Groin injury is one of the top 6 most frequently cited injuries in rugby union (3) and is the fourth most common injury sustained during rugby union training (20). Groin injuries are prevalent in field-based sports such a rugby union because of the specific biomechanical demands of match-play (10). The adductor muscles are constantly contracting during running, acting to stabilize the thigh (with respect to the pelvis) during the swing phase and the pelvis (with respect to the thigh) during stance (22). Moreover, actions such as twisting and kicking while running place increased load on the numerous anatomical structures local to the groin area (10).
To prevent groin injuries in field-based sport athletes, research has been undertaken to identify the modifiable risk factors associated with these injuries. One of the most prominent intrinsic risk factors is weakness of the hip adductor muscles (28). Various methods for assessing adductor muscle strength have been described in the literature including isokinetic dynamometry (1), hand-held dynamometry (13,30), and the adductor squeeze test (8,21). The latter is a simple, low-cost, reliable (9), and valid measure (8) of adductor muscle strength. Furthermore, the adductor squeeze test exhibits discriminative capacity whereby it can differentiate symptomatic from asymptomatic players (21,26).
In sport, the monitoring and screening of athletes is common practice to detect and thus avoid the development of fatigue and injury (12). The adductor squeeze test is a screening tool commonly used to identify changes in adductor strength, which may precede the onset of groin pain and injury. By monitoring an athlete's adductor squeeze on a regular basis, it is hoped that the early identification of potential groin pain can be made, and thus the progression to full injury can be prevented (6). The importance of such monitoring has been suggested in a study assessing adductor strength in 86 junior Australian Rules football players on a weekly basis over the course of a 9-week preseason period using a hand-held dynamometer (7). Twelve players (14%) developed groin pain during the study period and experienced decreases in adductor strength, 2 weeks (1.99 ± 4.28%; effect size [ES] = 0.26) and 1 week (5.83 ± 5.16%; ES = 0.55) before the onset of groin pain. These decreases became more pronounced during the week of groin pain onset (11.75 ± 2.5%; ES = 0.98); thus, it is suggested that the onset of groin pathology may be detected before the onset of symptoms through regular screening of adductor strength (7).
Although the development of groin pain has been attributed to the biomechanical demands of match-play, to date, no studies have reported changes in adductor strength after competitive match-play. However, recent research has evaluated changes in global lower-limb neuromuscular function, by the vertical jump test, demonstrating significant reductions in the first 24–48 hours postmatch in junior collision sport athletes (18,32). Based on these findings, it has been recommended that activities that may compound these deficits, such as repeated high velocity movements be avoided during this period to facilitate recovery (23). Equally, there is potential for the adductor muscles to exhibit similar postmatch fatigue and thus may require a period of recovery before returning to field-based training. Ensuring full recovery of adductor muscle strength before returning to training postmatch may reduce the likelihood of adductor injury. However, currently, no literature exists, which investigates this phenomenon. Therefore, the purpose of this study was to investigate the magnitude of change in adductor strength after match-play in academy rugby union players using the adductor squeeze test. The second purpose was to examine the relationship between locomotive match-play demands and changes in adductor strength.
Experimental Approach to the Problem
A within-group repeated measures design was used in this study. To examine the magnitude of change in adductor strength after a competitive match, adductor squeeze test scores were collected from academy rugby union players pre match and immediately, 24, 48, and 72 hours postmatch. Testing was conducted at the same time of day to avoid the potential effects of circadian rhythm on performance.
Fourteen players (age, 17.4 ± 0.8 years; height, 182.7 ± 7.6 cm; body mass, 86.2 ± 11.6 kg) were recruited from a professional rugby union academy. Players were excluded if they had a history of groin pain within the previous 3 months or had sustained a lower-limb injury during the match. Ethical approval was granted by the university ethics board, and written informed consent was acquired from all participants along with parental consent.
After a day of complete rest, players underwent baseline adductor strength testing at 11.00 AM, approximately 2 hours before kickoff. Further testing was undertaken immediately postmatch, and at 24, 48, and 72 hours after baseline testing. All testing was undertaken at the same time of the day. During the testing period, players did not engage in any training or strenuous activity in the days after the match. Players were advised on nutritional intake, but no recovery protocol was undertaken. Further adductor strength measurements were taken 6 days apart to determine the between-day reliability of the test.
Adductor Strength Testing
Adductor strength was assessed using the adductor squeeze test (8). The adductor squeeze test was performed with the participant supine on a plinth with hips at 45° and feet flat on the plinth. This position has the lowest standard error of measurement (1.6%), minimal detectable difference (10.62 mm Hg), highest reliability (intraclass correlation coefficient [ICC] = 0.92) (9) and greatest electromyographic activation of the adductor muscles (8). The position of the hip joint was confirmed using a goniometer. A sphygmomanometer (Welch Allyn Disytest, Skaneateles, NY, USA) was pumped to 10 mm Hg and allowed to settle of 30 seconds before being placed between the knees of the participant at the most prominent points of the medial condyles. The hip joints were positioned in neutral rotation. Participants were instructed to squeeze the sphygmomanometer as hard as possible. Participants performed 3 maximal efforts separated by 30 seconds, and the highest pressure achieved was recorded (9).
Internal Match Load
Subjective internal game load was obtained by the session rating of perceived exertion method (11) within 30 minutes of the match on a modified Borg scale, which has been previously validated in collision sport athletes (29). This rating was then multiplied by the time spent playing to give a game load in arbitrary units (AU).
External Match Load
External load was measured using Minimax x4 global positioning system (GPS) units (Catapult Innovations, Melbourne, Australia) with 10 Hz sampling to analyze the locomotive demands of the match. Each player wore a specialized vest designed to house and position a GPS unit in the upper-back region between the shoulder blades. The GPS units were turned on before the warm-up and turned off after the match. Global positioning system data were downloaded to a laptop and analyzed using Catapult Sprint software (Catapult Innovations). The GPS units also contained gyroscopes, magnetometers, and triaxial accelerometers sampling at 100 Hz, which provided information regarding collisions, accelerations, and decelerations.
To individualize the velocity bands used to categorize locomotive demands of the match, individual maximum velocities were established from speed testing undertaken a week before the match. Each player performed 3 maximal 40-m sprints with 3-minute rest in between each sprint while wearing the same Minimax x4 GPS unit that they wore during the match. The highest velocity (V[Combining Dot Above]O2max) achieved was used to classify locomotive match demands for each player as follows: walking and standing (<20% V[Combining Dot Above]O2max), jogging (20–50% V[Combining Dot Above]O2max), striding (51–80% V[Combining Dot Above]O2max), sprinting (81–95% V[Combining Dot Above]O2max), and maximum sprinting (96–100% V[Combining Dot Above]O2max) (5,31). However, as little distance was covered at maximum sprinting speed (1.43 ± 4.01 m), the sprinting and maximum sprinting categories were aggregated to form one sprinting category (81–100% V[Combining Dot Above]O2max).
All data were managed using Microsoft Excel 2011. Between-day reliability statistics of ICC, typical error (TE), and coefficient of variance (CV) were calculated for the adductor squeeze test using a Microsoft Excel spreadsheet (14). Prematch and postmatch scores after the match were log transformed to reduce bias as a result of nonuniformity error. Data were then analyzed for practical significance using magnitude-based inferences (2). This statistical approach was chosen because this study was concerned with magnitudes of change, which traditional hypothesis testing does not provide. The threshold for a change to be considered practically important (the smallest worthwhile change, SWC) was set at 0.2 × between-subject SD, based on Cohen's d ES principle. The probability that the magnitude of change was greater than the SWC was rated as <0.5%, almost certainly not; 0.5–5%, very unlikely; 5–25%, unlikely; 25–75%, possibly; 75–95%, likely; 95–99.5%, very likely; and >99.5%, almost certainly (15). The magnitude of increase or decrease in adductor squeeze scores was described as substantial when the probability of the effect being equal to or greater than the SWC (ES ≥ 0.2) was ≥75% (2). Those that were less than the SWC (ES ≤ 0.2) were described as trivial (2). Where the 90% confidence interval (CI) crossed both the upper and lower boundaries of the SWC (ES ± 0.2), the magnitude of change was described as unclear (2). Data were tested for normality using Shapiro-Wilk test, and the relationships between locomotive demands of match-play and changes in adductor strength were examined using Pearson's product-moment correlation coefficient using SPSS for Mac (version 21). The correlation coefficient was ranked as trivial (<0.1), small (0.1–0.29), moderate (0.3–0.49), large (0.5–0.69), very large (0.7–0.89), and nearly perfect (0.9–0.99) (15).
The total length of the match was 1 hour 13 minutes and 28 seconds. The first and second halves lasted 36 minutes and 30 seconds and 37 minutes and 7 seconds, respectively. The average match load (RPE × time) was 334 ± 121 AU. Locomotive demands of the match are presented in Table 1.
For the between-day reliability calculations, the mean and SD for testing day 1 and 2 were 297.4 ± 34.2 mm Hg and 304.9 ± 31.7 mm Hg, respectively. The TE was 7.5 mm Hg (90% CI = 5.70–11.11), and the SWC (0.2 × between-subject SD) was 6.5 mm Hg (2.1%), whereas the CV was 2.7 (90% CI = 2.1–4.1%), and the ICC was 0.95 (90% CI = 0.88–0.98).
Adductor Squeeze Test
Results for the magnitudes of change in adductor squeeze scores based on Cohen's d principle are presented in Figure 1. Trivial decreases in adductor squeeze strength scores occurred immediately (−1.3 ± 2.5%; ES = −0.11 ± 0.21; possibly, 74%) and 24 hours postmatch (−0.7 ± 3%; ES = −0.06 ± 0.25; likely, 78%), whereas a small but substantial increase occurred at 48 hours (3.8 ± 1.9%; ES = 0.32 ± 0.16; likely, 89%) before reducing to trivial at 72 hours postmatch (3.1 ± 2.2%; ES = 0.26 ± 0.18; possibly, 72%).
As the TE of the adductor squeeze was greater than the threshold set for the SWC (0.2 × between athlete SD), a separate analysis was conducted using the TE as the threshold (4) to determine whether the changes in adductor strength were also greater than the TE of the test. The standardized TE (based on Cohen's d ES principle) was calculated as 0.24 and is presented in Figure 1. Small changes to the probabilities occurred for all measurements, but the inferences stayed the same. Trivial decreases in adductor squeeze strength scores occurred immediately (−1.3 ± 2.5%; ES = −0.11 ± 0.21; likely, 84%) and 24 hours postmatch (−0.7 ± 3%; ES = −0.06 ± 0.25; likely, 86%), whereas a small but substantial increase occurred at 48 hours (3.8 ± 1.9%; ES = 0.32 ± 0.16; likely, 79%) before reducing to trivial at 72 hours postmatch (3.1 ± 2.2%; ES = 0.26 ± 0.18; possibly, 58%). Individual changes in adductor strength are shown in Figure 2.
Large, moderate, and very large relationships were observed between the change in adductor strength and the distance covered at sprinting speed immediately postmatch (p = 0.056, r = −0.521), 24 hours postmatch (p = 0.094, −0.465), and 48 hours postmatch (p = 0.005, −0.707), respectively. All other relationships between locomotive variables and changes in adductor strength were nonsignificant and small to moderate (p > 0.05, r = −0.373 to 0.249).
The aims of this study were to determine the magnitude of change in adductor strength after a competitive match in academy rugby union players using the adductor squeeze test and to examine any relationships that may exist between locomotive demands of match-play and changes in postmatch adductor strength. Trivial reductions were observed immediately and 24 hours postmatch, whereas there was a small but a substantial increase in adductor strength at 48 hours before reducing to trivial at 72 hours postmatch. To the author's knowledge, this is the first study to report changes in adductor strength after a competitive match in field-based sport athletes, despite anecdotally its wide use in practice. A previous study by Paul et al. (27) examined the acute effect of match-play on hip adductor strength and flexibility in junior football players, although only changes in flexibility were reported, thus making it impossible to compare changes in hip adductor strength with this study.
To use the adductor squeeze test as a monitoring tool, it must be reliable within a given athletic population. In this study, the adductor squeeze test demonstrated low TE (7.5 mm Hg [90% CI = 5.7–11.1]), a low CV (2.7% [90% CI = 2.1–4.1%]), and the ICC of 0.95 (90% CI = 0.88–0.98), which is similar to other adductor squeeze reliability studies (9). However, it must be pointed out that the TE was greater than the threshold set for the SWC (0.2 × between-subject SD), preventing the adductor squeeze test from detecting potentially important changes. When the threshold for the SWC was changed to the TE, minimal changes occurred in the results, potentially due to the small difference between the 2 thresholds (6.5 vs. 7.5 mm Hg). Nevertheless, when using the adductor squeeze as a monitoring tool in academy rugby union players, it may be prudent to use the TE as the SWC threshold to ensure that changes are real and not the result of testing error.
The magnitude of decrease in adductor strength immediately and 24 hours postmatch was trivial and did not reach the changes previously observed by Crow et al. (7) in the weeks preceding the onset of groin pain. Furthermore, adductor strength increased by a small but substantial amount at 48 hours. Based on the findings of this study, and the previous findings of Crow et al., players who present with substantially reduced adductor strength at 24–48 hours postmatch may require additional rest before returning to field-based training. Furthermore, it has been suggested that an athlete presenting with substantial reductions in adductor strength at this time point should be referred to the medical staff to determine whether any intervention is required (25).
Reductions in global lower-limb neuromuscular function have been reported after competition in field-based collision sport athletes. Decreases in countermovement jump peak power production have been reported for between 24 and 48 hours after competition (18,23). However, decrements in peak force have only been observed immediately postmatch (24) or not at all (17,19). Given that the adductor squeeze test measures isometric force, the trivial postmatch reductions in this study are similar to those found in the aforementioned studies measuring global lower-limb force production (17,19).
However, the results of this study demonstrate the individual nature of muscular recovery after match-play. Although the mean decreases in adductor strength did not reach substantial levels, it can be seen from Figure 2 that there was large variation in individual responses in the first 48 hours postmatch. Of particular note are the players who remained below the SWC at 24 and 48 hours postmatch. To investigate potential causes of such variation, the relationships between locomotive demands of match-play and changes in adductor strength were examined, of which the majority were small to moderate. However, the relationship between changes in adductor strength and distance covered at sprinting speed (V[Combining Dot Above]O2max ≥ 81%) was large immediately postmatch, moderate at 24 hours, and very large at 48 hours postmatch. Although this finding should be interpreted with caution due to the small sample size, this relationship suggests that players involved in a greater amount of sprinting during match-play have greater deficits in adductor strength postmatch. For example, the player who covered the greatest distance at sprinting speed during the match still demonstrated a reduction of 10 mm Hg at 48 hours postmatch, a deficit that was greater than the SWC (6.5 mm Hg) and TE (7.5 mm Hg). Nevertheless, future research involving larger sample sizes is needed to investigate this relationship further.
The increase in adductor strength 48 hours after match-play observed in this study is difficult to explain. Previous findings by Jensen et al. (16) showed increases in adductor muscle strength 24 hours after specific football kicking practices, indicating a potentiation or increased activation of these muscles after adductor related activity. Furthermore, vertical jump performance has been shown to increase 72 hours after completion in elite rugby league players (24). However, further research involving multiple matches is needed to investigate this phenomenon further.
A limitation of this study was the use of adductor squeeze in only 1 test position. The testing position was chosen to elicit maximal activation of the adductor muscles (8), with the greatest reliability (9), and to reduce the testing time before the match. However, due to the multiplanar function of the adductor muscles, it has been recommended that testing positions should also include 0 and 90° of hip flexion to determine whether strength deficits occur in different functional ranges (6). Future research is therefore needed to ascertain whether acute changes in adductor strength occur after competition in different adductor squeeze test positions. A further limitation of this study was the analysis of only 1 competitive match, without also examining the load accumulated during training and other competitive matches. Such data would have provided context regarding the load amassed during this match with respect to the typical load accumulated by this group of players in both training and competition. However, total distance covered in this study (4,847 ± 682 m) was similar to the average distance reported from 5 games (4,470 ± 292 m) by Venter et al. (31) in under-19 provincial players, suggesting this may represent academy rugby match demands. Nevertheless, future research investigating adductor strength responses to match-play after multiple matches is needed to fully establish the changes in strength that occur in this muscle group after competition.
In conclusion, findings from this study identified trivial reductions in adductor strength immediately and 24 hours after competition in academy rugby union players when measured with the adductor squeeze test at 45° hip flexion. The distance covered sprinting influenced adductor fatigue in the first 48 hours postmatch. Future research is needed to ascertain whether changes occur in adductor strength occur after competition when different adductor squeeze positions (0 or 90°) are used and to further understand the effect of sprinting on changes in postmatch adductor strength. The adductor squeeze test is a reliable tool for assessing adductor strength in academy rugby union players.
Practitioners can reliably monitor adductor strength in academy rugby union players using the adductor squeeze test and can be confident that the changes in adductor strength are the result of real changes and not due to testing error. The findings from this study highlight the importance of monitoring adductor squeeze strength in the days after competition in academy rugby union players. Players who present with substantial decreases in adductor strength beyond 24 hours may require additional rest or referral to medical staff before returning to field-based training. In addition, players who cover greater distances sprinting may experience greater adductor fatigue in the first 48 hours postmatch and should be monitored accordingly.
The author would like to thank Andrew Rock (Academy Director) and all the players who were involved in the project. This research was part funded by Leeds Rugby as part of the Carnegie Adolescent Rugby Research (CARR) project.
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