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Strength and Power Characteristics in English Elite Rugby League Players

Comfort, Paul; Graham-Smith, Phillip; Matthews, Martyn J; Bamber, Chris

Journal of Strength and Conditioning Research: May 2011 - Volume 25 - Issue 5 - p 1374-1384
doi: 10.1519/JSC.0b013e3181d687f5
Original Research
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Comfort, P, Graham-Smith, P, Matthews, MJ, and Bamber, C. Strength and power characteristics in English elite rugby league players. J Strength Cond Res 25(5): 1374-1384, 2011-The aim of this article is to present data on the strength and power characteristics of forwards and backs in a squad of elite English rugby league players and compare these findings to previously published literature from Australia. Participants were elite English rugby league players (n = 18; height 184.16 ± 5.76 cm; body mass 96.87 ± 10.92 kg, age 21.67 ± 4.10 years) who were all regular first team players for an English Superleague club. Testing included 5-, 10-, 20-m sprint times, agility, vertical jump, 40-kg squat jump, isometric squat, concentric and eccentric isokinetic knee flexion and extension. Independent t-tests were performed to compare results between forwards and backs, with paired samples t-tests used to compare bilateral differences from isokinetic assessments and agility tests. Forwards demonstrated significantly (p < 0.05) greater body mass (102.15 ± 7.5 kg), height (186.30 ± 5.47 cm), power during the 40-kg jump squat (2,106 ± 421 W), isometric force (3,122 ± 611 N) and peak torque during left concentric isokinetic knee extension (296.1 ± 54.2 N·m) compared to the backs (86.30 ± 8.97 kg; 179.87 ± 3.72 cm; 1,709 ± 286 W; 2,927 ± 607 N; 241.7 ± 35.2 N·m, respectively). However, no significant differences (p > 0.05) were noted between forwards and backs during right concentric isokinetic knee extension (274.8 ± 37.7 and 246.8 ± 25.8 N·m), concentric isokinetic knee flexion for both left (158.8 ± 28.6 and 141.0 ± 22. 7 N·m) and right legs (155.3 ± 22.9 and 128.0 ± 23.9 N·m), eccentric isokinetic knee flexion and extension, hamstring quadriceps ratios, or vertical jump (37.25 ± 4.35 and 40.33 ± 6.38 cm). In comparison, relative measures demonstrated that backs performed significantly better compared to the forwards during the 40-kg jump squat (20.71 ± 5.15 and 19.91 ± 3.91 W·kg−1) and the isometric squat (34.32 ± 7.9 and 30.65 ± 5.34 N·kg−1). Bilateral comparisons revealed no significant differences (p > 0.05) between left and right leg performances in the agility test (3.26 ± 0.18 and 3.24 ± 0.18 seconds), or between left (0.7 ± 0.10) and right (0.71 ± 0.17) leg eccentric hamstring concentric quadriceps ratios. The results demonstrate that absolute strength and power measures are generally higher in forwards compared to in backs; however, when body mass is taken into account and relative measures compared, the backs outperform the forwards.

Directorate of Sports, Exercise and Physiotherapy, University of Salford, Salford, United Kingdom

Address correspondence to Paul Comfort, p.comfort@salford.ac.uk.

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Introduction

Success in rugby league football is highly dependent on physical fitness characteristics including strength, power, speed, and agility (4,5,22,23,30-32). To perform consistently throughout the 10 km covered in the 80-minute game (20,30,31), and recover effectively between bouts of high-intensity exercise, rugby league players must also display a high level of aerobic fitness. This has been highlighted in a review by Gabbett (20) that found average heart rates during competition to be 78, 84, and 93% of maximum for amateur, semiprofessional, and professional athletes, respectively, with average o2max levels of 46.9 ± 5.8 ml·kg−1·min−1-although they have previously been reported to be as high as 56 ml·kg−1·min−1 (8).

Athletes must successfully complete multiple high-intensity short-duration sprints, cutting and turning maneuvers, and up to 40 tackles per game, all requiring high levels of concentric and eccentric force production (8).

The literature provides normative data for sprinting speed and agility (3,6,11,12,16,17,22), strength and power (3,4,7), and a range of other factors including anthropometric and aerobic capacity measurements (9,17,22,31,32) from Australia.

The majority of the literature reporting physiological and physical characteristics of rugby league players report findings for sprint and vertical jump performance across youth and senior teams and from amateur to elite level (Table 1).

Table 1

Table 1

In senior athletes, average times for 10-, 20-, and 40-m range from 1.77, 3.06, and 5.46 seconds, respectively, for amateur rugby league players (19), to 2.06, 3.36, and 5.83 seconds, respectively, for elite professional rugby league players (22). It is interesting to note, however, that the amateur athletes recorded average times that were noticeably faster than those of the elite professional players.

Average vertical jump performance demonstrates even greater variation, from 33.7 cm (16) to 53.6 cm in amateur players (19) and 50.7 cm in elite professional athletes (22).

More recently Baker and Newton (7) reported 1-repetition maximum parallel squat (175 ± 27.3 and 149.6 ± 14.3 kg) and maximal power during a loaded (40-kg) jump squat (1,897 ± 306 and 1,701 ± 187 W) in Australian national rugby league (NRL) and state rugby league (SRL) athletes. The same group of athletes also demonstrated quicker 10-m (NRL 1.61 ± 0.06 seconds, SRL 1.60 ± 0.05 seconds) and 40-m (NRL 5.15 ± 0.24 seconds, SRL 5.13 ± 0.17 seconds) sprint times than was previously reported.

Differences in demands of the game and physical characteristics between playing positions have also been reported. Forwards are involved in significantly more collisions and tackles compared to backs (25) and are also involved in a greater number of high-intensity actions, and cover a greater distance per game (9,929 vs. 8,458 m) than backs (31). Meir et al. (30,31) also concluded that forwards are involved in a greater number of offensive and defensive actions than backs. In contrast, backs tend to perform a greater percentage of their time running in free space (18). Therefore, forwards tend to have a higher body mass but reduced sprint and jumping ability compared to backs (3,7,9,11,16,17,31,32).

Most of the existing literature focuses on the physical characteristics of rugby league players in Australia, from youth to elite professional level (5-7,9,11,15-17,19,22,30-32). There are currently no ‘normative’ data available in the published literature regarding English Rugby League performances. The aim of this article, therefore, is to present strength, power, speed, and agility performance data in elite English rugby league, providing normative data to compare forwards and backs, and to allow comparison with published data on Australian rugby league players.

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Methods

Experimental Approach to the Problem

Assessments of speed, agility, strength, and power were conducted on a group of English elite rugby league players, immediately before the start of the season. Scores were analyzed to determine if there were any differences between forwards and backs and compared to previously published data on elite rugby league athletes.

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Subjects

Eighteen full-time, first team, professional rugby league players (12 Forwards; 6 Backs), competing in the English Superleague, participated in this investigation. All participants were regular first team players of the same rugby league club who performed similar training regimes, with small individual variations in training program to account for playing position and individual physical abilities. Participants had just completed a period of preseason training comprising of a 4-week strength mesocycle, followed by a 4-week power focused mesocycle.

All participants volunteered for the testing as part of their normal training and monitoring regime. Ethical approval was provided by the Institutional Review Board, and all participants provided written informed consent. The mean (SD). height, body mass, and age of the subjects were 184.16 (5.76) cm, 96.87 (10.92) kg, 21.67 (4.10) years, respectively. All subjects were familiar with testing protocols.

Assessments were selected to ensure that each component of the sport (power, speed, strength, agility) was assessed using reliable testing methods that the participants were familiar with. Isokinetic assessment was undertaken to assist with the identification of underlying muscle imbalances, which may increase the risk of lower limb injuries (10,12,14,29,35).

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Testing

Tests were selected to measure speed, agility, strength, power, and lower limb muscle balance. Anthropometric measurements, vertical jump, sprints (5, 10, 20 m) and agility were assessed on the first day, with isometric squat strength, power via squat jumps, and isokinetic strength, during knee flexion and extension, assessed 24 hours later.

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Speed Testing

Participants performed 3 20-m sprints on an indoor track (Mondo, SportsFlex-10 mm; Mondo America Inc, Conshohocken, PA, USA), wearing standard training shoes. Sprints were interspersed with a 1-minute rest period. Time to 5, 10, and 20 m was assessed using a Laveg (Jenoptik Jena, LDM 300C™; Haryana, India) speed gun. All subjects began with their front foot positioned 0.5 m behind the start line and were instructed to perform all sprints with a maximal effort.

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Agility

Agility was assessed via a standardized agility test (Figure 1), performed for both left and right sides on an indoor track (Mondo, SportsFlex-10 mm; Mondo America Inc.), with time to completion measured by timing gates (Newtest Powertimer 1.0 Testing System and Contact Mat; Oulu, Finland) positioned at the start and end of the drill. As with the sprint test agility tests were performed on the indoor track, with all subjects beginning with their front foot 0.5 m behind the start line and were instructed to perform all sprints with a maximal effort.

Figure 1

Figure 1

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Power Testing

To determine maximal power, athletes were asked to perform a countermovement jump and a loaded (40-kg) squat jump. The countermovement jump was performed with hands on hips, on a jump mat (Newtest Powertimer 1.0 Testing System and Contact Mat), with participants performing 3 jumps with 1-minute rest between each attempt.

The 40-kg (Werksan Olympic Bar and discs, Werksan, Moorsetown, NJ, USA) squat jump was performed on a force plate (Kistler, Winterthur, Winterthur, Switzerland, Model 9286AA, SN 1209740), with participants performing 3 maximal efforts with a 1-minute rest between each attempt. Participants were instructed to squat to approximately 45° of knee flexion, pause, and then instructed to jump. Attempts that included an increase in knee flexion before jumping were excluded and participants asked to repeat that attempt. The velocity of movement was measured via differentiation of the acceleration trace and subsequently multiplied with the vertical force trace to obtain instantaneous power in the movement. Peak power in the concentric phase of the movement was determined.

Only 9 of the 12 forwards participated in the squat jump, because of prior injury.

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Strength Testing

Maximal isometric force was assessed via an isometric squat. Participants stood on a force plate (Kistler Model 9286AA, SN 1209740) with the bar of a Smith machine resting on their upper trapezius at a height that resulted in an included knee angle of 135°. The bar was fixed to prevent any movement during the maximal efforts. Participants performed 3 3-second maximal efforts (isometric contractions) with a 1-minute rest between each effort. The peak force generated in the 3 trials was determined.

Isokinetic knee flexion and extension was assessed at 60°·s−1 using an isokinetic dynamometer (Kin-com model AP2, Software version 5.31HS3 Chattanooga Group Inc, Harrison, TN, USA). Participants first performed 3 incremental repetitions of concentric knee flexion and extension as a warm-up. Participants then performed 3 concentric and 3 eccentric maximal efforts for the quadriceps, with a 10-second rest between each effort-adopting the overlay method. After this, hamstrings were assessed using the same procedure as used for the quadriceps. Participants were verbally encouraged to perform maximally throughout the entire range of motion. Testing was repeated 72 hours later to assess test-retest reliability.

Gravity correction was performed, with the weight of the limb taken at full voluntary extension such that the limb was in a position close to horizontal, but without inducing excessive stretch in the hamstrings.

Peak concentric torques for the quadriceps and hamstrings were assessed. To identify possible risk of lower limb injury, peak torque of the hamstrings eccentrically relative to peak torque of quadriceps concentrically was also assessed because it is more representative of the functional roles of these muscles (1,2,26), with the hamstrings co-contracting during knee flexion to minimize both anterior and lateral tibial translation (2,13,28).

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Statistical Analyses

To determine the reliability of each assessment intraclass correlations between trials of each assessment were conducted. Independent t-tests (SPSS version 16.1) were performed to compare results between forwards and backs, with paired samples t-test used to compare bilateral differences from isokinetic assessments and agility tests. The level of significance was determined a priori as p ≤ 0.05.

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Results

Intraclass correlations demonstrated a high level of reliability between trials for the 5-, 10-, and 20-m sprints (r = 0.98, p < 0.001; r = 0.97, p = <0.001; r = 0.98, p < 0.001, respectively), agility drill (r = 0.98, p < 0.001; for both left and right legs), vertical jump height (r = 0.96, p < 0.01), peak power output during the loaded squat jump (r = 0.91, p < 0.01), isometric squat (r = 0.89, p < 0.01), and isokinetic knee flexion (r = 0.995, p < 0.01; r = 0952, p < 0.001) and extension (r = 0.928, p < 0.01; r = 0.937, p < 0.01) for concentric and eccentric measurements, respectively.

Results highlighted significantly greater body mass (p = 0.001) and height (p = 0.02) of forwards (102.15 ± 7.50 kg; 186.30 ± 5.47 cm) compared to backs (86.30 ± 8.97 kg; 179.87 ± 3.72 cm) (Figures 2 and 3).

Figure 2

Figure 2

Figure 3

Figure 3

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Speed and Agility

No significant differences (p > 0.05) were found for sprint times between forwards (1.03 ± 0.03; 1.66 ± 0.20; and 3.00 ± 0.08 seconds) and backs (1.00 ± 0.03; 1.65 ± 0.15; and 2.91 ± 0.10 seconds) across all 3 distances (5, 10, and 20 m), respectively (Figure 4).

Figure 4

Figure 4

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Agility

Backs were significantly faster (p < 0.01) during the agility test compared to forwards for both left (3.10 ± 0.11 and 3.33 ± 0.16 seconds) and right legs (3.07 ± 0.14 and 3.30 ± 0.14 seconds), respectively (Figure 5). The right leg performances (3.24 ± 0.18 seconds) were consistently quicker (0.02 seconds) than left leg performances (3.26 ± 0.18 seconds), although this was not significant (p > 0.05).

Figure 5

Figure 5

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Strength and Power

Backs demonstrated a greater mean vertical jump height (40.33 ± 6.38 cm) compared to the forwards (37.25 ± 4.35 cm), although this was not statistically significant (p > 0.05) (Figure 6).

Figure 6

Figure 6

Forwards demonstrated a significantly (p = 0.049) greater (396-W) peak power output (2,106 ± 421 W), during the 40-kg squat jump compared to the backs (1,709 ± 286 W) (Figure 7).

Figure 7

Figure 7

In contrast to the absolute power output, when relative power is compared, backs exhibit significantly (p = 0.01) greater relative power output (20.71 ± 5.15 W·kg−1) compared to the forwards (19.91 ± 3.91) (Figure 8).

Figure 8

Figure 8

Forwards demonstrated greater mean isometric squat force (3,121 ± 611 N) compared to backs (2,927 ± 607 N), although this was not statistically significant (p > 0.05) (Figure 9).

Figure 9

Figure 9

By contrast, however, when comparisons of relative force are made between forwards and backs, the backs demonstrate significantly (p < 0.001) greater force production relative to body mass (34.32 ± 7.90 N·kg−1) compared to the forwards (30.65 ± 5.34 N·kg−1) (Figure 10).

Figure 10

Figure 10

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Isokinetic Assessment

There was a significantly (p = 0.014) greater (54.4 N·m) concentric peak torque for the forwards' left quadriceps (296.1 ± 54.2 N·m) compared to the backs (241.7 ± 35.2N·m). In comparison, concentric peak torque for the right quadriceps was greater (28.0 N·m) in the forwards (274.8 ± 37.7 N·m) compared to the backs (246.8 ± 25.8), although this was not statistically significant (p > 0.05) (Table 1).

No significant differences (p > 0.05) were noted between forwards and backs for concentric peak torque of the hamstring for left leg (158.8 ± 28.6 and 141.0 ± 22.7 N·m) or right leg (155.3 ± 22.9 and 128.0 ± 23.9 N·m), respectively (Table 2).

Table 2

Table 2

In comparison to the concentric isokinetic data, there were no significant (p > 0.05) differences for eccentric peak torque of the quadriceps or hamstrings between forwards and backs, although there was a trend for higher eccentric peak torque in the forwards (Table 3).

Table 3

Table 3

Statistical analysis revealed no significant differences (p > 0.05) between the eccentric hamstring/concentric quadriceps ratios of forwards (0.7 ± 0.09) and backs (0.7 ± 0.19), or between left (0.7 ± 0.10) and right (0.71 ± 0.17) leg ratios (Figures 11 and 12).

Figure 11

Figure 11

Figure 12

Figure 12

Individual results for eccentric hamstring versus quadriceps, however, illustrate a large variance (0.57-0.96) between players, with 11 athletes demonstrating ratios below the recommended minimum level (0.70) (1) (Figure 13).

Figure 13

Figure 13

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Discussion

Because of the varying roles within the game, forwards and backs were expected to demonstrate a variation in physiological characteristics and physical ability. Data presented above supports this, demonstrating significantly greater body mass (p = 0.001) and height (p = 0.02) of forwards (102.15 ± 7.50 kg; 186.30 ± 5.47 cm) compared to backs (86.30 ± 8.97 kg; 179.87 ± 3.72 cm). This is likely because of selection bias, as the specific requirements of the players in each position dictate that forwards require greater body mass-they are involved in a greater number of offensive and defensive actions (31) resulting in significantly more collisions and tackles compared to backs (25).

English forwards exhibit noticeably greater body mass (5.15 kg) in comparison to data for semiprofessional Australian forwards (102.15 ± 7.50 and 97.0 ± 10.00 kg, respectively) (16). Conversely, semiprofessional Australian backs tend to have a slightly greater body mass (1.7 kg) compared to English backs (88.0 ± 7.0 and 86.3 ± 8.97 kg, respectively) (16). In comparison to the body mass of elite professional Australian players (92.2 ± 11.4 kg) (22), English players exhibit a noticeably greater (4.65 kg) mean body mass (96.87 ± 10.92 kg). However, Baker and Newton (7) have more recently shown a higher body mass (98.2 ± 9.9 kg) in elite Australian (NRL) players, which is slightly higher (1.33 kg) than the mean body mass reported in this study.

Sprint performance, across all 3 distances (5, 10, 20 m), showed no significant differences (p > 0.05) between forwards (1.03 ± 0.03; 1.66 ± 0.20; and 3.00 ± 0.08 seconds) and backs (1.00 ± 0.03; 1.65 ± 0.15; and 2.91 ± 0.10 seconds), which is in contrast to the findings of Gabbett (15-17). However, this may be partly explained by the similarity in training regimes between the athletes in this study.

Sprint times for the English players are faster than the majority of 10-m (range = 1.77-1.83 seconds) and 20-m (range = 3.06-3.14 seconds) sprint times available in the published literature (15-17,19). Moreover the sprint times for elite English players are faster than the times for elite Australian players (22) over both 10 m (1.66 = 0.18 and 2.06 ± 0.18 seconds) and 20 m (2.97 ± 0.09 and 3.36 ± 0.23 seconds). However, in comparison to the 10-m sprint times of NRL and SRL players (1.61 ± 0.06 and 1.60 ± 0.05 seconds) more recently reported by Baker and Newton (7), the English athletes demonstrate similar performances.

The combination of greater body mass in English athletes, along with the similar or faster sprint times, leads to greater momentum, which is likely to result in an increase in impact force during tackles, in turn increasing the difficulty for an opponent to stop or tackle the player. The implications for this would be the requirement for players to condition themselves to accept higher impact forces and offset a possible increase in the risk of collision injury. Furthermore, because of these increased forces, greater focus on eccentric hamstring (via Nordic curls) and quadriceps strength and the ability to decelerate (jump landings and agility drills) is recommended to ensure optimal performance in agility and cutting maneuvers.

Backs demonstrated significantly faster (p > 0.01) performances in the agility test compared to forwards for both left (3.10 ± 0.11 and 3.33 ± 0.16 seconds) and right legs (3.07 ± 0.14 and 3.30 ± 0.14 seconds). Because there was no significant difference (p > 0.05) in sprint ability between forwards and backs, sprint ability cannot account for the differences in agility performance. The backs ability to decelerate a lower body mass more effectively may provide the explanation for improved agility; however, their ability to decelerate was not assessed. These findings are in line with those of previous research, which demonstrated faster performances in backs compared to forwards during the agility test (21).

Backs demonstrated a greater mean vertical jump height (40.3 ± 6.4 cm) compared to the forwards (37.3 ± 4.4 cm), although this was not statistically significant (p > 0.05). This may be because the backs exhibit a significantly (p < 0.01) greater relative power output (20.71 ± 5.15 W·kg−1) compared to the forwards (19.91 ± 3.91 W·kg−1).

In comparison to the most recent Australian amateur (53.6-57.4 cm) (19) and elite professional (50.7 ± 9.8 cm) (22) data, the performance of the English players (38.3 ± 5.1 cm) is dramatically lower (>12 cm). It is worth noting, however, that Gabbett et al. (22) did allow subjects to use their arms during the countermovement. This, and differences in measurement methods used, may account for some of the difference in height jumped.

In contrast to the vertical jump results, the loaded (40-kg) squat jump resulted in a significantly greater (p = 0.049) absolute peak power output (2,105.6 ± 420.9 W) for the forwards compared to the backs (1,709.2 ± 285.5 W). The differences in vertical jump height, therefore, may be a result of the higher mean body mass in the forwards reducing height jumped, which is supported by the fact that backs exhibit significantly (p < 0.01) greater relative power output (20.71 ± 5.15 W·kg−1) compared to the forwards (19.91 ± 3.91), as previously mentioned.

When compared to the peak power outputs of NRL (1,897 ± 306 W) and SRL (1,701 ± 187 W) players reported by Baker and Newton (7), the mean peak power output for the whole squad (1,947 ± 413.2 W) is noticeably higher in the English players.

Forwards demonstrated greater mean isometric squat force (3,121 ± 611 N) compared to backs (2,927 ± 607 N), although this was not statistically significant (p > 0.05). However, in contrast, backs demonstrated significantly (p < 0.001) greater force production relative to body mass (34.32 ± 7.90 N·kg−1) compared to the forwards (30.65 ± 5.34 N·kg−1). Because backs achieved a higher vertical jump, while exhibiting a lower absolute force and absolute peak power output, it is feasible that these differences occurred as a result of lower body mass in the backs, which results in the a higher relative force (34.32 ± 7.90 N·kg−1) and relative power output (20.71 ± 5.15 W·kg−1) compared to the forwards (30.65 ± 5.34 N·kg−1; 19.91 ± 3.91 W·kg−1, respectively).

Even though there were no significant differences (p > 0.05) between the quadriceps hamstring ratios of forwards (0.7 ± 0.09) and backs (0.7 ± 0.19), or between left (0.7 ± 0.10) and right (0.71 ± 0.17) leg ratios, there was a large variance (0.57-0.96) between players. Eleven athletes demonstrated concentric quadriceps eccentric hamstring ratios below the recommended minimum level (0.70) (1). This is most likely because of a training program that appears dominated by movements that target the quadriceps, with a limited volume of eccentric training for the hamstrings. As mentioned previously, it is recommended that English Rugby League teams amend current training practices to increase the focus on eccentric hamstring (via Nordic curls) and quadriceps strength, and the ability to decelerate (jump landings and agility drills). This is particularly relevant considering the high sprint speeds and high body mass exhibited by these players, resulting in greater momentum and the requirement to accept greater braking forces when decelerating and changing direction.

Furthermore, the most common noncontact injuries in rugby league are musculotendinous injuries to the lower limbs (27), with hamstring and groin injuries making up 8.0-19.7% (21,24,33,34) of injuries. Hamstring/quadriceps ratio, therefore, is of great importance as thigh muscle imbalance may increase the risk of hamstring injuries (10,12,14,29,35).

In comparison to the published data from elite Australian athletes (22) regarding Australian rugby league performances, it appears that body mass, sprint speed (10 and 20 m), and peak power output during a 40-kg jump squat are higher in the English athletes in this study. However, when compared to the data from subelite Australian athletes, more recently reported by Baker and Newton (7), mean body mass in the English athletes is slightly lower, with similar 10- and 20-m sprint performances.

Conflicting results appear between the vertical jump performances where English athletes are noticeably out performed by the Australian athletes (19,22), although some of this difference may be related to the differing methods of performing and assessing vertical jump, as previously mentioned. In contrast, peak power output during the 40-kg jump squats is noticeably higher in the English athletes compared to the Australian athletes (7). It is possible that these differences may be because of differences in training methods between English and Australian athletes, where English athletes may have a greater emphasis on strength, as indicated by the high forces generated during the isometric squat, and Australian athletes a greater emphasis on power, resulting in differences in rates of force development and peak power output at different loads.

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Practical Applications

To improve agility performance and reduce the risk of noncontact hamstring injuries, based on the current findings, it is advisable that English Rugby League teams amend current training practices to increase the focus on eccentric hamstring (via Nordic curls) and quadriceps strength and the ability to decelerate (jump landings and agility drills).

It is also advisable for current training practices to be manipulated to increase emphasis on low load (e.g., ≤40% 1repetition maximum during jump squats) high-velocity training, including plyometrics to increase power output at higher velocities, which may increase vertical jump ability, and further enhance sprint ability (>5 m).

It is further suggested that when comparing performance in strength and power output, in impact sports such as rugby league, relative measures may be a better indicator of sports performance than absolute measures.

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Keywords:

performance; isokinetic; isometric; dynamic

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