already known on this topic

  • Measures that quantify hip flexibility and strength are often used to examine athletes with longstanding groin pain.

  • Little is known about the reliability of some of these measures and whether they discriminate between athletes with and without longstanding groin pain.

What this study adds

  • Reliable measures of hip flexibility (bent knee fall out, hip internal and external rotation) and strength (squeeze test, hip abduction) were identified among junior football players.

  • Force production on the squeeze test may discriminate between junior football players with and without longstanding groin pain.

Acknowledgments

The authors would like to acknowledge the Australian Physiotherapy Association for providing the funding for this project.

REFERENCES

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Footnotes

  • Funding The Australian Physiotherapy Association funded this study.

  • Competing interests None.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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