Strength ratios are affected by years of experience in American collegiate rugby athletes: A preliminary study AUTHOR COPY

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1 Isokinetics and Exercise Science 24 (2016) DOI /IES IOS Press Strength ratios are affected by years of experience in American collegiate rugby athletes: A preliminary study Kyle S. Beyer a, David H. Fukuda a,,ameliam.miramonti a, David D. Church a,satorutanigawa a,b, Jeffrey R. Stout a and Jay R. Hoffman a a Institute of Exercise Physiology and Wellness, University of Central Florida, Orlando, FL, USA b Faculty of Health and Sport Sciences, University of Tsukuba, Ibaraki, Japan Received 4 January 2016 Accepted 29 March 2016 Abstract. BACKGROUND: Hamstring:quadriceps ratios (HQRs) have been studied as potential markers for an athlete s risk of injury, but little is known about how sport-specific experience affects HQRs. OBJECTIVE: This study compared isokinetic and isotonic strength measures, including determining HQRs, between American collegiate male rugby athletes with at least two years of experience (XP; n = 10) and those with less than two years of experience (IN; n = 14). METHODS: Isokinetic concentric (ConH) and eccentric (EccH) hamstring and concentric only quadriceps (ConQ) strength was measured at 60 and 180 /s. The strength ratio: Hcon/Qcon (HQR), was calculated for both velocities, HQR 60, HQR 180, while the strength ratio: ConH/ConQ, the dynamic control ratio (DCR), was likewise calculated, DCR 60 and DCR 180. The isotonic strength was assessed via squat 1-repetition maximum. Analysis of covariance was used to examine differences between IN and XP while body weight and age served as the covariates. RESULTS: No significant differences were noted between groups in isokinetic or isotonic strength. However, a significant difference was observed in DCR 60 (p = 0.047) between the two groups, with XP (0.94 ± 0.11) being greater than IN (0.81 ± 0.16). CONCLUSIONS: In view of the preliminary nature of this study, the implication of this finding is yet obscure. However, given the role of the DCR in an underlying proneness to injury, we suggest that a more comprehensive study with a substantial followup period is undertaken for uncovering the potential importance of this ratio vis-à-vis other major functional parameters as well as injury prediction and assessment of rehabilitation. Keywords: Muscle imbalance, injury risk, rugby union, isokinetic strength, agonist-antagonist ratio 1. Introduction Rugby union is a physically demanding sport played all over the world. Like most field-based team sports, Corresponding author: David H. Fukuda, Institute of Exercise Physiology and Wellness, University of Central Florida, Orlando, FL 32817, USA. Tel.: ; Fax: ; david.fukuda@ucf.edu. it requires numerous bouts of high intensity running, tackling and rapid changes of direction, interspersed by periods of lower intensity activity such as walking or jogging [1]. Rugby is a full contact sport with high physical demands, and, consequently, a high injury rate [2]. The lower limb is the most common site for an injury, and regardless of the affected site, the ligaments, muscles, and tendons are the most commonly injured tissues [3]. Therefore, it can be inferred that the ISSN /16/$35.00 c 2016 IOS Press and the authors. All rights reserved

2 258 K.S. Beyer et al. / Strength ratios are affected by years of experience in American collegiate rugby athletes largest potential areas of injury are to the musculature and connective tissue of the knee and ankle. Regardless of playing position, hamstring injuries are the first and second most common injuries in training and match play, respectively [4,5]. Additionally, anterior cruciate ligament (ACL) injuries resulted in the most missed time amongst rugby union players in the English Premiership [4,5]. There are many factors associated with an athlete s risk for sustaining a lower body injury, such as a hamstring strain or an ACL sprain [6,7]. A proposed risk factor is strength imbalances between the hamstring and quadriceps muscle groups [8 10]. Typically, strength imbalances are assessed via isokinetic dynamometry in the form of a hamstring:quadriceps ratio (HQR). This ratio has two variants: the conventional ratio (HQR), which is widely described in the isokinetic muscle performance literature, and the dynamic control ratio (DCR [11,12]). While the clinical significance of the HQR has never been validated, the DCR has been shown to vary significantly both in individuals with compromised ACL [13] as well as individuals with strain of the hamstring [9]. Previous research has shown that as athletes age, there is a greater incidence of hamstring injury, but this may be due to a greater exposure (minutes played) to injury or to the development of a recurring injury earlier in their career rather than a muscle imbalance at the knee joint [14]. The effects of aging on HQRs are unclear [15,16]. However, the age-related difference in these ratios may be a product of increased sportspecific experience. It is expected, that the years of sport-specific experience increases with age, but the physiological changes associated with normal growth and development may mask any experience-related differences [15]; therefore, it is important to control for age when trying to compare novice and experienced athletes. Previous research has shown that highly trained female runners have greater HQRs than their recreational counterparts, but age was not controlled in this study [17]. The relationship between HQRs and sport-specific experience has not been investigated in rugby union, which may be of particular relevance due to the nature of the sport, including the use of rapid accelerations, decelerations, and changes of direction [1]. The repeated loading of the knee joint with each practice and game may result in sport-specific adaptations, such as variable increases in lower body strength, which could lead to muscle imbalances [18]. The purpose of the current study is to compare the strength and strength ratios between rugby athletes with at least two years of playing experience and those with less than two years of playing experience. Only 31 US high schools offer rugby as a scholastic sport [19], while over 300 universities field club teams [20]. Therefore, many individuals in the USA are not exposed to rugby until they enroll in college/ university. The selected two-year cutoff point likely demarcates athletes being introduced to rugby union and those who have been engaged in the sport. We hypothesize that individuals with at least two years of playing experience will have greater isotonic and isokinetic strength and better HQRs than those with less than two years of playing experience. 2. Methods 2.1. Subjects Twenty-five members of the university men s rugby team provided verbal consent and volunteered to participate in the current research study. One participant was excluded from data analysis due to pre-existing injury. The age, height, weight and years of competitive rugby experience were recorded for all participants. The participants were then grouped based on years of experience, with two years of experience selected as the cutoff, to create XP (n = 10; ± 1.95y; 1.79 ± 0.08 m; ± kg; 4.00 ± 1.56 years of experience) and IN (n = 14; ± 1.31y; 1.77 ± 0.05 m; ± kg; 0.29 ± 0.47 years of experience) groups. This study was approved by the university Institutional Review Board and was in accordance with the Declaration of Helsinki Procedures Isokinetic strength procedures Prior to beginning the isokinetic testing, participants completed a general warm-up, which consisted of: five minutes of sub-maximal cycling at a selfselected resistance, ten body-weight squats, ten alternating lunges, ten walking knee hugs, and ten walking butt kicks. Following the general warm-up, participants were seated in an isokinetic dynamometer (S4, Biodex Medical System, Inc., New York, NY, USA) with a hip angle of 110 and strapped to the chair at the waist, shoulders and across the thigh to complete a series of isokinetic strength assessments. Chair and dynamometer settings were adjusted for each participant to properly align the axis of rotation with the lat-

3 K.S. Beyer et al. / Strength ratios are affected by years of experience in American collegiate rugby athletes 259 eral condyle of the femur. All participants were tested on their dominant leg (kicking preference) which was secured to the dynamometer arm just above the medial and lateral malleoli. Range of motion was assessed for each participant. All participants were able to achieve a range of motion of without discomfort. The gravity effect of moment was measured at 30 of knee flexion (0 representing full extension) and subsequently corrected for during testing using a previously published equation by Nelson and Duncan [21]. For each muscle action, participants completed 3 warm-up repetitions (50% of perceived maximum) and 3 maximal repetitions separated by a 10- s rest period. Quadriceps peak moment (PM) was assessed concentrically at 60 /s (Q 60 ) and 180 /s (Q 180 ). Hamstring PM was measured both concentrically and eccentrically at 60 /s (ConH 60 and EccH 60, respectively) and 180 /s (ConH 180 and EccH 180, respectively). The muscle action/speed sets were completed in a random order and participants were given two minutes of rest between sets in order to limit the effects of testing. Peak moment was recorded for all repetitions and the greatest value was used for subsequent analysis, including the calculation of HQR and DCR. Previous unpublished reliability measures within our laboratory have shown intraclass correlations (ICC; model 3,1) [22] of greater than 0.90 for all peak moment data and greater than 0.89 for HQR and DCR Isotonic strength procedures On a separate testing day, participants reported to the Strength and Conditioning Laboratory to complete a 1- repetition maximum (1RM) squat. Prior to beginning the 1RM test, each participant completed a general and specific warm up. The general warm up consisted of riding a cycle ergometer for 5 minutes at the participant s preferred resistance. The specific warm up consisted of 10 body weight squats, 10 alternating lunges, 10 walking knee hugs and 10 walking butt kicks. Each participant performed 2 warm-up trials using a resistance that was approximately 40 60% and 60 80% of their perceived maximum, respectively. The third trial served as the first attempt at the participant s 1RM. If the trial was successfully completed, then weight was added and another trial was attempted. If the trial was not successfully completed, then the weight was reduced and another trial was attempted. The amount of weight that was added or removed was at the discretion of the Certified Strength and Conditioning Specialist supervising the testing session. A 3 5 minute rest period was provided between each trial. This process of adding and removing weight continued until a 1RM was reached. Attempts not meeting the range of motion criterion for each exercise, as determined by the trainer following previously established guidelines [23], were discarded Statistical analysis All peak moment data were assessed for normality using Shapiro-Wilk tests of normality. All descriptive data (age, height, and weight) were analyzed using one-way analysis of variance (ANOVA). All isotonic and isokinetic PM and ratio data were analyzed using a one-way analysis of covariance (ANCOVA) with age and body weight serving as the covariates. Homogeneity of variance was assessed for each ANOVA and AN- COVA via Levene s test for equality of variance. For effect size, the partial eta squared (η 2 ) statistic was calculated, and according to Green and Salkind [24], 0.01, 0.06, and 0.14 were interpreted as small, medium, and large effect sizes, respectively. Statistical software (SPSS; V. 20.0; SPSS, Inc, Chicago, IL, USA) was used for all analyses. Results were considered significantatanalphalevelofp All data are reported as mean ± SD. 3. Results 3.1. Descriptives Age, height, weight, and years of experience data can be seen in Table 1. Statistical analysis revealed no significant differences between XP and IN groups for age (F = 1.900, p = 0.182, η 2 = 0.08), height (F = 0.655, p = 0.427, η 2 = 0.029), and weight (F = 2.403, p = 0.135, η 2 = 0.098). The medium effect sizes noted for age and weight were used as partial justification for their use as covariates. As expected, there was a significant difference in years of experience between XP and IN groups (F = , p<0.000, η 2 = 0.764) Isokinetic and isotonic strength Isokinetic and isotonic strength data are outlined in Table 2. The assumptions of normality and equality of variance were met for all data. When controlling for age and body weight, ANCOVA revealed no significant difference between XP and IN groups in terms of Q 60 (F = 1.226, p = 0.281, η 2 = 0.058) or Q 180 (F = 1.693, p = 0.208, η 2 = 0.078). When control-

4 260 K.S. Beyer et al. / Strength ratios are affected by years of experience in American collegiate rugby athletes Table 1 Age, height, body weight and years of experience (mean ± SD) for the inexperienced and experienced groups Inexperienced (n = 14) Experienced (n = 10) Effect size interpretation Age (y) ± ± 1.95 Medium Height (m) 1.77 ± ± 0.08 Small Weight (kg) ± ± Medium Years of Experience (y) 0.29 ± ± 1.56 Large Denotes significant difference between inexperienced and experienced (p <0.05). Table 2 Isokinetic and isotonic strength (PM, mean ± SEM) for the inexperienced and experienced groups adjusted for age and body weight (covariate: age = y; body weight = kg) Inexperienced (n = 14) Experienced (n = 10) Effect size interpretation Q 60 (Nm) ± ± Small Q 180 (Nm) ± ± 6.94 Medium ConH 60 (Nm) ± ± 5.82 Small ConH 180 (Nm) ± ± 4.73 None EccH 60 (Nm) ± ± 9.89 Medium EccH 180 (Nm) ± ± Small Squat 1RM (kg) ± ± Small Denotes different sample sizes for inexperienced (n = 12) and experienced (n = 8). ling for age and body weight, ANCOVA revealed no significant difference between XP and IN groups in terms of ConH 60 (F = 0.419, p = 0.525, η 2 = 0.021) or ConH 180 (F = 0.023, p = 0.881, η 2 = 0.001). Furthermore, there were no significant differences between XP and IN groups for EccH 60 (F = 1.941, p = 0.179, η 2 = 0.088) or EccH 180 (F = 0.563, p = 0.462, η 2 = 0.027). In congruence with isokinetic strength data, there was no significant different between XP and IN in squat 1RM when controlling for age and body weight (F = 0.523, p = 0.480, η 2 = 0.032) Hamstring:quadriceps ratios HQR ratio data can be seen in Table 3. When controlling for age and body weight, ANCOVA revealed no significant difference between XP and IN groups in terms of HQR 60 (F = 3.607, p = 0.072, η 2 = 0.153), HQR 180 (F = 0.873, p = 0.361, η 2 = 0.042), or DCR 180 (F = 1.279, p = 0.271, η 2 = 0.060). However, the XP group had a significantly higher DCR 60 when compared to the inexperienced group (F = 4.502, p = 0.047, η 2 = 0.184). 4. Discussion The main findings of this study were no significant differences between XP and IN in quadriceps and hamstring strength, or isotonic squat 1RM when controlling for age and body weight but a significantly higher DCR 60 in the former compared to the latter group. In addition, there were large effect sizes for both ratios at 60 /s and a medium effect for the DCR 180.Theeffect sizes for Q 60 and Q 180 revealed small to medium effects favoring IN after controlling for age and body weight. Conversely, hamstring isokinetic strength data revealed small to medium effects favoring XP after controlling for age and body weight. Despite the significant difference in DCR 60 between XP and IN, there was no significant difference in DCR 180. This discrepancy in results may be due to speed-specific adaptations to rugby union or larger degrees of variance at faster testing speeds. However, considering rugby union is a fast-paced sport with repeated sprints [1], it is expected that adaptations would have occurred at high velocities. Therefore, the lack of significance may be due to the larger standard deviations observed at 180 /s which in turn may be due to a lack of familiarization with the test velocity. Previous research in basketball [16] and soccer [25] athletes have observed higher standard deviations in DCR at 180 /s than at 60 /s regardless of age. Despite the lack of significant differences in DCR 180 between groups, the current preliminary study did observe a medium effect, which should be further investigated in a future study. Rugby union experience may be causing adaptations specifically to the hamstring muscle group during eccentric muscle action. Indeed, rugby union athletes have been shown to have stronger hamstring muscles than soccer [26] and rugby league athletes [18], which may indicate the high stresses being placed on the hamstring muscle of these players. Previous research

5 K.S. Beyer et al. / Strength ratios are affected by years of experience in American collegiate rugby athletes 261 Table 3 Hamstring: quadriceps ratios (mean ± SEM) for the inexperienced and experienced groups adjusted for age and body weight (covariate: age = y; body weight = kg) Inexperienced (n = 14) Experienced (n = 10) Effect size interpretation HQR ± ± 0.02 Large HQR ± ± 0.03 Small DCR ± ± 0.05 Large DCR ± ± 0.06 Medium Denotes significant difference between inexperienced and experienced (p <0.05). has examined isokinetic leg extensor and leg flexor strength testing at the same speeds in different populations. When compared to a study conducted in professional Australian Rules footballers [27], the current study observed lower conventional HQRs but higher DCRs which may be due to the differences in the physical demands of the sport. Similarly, Cometti and colleagues [28] observed greater conventional HQRs in professional French soccer players, but the DCR 60 was similar to the findings of the current study. Furthermore, in comparison to a similar population of English Premiership Rugby Union Club athletes [29], our study yielded similar conventional HQRs, but higher DCRs. The proposed effect of sport-specific experience on HQRs is that by participating in practices and games over numerous years, an athlete may adapt to the rapid accelerations, decelerations, changes of directions and landings [1]. Previous research on experience-related differences in HQRs is limited; however, lower DCRs have been reported in youth soccer players when compared to age-matched controls [30]. Conversely, Evangelidis and colleagues [31] observed no differences in HQRs between soccer athletes and recreational males. However, with additional sport experience, there may be differences in level of play or playing time. With regard to level of play, Cometti and colleagues [28] found that elite and subelite athletes, with similar playing experience over the last 5 years, had greater HQRs than amateur athletes. Conversely, a study in NCAA women s soccer athletes showed that there was no difference in DCRs between starters and nonstarters, despite a greater eccentric hamstring strength in starters [32]. Althoughplayingtimedatawas notincluded in the current study, we can conclude that all athletes were competing at the same level of play as they were all on the same team. While the current study is reporting years of rugbyspecific experience to have an effect on DCR 60,it is important to consider that different sports [33 35] and genders [36,37] display varying HQRs. Future research should examine experience-related differences in muscle imbalances in athletes of differing playing time/talent levels, varied sporting contexts, and amongst genders. Limitations to this study include a very small sample in each group, an arbitrarily set divider (2y) between the groups, and the lack of injury data and longitudinal tracking of HQRs with increased rugby experience. Future studies should investigate how these ratios adapt to increased sporting experience, and if those adaptations affect risk of injury. 5. Conclusion In conclusion, this preliminary study observed a significantly greater DCR 60 in American collegiate rugby athletes with at least two years from sport-specific experience when compared to athletes of the same team with less than two years of experience. Furthermore, by statistically controlling for age and body weight, we have inferred that the difference is likely due to the years of experience playing the sport of rugby. Interestingly, there were no statistically significant differences between groups in terms of absolute measures of isokinetic strength or squat 1RM. Thus, absolute measures of strength may not be able to differentiate inexperienced and experienced US collegiate rugby athletes. While this study was preliminary in nature, future research should further investigate the role of sport-specific experience on HQR and DCR. Findings from this and future studies may assist professionals working with athletes to identify injury risk and prevent subsequent injury. Conflict of interest The authors declare no conflict of interest. References [1] Roberts SP, Trewartha G, Higgitt RJ, El-Abd J, Stokes KA. The physical demands of elite English rugby union. J Sports Sci. 2008; 26(8):

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