Rugby league is an intermittent collision sport that involves frequent bouts of high-intensity activity (e.g., sprinting, tackling, and wrestling) with short bouts of low-intensity activity (e.g., walking and jogging) (6). These high-intensity activities are highly dependent on fitness characteristics including strength, speed, and power (4–6,11). Assessing strength, speed, and power allows strength and conditioning practitioners to create a profile of each individual athlete using these characteristics and track progress over time.
In rugby league, forwards and backs need similar functional skills such as tackling, passing, and catching. However, these playing positions differ in physical and physiological characteristics (8,10). Forwards generally carry a greater body mass and have greater strength most likely because of their position in the middle of the field, thus having to spend more time in tackling the opposition (3,8,11). Conversely, it has been shown that backs can generally jump higher and sprint faster, most likely because of their position on the edge of the field where these attributes serve an important purpose on offense and defense (3,11). To date, there is minimal research comparing strength, speed, and power characteristics between forwards and backs in elite rugby league. The existing literature has only published findings from simple field tests such as sprint times and vertical jump. Furthermore, there is very little research to date using isokinetic dynamometry at the knee and hip joints and instrumented treadmills for assessing sprint kinetics in elite rugby league (3,11). Such information is vital for gaining a more accurate understanding of the positional differences between forwards and backs. This could potentially inform coaches of suitable positions for up and coming rugby league players and influence strength and conditioning coaches in how they program for positional and individual differences in rugby league. Therefore, the purpose of this study was to compare strength, speed, and power characteristics between playing position (forwards and backs) in elite rugby league players.
Experimental Approach to the Problem
This cross-sectional analysis investigated player position differences (i.e., forwards vs. backs) for isokinetic hip and knee strength, sprint kinetics and kinematics on a nonmotorized treadmill, and sprint times during overground sprinting with and without external resistance. Testing was completed during the National Rugby League (NRL) off-season.
Thirty-nine professional NRL players (mean ± SD: height = 183.8 ± 5.95 cm; body mass = 100.3 ± 10.74 kg; age = 24 ± 3 years) volunteered as participants for this research. Over the last 2 seasons, this team has placed second and 14th in the NRL competition. Of the 39 subjects, 22 were forwards and 17 were backs, and a total of 12 competed internationally during the 2013 Rugby League World Cup in the United Kingdom and France. All participants were free from any injury that would prevent maximal efforts during testing. The procedures for this study were approved by the Auckland University of Technology Ethics Committee. Subjects were informed of risks and benefits of participation in the study and signed informed consent.
Before testing, all subjects completed a standardized warm-up and familiarization protocol, which consisted of 3 movements at an individually perceived 50, 70, and 90% of maximum exertion (2). After warm-up, participants were fastened to a Humac Norm dynamometer (Lumex, Ronkonkoma, NY, USA) to assess isokinetic concentric knee and hip extensor and flexor strength on the dominant leg at 100 Hz. The leg that the player preferred to kick the ball was defined as the dominant leg (1). The dynamometer was set up in 2 separate positions using a standardized protocol for knee and hip actions (12). The dominant leg was tested at a fixed angular velocity of 60°·s−1 for 5 extension and 5 flexion actions (9). A custom-made LabVIEW program (Version 11.0; National Instruments Corp., Austin, TX, USA) was used to fit the torque-angle curves with a fourth order polynomial to identify peak torque using the average of the last 4 repetitions for the final value (2). Peak flexion torque was divided by the peak extension torque to determine hamstring to quadriceps ratios (H/Q ratio).
A nonmotorized treadmill (NMT) (Woodway, Force 3.0; Waukesha, WI, USA) was used to measure maximum velocity sprint kinetics and kinematics. After a warm-up, the subjects performed maximum effort sprints on the NMT. The NMT was instrumented with 4 embedded vertical load cells mounted under the running surface, and a mounted horizontal load cell connected to a nonelastic tether and waist harness. Subjects were instructed to sprint maximally from a standing start and to reach and maintain maximum velocity for greater than 5 seconds. The data were collected at a sampling rate of 200 Hz allowing collection of vertical forces (Fv), horizontal forces (Fh), and power (P). The recorded data were filtered with a fourth order, low-pass Butterworth filter with a cutoff frequency of 50 Hz. Power was calculated as horizontal force multiplied by sprint velocity. Peak values of force and power over 10 steps at constant maximum velocity were averaged for a final value. Contact times (Ct) and flight times (Ft) were also averaged over the 10 steps. Contact times were determined from the time of force applied to the treadmill was greater than 0 N, whereas Ft were determined from the time between the ground contact periods. Stride frequency was determined from: 1/(Ct + Ft), whereas stride length was determined from: running velocity/step frequency. The subjects completed 2 trials to determine within-session reliability for all variables on the NMT. The reliability of this testing in our laboratory was moderate-to-high with interclass coefficient correlation ranging between 0.79 and 0.97 and coefficient of variation ranging between 1.4 and 4.7%.
Sprint times were assessed using timing lights (Fusion SmartSpeed Timing Gates System, Queensland, Australia). After a thorough warm-up of dynamic flexibility and running at progressively faster speeds, participants completed two 40-m sprints through the timing lights with at least 2 minutes of passive recovery. Participants were instructed to start the sprint from a standing split stance (left leg forward) and to maintain their effort past the last timing light. Sprint times were collected at 10 and 40 m.
All data are reported as mean and SD, and statistical significance was set at p ≤ 0.05. Independent t-tests were used to determine differences between the forwards and backs for all data. Significant differences were further analyzed using effect size (ES). Effect size of <0.2, <0.6, <1.2, and >1.2 were considered trivial, small, moderate, and large, respectively (7).
Significant differences were found between forwards and backs in the following tests and variables: body mass, 10-m and 40-m sprint times, relative horizontal force, and relative power (Table 1). Forwards were significantly heavier than backs (+9%; ES = 0.98). Backs were significantly faster over 10 m (+3.6%; ES = 1.26) and 40 m (+5.3%; ES = 1.61) compared with forwards. Furthermore, when horizontal force and power were made relative to body mass, backs displayed significantly greater values than forwards (+23.3%; ES = 0.87; +25.4%; ES = 1.04, respectively).
The results of this investigation confirmed that there are moderate-to-large differences in selected physical qualities between positions in elite rugby league players. Backs, observed to have significantly lighter body mass (ES = 0.98), were significantly faster (10-m ES = 1.26; 40-m ES = 1.61) and produced significantly greater relative horizontal force and power (ES = 0.87 and 1.04) compared with forwards. Forwards were found to be generally taller and significantly heavier than their back counterparts. However, no significant differences were found between forwards and backs during relative isokinetic knee extension, knee flexion, relative isokinetic hip extension, flexion, sprint velocity, contact time, or flight time. These physical differences are likely due to selection bias and the demands of the separate positions. Forwards are involved in significantly more tackles than backs and travel slightly less distance per game (8,10). The greater body mass of forwards allows them to win the collision; and thus, potentially restart play before the opposition defensive line is ready. However, the lighter backs average significantly greater sprinting distances and durations at high velocity during a rugby league match (10).
Significant differences in sprinting performance over 10 and 40 m were found between forwards and backs (Table 1). Backs were significantly faster than forwards, which aligns with the findings of Meir et al. (11) on professional rugby league players but contrasts with Comfort et al. (3) where the authors found no significant differences between forwards and backs during the sprint. This may have been because of the different equipment used in measuring time over the sprint. This study and Meir et al. (11) used timing lights, whereas Comfort et al. (3) used a speed gun.
Although no significant differences were observed in vertical force or sprint kinematics, backs showed significantly faster 10- and 40-m sprint times overground. Significant differences in relative horizontal force (ES = 0.87) and relative power (ES = 1.04) were also found, and these factors may be the contributing factors to the faster sprint times recorded by the backs. Morin et al. (13) reported that maximal power was found to be strongly correlated with maximal speed and 4-s distance (r = 0.863 and 0.892, respectively). Furthermore, the authors found a significant correlation between the index of force application and maximal speed and 4-s distance (r = 0.875 and 0.683, respectively). Thus, it could be suggested that the magnitude of force and power produced in the intended direction of movement has an influence on performance. Additionally, this study found that athletes who can display high levels of horizontal force and power relative to body mass had superior sprint times.
There were no significant differences in relative isokinetic values between forwards and backs. Comfort et al. (3) also observed similar findings in isokinetic strength in elite English rugby league players. However, sprint times and sprint kinetics were different. Thus, despite similar strength levels, faster athletes were able to produce more force in the ground in the horizontal direction. This finding suggests that with relative force being equal between athletes, further improving horizontal force production may potentially improve sprint performance.
Backs were faster than forwards over 10 and 40 m; and furthermore, backs produced greater relative horizontal force and power while no other variables were found to be significantly different. These findings suggest that developing rugby league players with greater horizontal force and greater sprinting abilities may be best suited to positions in the backs. The magnitude of force produced may not be as important as the direction the force is applied in during short distance sprinting. Thus, for backs to improve sprinting performance and therefore potentially on field performance, developing force and power capabilities in the horizontal direction may be beneficial in addition to improving overall force and power capabilities. Certain loaded and unloaded movements can be used to develop horizontal force and power. For example, some exercises to develop horizontal force are hip thrusts, back extensions, reverse hypers, heavy sled drags, and heavy sled pushes. Horizontal power can be developed using kettlebell swings and various weighted ball throws. Plyometric exercises such as broad jumps and bounding also assist in developing force and power in the horizontal direction.
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