Introduction
Rugby league is an international collision sport played at both junior and senior levels (6,16). The sport has similar rules and movement patterns to rugby union; however, unlike rugby union, rugby league does not have a line-out, involves 13 players per team (rather than 15), and involves an immediate play-the-ball after each tackle (6,16). A typical rugby league match requires players to compete in a challenging contest, comprising intense bouts of sprinting and tackling, separated by short bouts of lower intensity activity. During the course of a rugby league match, each team will perform an average of 300 tackles; forwards are exposed to an average of 55 physical collisions (39 tackles, 16 hit-ups), whereas backs are exposed to an average of 29 physical collisions (16 tackles, 13 hit-ups) (22,23). As a result, rugby league players are required to have well-developed physiological and anthropometric qualities, combined with a wide range of offensive and defensive skills.
Several studies have investigated the physiological and anthropometric qualities of rugby league players, with performance on a wide range of field tests typically improving with increases in playing level (3,5,8,11). The superior playing performance of elite level rugby league players in comparison to their subelite counterparts is often attributed to the greater physiological capacities of these athletes (16). Although successful performance in rugby league is dependent (at least in part) on well-developed physiological capacities, players also require the ability to exhibit high levels of skill under pressure and fatigue (10). Indeed, the significance of high physical fitness levels is reduced if the physiological parameter does not transfer to improved playing performance. For example, an increase in muscular power is redundant unless the enhanced physiological capacity transfers to improved leg drive in tackles, or greater play-the-ball speed. In addition, the value of an increase in aerobic fitness is negated if the ability of players to defend for multiple sets is unchanged or players are unable to exhibit a high level of skill while fatigued.
To date, few studies have investigated the skill qualities that differentiate highly skilled and lesser-skilled rugby league players (7). It has been suggested that skillful performance is constrained by physiological limitations (26). However, in a study of 86 first-, second-, and third-grade rugby league players, Gabbett et al. (4) found that skill, but not physiological or anthropometric characteristics, discriminated between successful and less successful players. Although monitoring the physiological and anthropometric qualities of players is important, the relationship between physical fitness and skill remains unclear.
Success in rugby league is dependent, at least in part, on tackling proficiency, the ability to tolerate physical collisions, and the skill to ‘win’ the tackle contest (18). Despite the fundamental importance of tackling in rugby league, very little is known about the tackle contest (3,7,18,19). Furthermore, there are limited published data on the physical (i.e., physiological and anthropometric) factors that contribute to, or limit, tackling ability in rugby league players (3,19). In a study of amateur rugby league players, it was reported that players with better tackling proficiency were older, more experienced, shorter, lighter, and leaner than players with poor tackling proficiency (19). Better tacklers also had greater levels of mesomorphy, acceleration, and change of direction speed than did poorer tacklers. In a subsequent study, the relationship between physiological and anthropometric qualities and tackling ability in junior elite and subelite rugby league players was investigated (3). The strongest individual correlates of tackling ability were acceleration (r = 0.60) and lower body muscular power (r = 0.38). When multiple linear regression analysis was performed to determine which of the physiological and anthropometric characteristics predicted tackling ability, fast acceleration was the only variable that contributed significantly (r2 = 0.24) to the predictive model. Gabbett (18) investigated the influence of fatigue on tackling proficiency in subelite rugby league players and examined the relationship between selected physiological capacities and fatigue-induced decrements in tackling ability. Progressive increases in fatigue resulted in progressive reductions in tackling ability. An inverse relationship (r = −0.62) was found between maximal aerobic power and fatigue-induced decrements in tackling proficiency, suggesting that well developed endurance qualities may minimize fatigue-induced reductions in skill in rugby league players. In the only study to investigate tackling ability in professional rugby league, players who had at least 150 National Rugby League (NRL) matches experience had a significantly greater tackling proficiency than players who had played up to 49 NRL matches, 50-99 NRL matches, or 100-149 NRL matches (7). Collectively, these findings demonstrate that well-developed physiological and anthropometric qualities contribute to effective tackling ability in rugby league players and that accumulated playing experience also contributes to the expertise exhibited in this skill.
To date, few studies have attempted to determine the relationship between physiological and anthropometric qualities and tackling ability in rugby league players (3,19). These studies have been limited to junior players (3) or senior players competing at the subelite level (19). Although these studies have provided important information on the physical factors that contribute to, and limit, tackling ability, the extent to which these findings can be applied to high-performance rugby league players is questionable. With this in mind, the purpose of this study was to investigate the tackling ability of professional and semiprofessional rugby league players and to determine the relationship between selected physiological and anthropometric characteristics and tackling ability in these athletes.
Methods
Experimental Approach to the Problem
This study used a cross-sectional experimental design to compare the physiological and anthropometric qualities and tackling ability of professional and semiprofessional rugby league players. In addition, Pearson's product-moment correlation coefficients and hierarchical multiple regression analysis were used to determine the relationship among physiological and anthropometric qualities and tackling proficiency. It was hypothesized that a positive relationship would be detected between physical qualities (e.g., high muscular power, fast acceleration, low levels of adiposity) and tackling ability in professional rugby league players.
Subjects
Thirty-seven male high-performance rugby league players (mean ± SE age, 25.6 ± 0.7 years) participated in this study. All players were from the same NRL club and were competing in the elite NRL competition (N = 20) or the state-based Queensland Cup competition (N = 17). The NRL is the highest standard of rugby league competition in the world, whereas the Queensland Cup serves as a feeder competition to the NRL. All players were free from injury and had completed a 12-week preseason conditioning program at the commencement of the study. Testing was performed 3 weeks before the first competition game. All testing was performed at the same time of the day (9.00 am), with players in a nonfatigued state. Players were instructed to consume their normal pretraining breakfast and to ensure adequate hydration at the time of testing. All players received a clear explanation of the study, including the risks and benefits of participation and written consent was obtained. The Institutional Review Board for Human Investigation approved all experimental procedures.
Anthropometry
Skinfold thickness was measured at 7 sites (biceps, triceps, subscapular, suprailiac, abdomen, thigh, and calf) using a Harpenden skinfold caliper (British Indicators Ltd, West Sussex, United Kingdom). The exact positioning of each skinfold measurement followed procedures described previously (25). Stature was measured using a stadiometer (Hart Sport, Queensland, Australia), and body mass was measured using calibrated digital scales (A & D Company Limited, Tokyo Japan). The intraclass correlation coefficient for test-retest reliability and typical error of measurement for stature, body mass, and sum of 7 skinfolds measurements were 0.99, 0.99, and 0.99; and 0.2, 0.8, and 1.1%, respectively.
Lower Body Muscular Power
Lower body muscular power was estimated using a vertical jump test (Yardstick, Swift Performance Equipment, New South Wales, Australia). Players were requested to stand with feet flat on the ground, extend their arm and hand vertically, and mark the standing reach height. After assuming a crouch position, each subject was instructed to spring upward and touch the Yardstick device at the highest possible point. Vertical jump height was calculated as the distance from the highest point reached during standing and the highest point reached during the vertical jump. Vertical jump height was measured to the nearest 1 cm with the highest value obtained from 3 trials used as the vertical jump score. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the vertical jump test was 0.92 and 2.9%, respectively.
Acceleration
The acceleration of players was evaluated with a 10-m sprint effort using dual beam electronic timing gates (Swift Performance Equipment, New South Wales, Australia). The timing gates were positioned 10 m cross wind from a predetermined starting point. Players were instructed to run as quickly as possible along the 10-m distance from a standing start. All tests were conducted on a synthetic surface. Participants started from a stationary, upright position with the front foot on the 0-m point, in line with the start gate. Sprint times were measured to the nearest 0.01 seconds with the fastest value obtained from 3 trials used as the sprint score. Sprint times were subsequently converted to velocity and acceleration scores. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the 10-m sprint test was 0.95 and 1.8%, respectively.
Change of Direction Speed
The speed of players in changing directions was evaluated using the 505 test (5). Two timing gates were placed 5 m from a designated turning point. Players assumed a starting position 10 m from the timing gates (and therefore 15 m from the turning point). Players were instructed to accelerate as quickly as possible along the 15-m distance, pivot on the 5-m line, and return as quickly as possible through the timing gates (Figure 1). Change of direction times were measured to the nearest 0.01 seconds with the fastest value obtained from 3 trials used as the change of direction score. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the 505 test was 0.92 and 2.5%, respectively.
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Figure 1: Schematic illustration of the 505 test (
21).
Tackling Proficiency
Players underwent a standardized one-on-one tackling drill in a 10-m grid. Video footage was taken from the rear, side, and front of the defending player using 37-mm digital video cameras (Sony, DCR-TRV 950E, Nagasaki, Japan). Players performed 8 trials each in the one-on-one tackling drill. Tackling proficiency was assessed by a sport scientist using standardized technical criteria. The technical criteria were developed by 2 expert coaches who also used these criteria as cues when coaching tackling technique in rugby league players (7).
The technical criteria included the following: (a) contacting the target in the center of gravity, (b) contacting the target with the shoulder, (c) body position square and aligned, (d) leg drive upon contact, (e) watching the target onto the shoulder, and (f) center of gravity forward of base of support.
Players were awarded one point for each occasion they achieved the relevant criteria and a score of zero if they failed to achieve the criteria. A total score (out of 8, and reported in arbitrary units) was awarded for each of the criteria. In addition, a total tackling proficiency score (also reported in arbitrary units) was awarded based on the aggregate of all technical criteria. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for assessments of tackling technique was 0.83 and 3.3%, respectively.
Statistical Analyses
Data were tested for normality using a Shapiro-Wilk test. Differences in tackling ability, and physiological and anthropometric qualities between professional and semiprofessional players were compared using an independent t-test and Cohen's effect size (ES) statistic (2). Effect sizes of <0.09, 0.10-0.49, 0.50-0.79, and >0.80 were considered trivial, small, moderate, and large, respectively (1). Pearson's product-moment correlation coefficients were used to determine the relationship among physiological and anthropometric qualities and tackling proficiency. Hierarchical multiple regression analysis was performed to determine which of the physiological and anthropometric characteristics predicted tackling ability. A Mahalanobis distance test was used to detect the presence of outliers. A Goldfeld-Quandt test was used to assess heteroscedasticity. The absence of autocorrelation and multicollinearity of the regression model was established before all variables were incorporated. Based on an alpha level of 0.05, the inclusion of 8 independent variables in the multiple regression model, and a sample size of 37, our beta level (statistical power) was ≥0.75 for detecting a moderate r2 value from the multiple regression analysis. The level of significance was set at p ≤ 0.05, and all data are reported as mean ± SE.
Results
Tackling Proficiency
The mean tackling proficiency for the professional and semiprofessional players is shown in Table 1: Professional players had greater (ES = 1.10, p ≤ 0.05) tackling proficiency than did semiprofessional players. Professional players more regularly watched the target onto their shoulder (ES = 1.33, p ≤ 0.05), maintained a square and aligned body position (ES = 0.91, p > 0.05), had their center of gravity forward of their base of support upon contact (ES = 0.57, p > 0.05), and made contact in the target's center of gravity (ES = 0.62, p > 0.05) than semiprofessional players.
Table 1: Tackling ability of high-performance rugby league players.*†‡§
Physiological and Anthropometric Qualities
Professional players were older (ES = 1.29, p ≤ 0.05), more experienced (ES = 1.59, p ≤ 0.05), leaner (ES = 1.81, p ≤ 0.05), and had faster acceleration (ES = 0.82, p ≤ 0.05) than semiprofessional players (Table 2).
Table 2: Age, playing experience, and physiological and anthropometric qualities of high-performance rugby league players.*†‡
Relationship among Physiological and Anthropometric Qualities and Tackling Proficiency
Table 3 shows the relationship between the physiological and anthropometric qualities of players and tackling proficiency. The strongest individual correlates of tackling ability were age (r = 0.41, p ≤ 0.05), playing experience (r = 0.70, p ≤ 0.01), skinfold thickness (r = −0.59, p ≤ 0.01), acceleration (r = 0.41, p ≤ 0.05), and lower body muscular power (r = 0.38, p ≤ 0.05).
Table 3: Relationship among physiological and anthropometric characteristics and tackling ability in high-performance rugby league players.*†
Hierarchical Multiple Regression Analysis
Table 4 shows the hierarchical multiple regression analysis that determined which of the physiological and anthropometric characteristics predicted tackling ability. Playing experience and lower body muscular power were the only variables that contributed significantly to the predictive model (r2 = 0.60, F = 20.798, p ≤ 0.01).
Table 4: Hierarchical multiple regression analysis to predict tackling ability in high-performance rugby league players.
Discussion
This study investigated the tackling ability of professional and semiprofessional rugby league players and determined the relationship between physiological and anthropometric qualities and tackling ability in these athletes. Although previous studies have investigated the relationship between physical qualities and tackling proficiency in junior (3) and amateur (19) rugby league players, this is the first study to examine the relationship between measures of physical qualities and skill in high-performance rugby league players. The results of this study demonstrate that highly skilled professional players have better tackling ability than lesser-skilled semiprofessional players, and the strongest correlates of tackling ability are age (r = 0.41), playing experience (r = 0.70), skinfold thickness (r = −0.59), acceleration (r = 0.41), and lower body muscular power (r = 0.38). These findings demonstrate that well-developed physical qualities contribute to effective tackling ability in high-performance rugby league players. From a practical perspective, strength and conditioning coaches should emphasize the development of acceleration, lower body muscular power, and lean muscle mass to improve tackling ability in high-performance rugby league players.
Tackling is arguably the most important skill in rugby league with playing success depending, at least in part, on tackling ability, the capacity to tolerate physical collisions, and the ability to ‘win’ the tackle contest. Furthermore, the majority of rugby league injuries have been shown to occur within the tackle (9,12,13,15,20,22), and the physiological demands significantly increase through the large amounts of contact (i.e., tackling, wrestling, grappling, and incidental collisions) performed during the game (6). Despite the reported importance of tackling in rugby league, few studies have investigated the tackle contest and its relevance to playing performance. A greater understanding of the factors that limit tackling performance is therefore warranted, from both an injury prevention and performance perspective. Consistent with previous studies (3,7,19), this study found better tackling proficiency in the highly skilled professional players than in their lesser-skilled semiprofessional counterparts. These findings provide further support for the tackle assessment as a valid method of detecting differences in tackling ability in rugby league players with varying levels of expertise. Indeed, assessing the tackling ability of a high-skilled and lesser-skilled playing group provided an insight into the criteria that were the most reliable discriminators of tackling ability. Specifically, professional players had superior tackling proficiency because they more regularly watched the target onto their shoulder, maintained a square and aligned body position, had their center of gravity forward of their base of support upon contact and made contact in the target's center of gravity.
Professional players were older and had significantly more playing experience than semiprofessional players. In addition, significant associations were observed among age, playing experience, and tackling ability. Playing experience has consistently been shown to be a significant predictor of playing performance and success in senior rugby league players (4,7,10,19). In a series of studies (4,7,10,19), it was demonstrated that playing experience was a critical variable determining selection into a first-grade rugby league team. It has also been shown that players with 150 matches or greater NRL experience had superior tackling technique to players with <150 matches NRL experience (7), providing further support that sporting expertise develops, at least in part, as a result of accumulated experience.
No significant differences were found between professional and semiprofessional players for body mass. However, the lean body mass (as estimated from skinfold thickness) was higher in professional players. Furthermore, a significant inverse relationship (r = −0.59) was observed between tackling proficiency and the sum of 7 skinfolds. These findings are in agreement with some (19) but not all (3) studies that have investigated the relationship between anthropometric characteristics and tackling proficiency. Although a higher percentage body fat has been suggested to provide protection against contact injuries, by providing an energy absorbing barrier (24), previous studies have reported that excess body fat has the potential to reduce performance by reducing the power to body mass ratio, decreasing aerobic capacity, and by increasing the thermoregulatory strain on players (6). Excess body fat is likely to be detrimental to playing performance given that rugby league players must develop large impact forces in tackles. The lower skinfold thickness (and higher estimated lean muscle mass) in the professional players of this study most likely contributed to the superior acceleration and lower body muscular power qualities in these players. Although the reported influence of anthropometric qualities on tackling ability are equivocal (3,19), the results of this study extend and confirm those of others by demonstrating that excess body fat is associated with poorer tackling ability in high-performance senior rugby league players.
The professional rugby league players in this study had superior acceleration over 10 m than the semiprofessional players, with this physical quality associated with better tackling ability. These findings are consistent with a recent study that reported better tackling proficiency and faster acceleration over 10 m, in junior elite than in subelite players (3), emphasizing that the ability to accelerate into the contact zone is an important prerequisite for effective tackling ability. Of interest were the similar performances between professional and semiprofessional players for change of direction speed and the lack of association between this quality and tackling proficiency. It is possible that the poor relationship between change of direction speed and tackling proficiency reflected the change of direction speed test employed. Although the 505 test has been shown to discriminate higher- and lesser-skilled rugby league players (21), the movement patterns it requires does not reflect the typical changes in direction observed while tackling. A change of direction speed test that assessed forward movement followed by a 45° change of direction (to mimic the changes in direction commonly seen in tackling) may have resulted in a better association between change of direction speed and tackling ability. Alternatively, it is possible that tackling proficiency is more closely related to the perceptual components of agility (e.g., visual scanning, anticipation, pattern recognition, and situational knowledge) (27), which have been shown to discriminate higher- and lesser-skilled players but are also reported to have a poor association with traditional change of direction speed results (21). In this study, only the technical components of the tackle were assessed, with assessments of perceptual skill, where defending players are required to anticipate and rapidly respond to the movements of the attacking player not included. Although consideration was given to the decision-making component of tackling, the development of a one-on-one tackling drill emphasizing the important technical components of the tackle represented the most reliable method of assessing this skill. Moreover, the technical criterion offered a valid method of discriminating players with good and poor tackling proficiency. Despite the practical utility of the technical assessment of tackling skill, future studies investigating the influence of a decision-making task on tackling proficiency are warranted.
Playing experience and lower body muscular power were the only variables that contributed significantly to the hierarchical multiple regression model of tackling ability. Playing experience alone explained over half (51%) of the variance in tackling proficiency. The addition of lower body muscular power to the multiple regression model only contributed a further 9% to the common variance. These findings suggest that the ability to perform an effective tackle in rugby league is dependent on factors in addition to, or other than playing experience and lower body muscular power. Given that a limited number of physical qualities were measured in this study, we cannot preclude the possibility that other physical qualities (e.g., strength, upper body muscular power), or other qualities that are more difficult to measure (e.g., attitude, mental resilience) may have contributed to the tackling proficiency regression model.
Although several studies have investigated the physiological and anthropometric qualities of rugby league players, the majority of the research performed to date has been limited to subelite competitors (8,10,11,14,17). Few studies have investigated the skill qualities of rugby league players (4). In a study of subelite rugby league players, Gabbett et al. (4) investigated the physiological, anthropometric, and skill characteristics of rugby league players competing at 3 distinct playing levels to determine which, if any factors, discriminated between successful and less successful athletes. In comparison to second and third-grade players, first-grade players had greater basic passing and ball carrying ability, tackling and defensive skills, evasion skills (i.e., ability to beat a player and 2 vs. 1 skills), and superior skills while fatigued. However, no significant differences were found among playing levels for body mass, skinfold thickness, height, 10-, 20-, or 40-m speed, change of direction speed, lower body muscular power, or estimated maximal aerobic power. These results demonstrate that selected skill characteristics, but not physiological or anthropometric characteristics discriminate between successful and less successful rugby league players competing at the subelite level. Although the study of Gabbett et al. (4) provided an insight into the relative importance of physical and skill qualities in rugby league, participants were subelite players, making generalizations to an elite playing population problematic. The influence of physiological, anthropometric, and skill qualities on team selection and playing performance is further complicated by the fact that to date, only one study has investigated the relationship between standardized assessments of skill (e.g., tackling ability) and game-specific playing performance (e.g., total tackle attempts, tackles completed, missed tackles, dominant tackles) (7). The relative importance of physical qualities and skill to selection in an elite NRL team and subsequent playing performance has obvious implications for high-performance coaches. Clearly, studies of the relative importance of physiological, anthropometric, and skill qualities on (1) team selection (e.g., qualities that discriminate selected and nonselected players, and starters and nonstarters) and (2) playing performance (e.g., tackle efficiency, meters gained, support and decoy runs, line breaks, tackle breaks, and offloads, etc.) are warranted.
In conclusion, this study is the first to investigate the tackling ability of high-performance rugby league players and determine the relationship between physiological and anthropometric qualities and tackling ability in these athletes. The results of this study demonstrate that highly skilled professional players have better tackling ability than lesser-skilled semiprofessional players, and the strongest correlates of tackling ability are age (r = 0.41), playing experience (r = 0.70), skinfold thickness (r = −0.59), acceleration (r = 0.41), and lower body muscular power (r = 0.38). From a practical perspective, strength and conditioning coaches should emphasize the development of acceleration, lower body muscular power, and lean muscle mass to improve tackling ability in high-performance rugby league players.
Practical Applications
There are several practical applications from this study that are relevant to the applied sport scientist, strength and conditioning practitioner, and rugby league coach. Few studies have investigated the tackling proficiency of rugby league players, whereas even less work has been devoted to high-performance players competing at an elite level. No study has investigated correlates of tackling ability in high-performance rugby league players. Although a significant correlation does not imply cause and effect, the strength of the association of these qualities provides an insight into the factors that contribute to, and limit tackling proficiency. Importantly, this study provides empirical evidence to support the suggestion that skillful performance is constrained by physical limitations (26).
First, lean muscle mass (as estimated from skinfold thickness) was significantly associated with tackling proficiency. Players with higher levels of adiposity had poorer tackling proficiency. These findings suggest that the development of lean muscle mass, while also maintaining body mass, may facilitate effective tackling technique.
Second, well-developed lower body muscular power and acceleration qualities were associated with better tackling proficiency. It is likely that lower body muscular power enables effective leg drive in tackles, whereas fast acceleration promotes the ability of defenders to accelerate into the contact zone. Assuming that appropriate training time is devoted to the skill of tackling, it is likely that increases in acceleration and lower body muscular power will transfer to improvements in tackling proficiency.
Third, better tacklers were older and had greater playing experience than poorer tacklers. Furthermore, the significant association between age, playing experience, and tackling proficiency has implications for team selection. Given that tackling proficiency has previously been shown to be significantly associated with the proportion of missed tackles (negative) and dominant tackles (positive) performed during a match (7), coaches may expect superior tackling performances (i.e., fewer missed tackles and more dominant tackles) from older and more experienced players.
Finally, individually, playing experience, skinfold thickness, acceleration, and lower body muscular power explained 14-49% (r = 0.38-0.70) of the variance in tackling ability, and only playing experience and lower body muscular power contributed significantly to the multiple regression model to predict tackling ability. These findings demonstrate that although selected physical qualities are associated with effective tackling ability, other unmeasured qualities (e.g., attitude, mental resilience, coordination) may also contribute to the tackling proficiency regression model.
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