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Original Research

Influence of Fatigue on Tackling Technique in Rugby League Players

Gabbett, Tim J

Author Information
Journal of Strength and Conditioning Research: March 2008 - Volume 22 - Issue 2 - p 625-632
doi: 10.1519/JSC.0b013e3181635a6a
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Abstract

Introduction

Rugby league is an international collision sport played by sub-elite and elite competitors (17). 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 has an immediate play-the-ball after each tackle (17). A typical senior rugby league match is 80 minutes in duration, requiring 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, players are exposed to multiple physical collisions and tackles (17). As a result, musculoskeletal injuries are extremely common (14).

The majority of rugby league injuries occur in the tackle (8,13,15,16,20,21) and during the second half of matches (7), suggesting that player fatigue may contribute to tackle-related injuries in rugby league (7,9,23). In addition, it has been shown that players with a low maximal aerobic power are at an increased risk of sustaining a tackle-related injury (18). These findings suggest that a high aerobic fitness may protect against fatigue-related tackling injuries in rugby league players. However, despite the evidence that fatigue may contribute to injuries in rugby league, it has also been suggested that fatigue may play a protective role against injury by reducing match speed and impact forces associated with physical collisions and tackles (16).

Success in rugby league is dependent, at least in part, on tackling ability, the ability to tolerate physical collisions, and the ability to “win” the tackle contest. However, an objective method of assessing tackling technique in rugby league players is currently not available. In addition, while there is evidence that fatigue may contribute to injuries in rugby league (7,9,23), no study has demonstrated a reduction in tackling proficiency under fatigued conditions. With this in mind, the purpose of this study was to investigate the influence of fatigue on tackling technique in rugby league players. In addition, the relationship between selected physiological capacities and fatigue-induced decrements in tackling technique was also determined.

Methods

Experimental Approach to the Problem

The present study used a repeated-measures design to investigate the influence of progressive increases in game-specific fatigue on tackling technique in rugby league players. In addition, Pearson product moment correlation coefficients were used to determine the relationship between selected physiological capacities and decrement in tackling technique under fatigued conditions. It was hypothesized that fatigue would elicit reductions in tackling technique and that the greatest fatigue-induced decrements in tackling technique would occur in players with the poorest physiological capacities.

Subjects

Eight rugby league players (mean ± SE age, 23.0 ± 1.2 years) participated in this study (Table 1). All subjects were first-grade players from the same rugby league club. The club was the reigning premiers in the Gold Coast Rugby League (Queensland, Australia) competition. Players had undergone a 6-month conditioning program, participated in 12 competitive matches, and were in peak physical condition at the time of testing. All players refrained from strenuous exercise for at least 48 hours before testing. Informed written consent for testing was obtained from each player before participation.

T1-41
Table 1:
Physical, physiological, and anthropometric characteristics of rugby league players.

Tackling Technique

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. Players performed 6 trials each in the one-on-one tackling drill. Tackling technique was objectively 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.

The technical criteria included (a) accelerating into the contact zone, (b) contacting the target in the center of gravity, (c) contacting the target with the opposite shoulder to leading leg, (d) body position square/aligned, (e) arms wrapping around the target on contact, (f) leg drive on contact, (g) watching the target onto the shoulder, and (h) center of gravity forward of base of support.

Players were awarded 1 point for each occasion in which they achieved the relevant criteria and a score of 0 if they failed to achieve the criteria. A total score (out of 6 and reported in arbitrary units) was awarded for each of the criteria (excluding accelerating into the contact zone). In addition, a total tackling technique score (also reported in arbitrary units) was awarded based on the aggregate of all technical criteria (excluding accelerating into the contact zone). The intraclass correlation coefficient for test-retest reliability and typical error of measurement for assessments of individual tackling technique criteria ranged from 0.76 to 0.99 and 4.7% to 8.7%, respectively, while the intraclass correlation coefficient for test-retest reliability and typical error of measurement for total tackling technique was 0.82 and 6.7%, respectively. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for measures of acceleration into the contact zone was 0.97 and 4.1%, respectively.

Repeated-Effort Protocol

Players performed the one-on-one tackling drill before strenuous exercise and after game-specific, repeated-effort exercise of progressively increasing intensities (corresponding to moderate, heavy, and very heavy intensity) in order to induce fatigue that was representative of match conditions (35). Given that the longest period of repeated-effort, high-intensity exercise in rugby league may last as long as 35 seconds and include high-intensity sprinting, tackling, and lateral movements (24), the repeated-effort protocol used 3 sets of 6 repetitions on a repeated-effort tackle test and was designed to reflect the most extreme demands of competition. The repeated-effort protocol was 30-45 seconds in duration, with rest periods between sets manipulated to control the level of exertion. Rest durations of 40, 20, and 10 seconds between sets were used to simulate moderate, heavy, and very heavy exercise, respectively. The rest periods reflected specific activities that players were likely to experience during game-specific situations, such as resuming play from an in-goal drop-out (approximate recovery of 40 seconds), resuming play from a scrum (approximate recovery of 20 seconds), and resuming play immediately after a skill error (e.g., dropped ball) where the opposition have gained possession of the ball (approximate recovery of 10 seconds or less).

The intensity of exercise was verified by measuring total repeated effort time, heart rate, blood lactate concentration, and ratings of perceived exertion. Heart rate was recorded using Polar S610i heart rate monitors. Once the raw data were collected, it was downloaded to a computer using Polar Precision Performance software. Blood lactate concentration was measured from the earlobe before exercise and at the completion of each bout of exercise using a Lactate Pro portable lactate analyzer (Arkray Inc., Kyoto, Japan). Perceived effort was recorded at the completion of each bout of exercise using a 6-20 ratings of perceived exertion scale (2).

Fitness Testing Battery

All fitness testing was conducted 1 week after the tackle assessment. Players were well motivated, free from injury, and in peak physical condition at the time of testing. The coach of the squad stated that he was prepared to devote 1 training session (approximately 2 hours) to the field testing. Although consideration was given to the specificity of the field test, the selection of tests included in the field testing battery was influenced by this time constraint. Standard anthropometry (height, body mass, and sum of 7 skinfold measurements) (32), muscular power (vertical jump) (10), speed (10-, 20-, and 40-m sprint) (11), agility (L run) (36), and maximal aerobic power (multistage fitness test) (34) were the fitness tests selected. At the beginning of the field testing session, anthropometric measurements were taken for each subject. After anthropometric measurements were taken, subjects underwent a standardized warm-up (which included low-intensity running activities and moderate- to high-intensity striding and sprinting activities) and stretching routine. Players were then randomly allocated into 3 groups consisting of approximately equal numbers. Players in group 1 underwent measurements of muscular power (vertical jump), while players in groups 2 and 3 underwent agility (L run) and speed (10-, 20-, and 40-m sprint) measurements, respectively. To encourage a maximal effort, players received verbal encouragement throughout all physiological tests. All tests were conducted on a firm, well-grassed surface. On completion of the respective tests, each group rotated until all tests had been performed. Following the speed, agility, and muscular power tests, the field testing session concluded with players performing the multistage fitness test (estimated maximal aerobic power).

Anthropometry

Excess body mass and body fat have been shown to negatively influence performance (e.g., power-to-body mass ratio, thermoregulation, and aerobic capacity) (32). As an estimate of adiposity, skinfold thickness was measured at 7 sites using a Harpenden skinfold caliper. Biceps, triceps, subscapular, suprailiac, abdomen, thigh, and calf were the sites selected. The exact positioning of each skinfold measurement was in accordance with procedures described by Norton et al. (32). Height was measured using a stadiometer, 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 height, body mass, and sum of 7 skinfold measurements were 0.99, 0.99, and 0.99 and 0.2%, 0.8%, and 3.0%, respectively.

Muscular Power

The ability to generate high levels of muscular power is an important attribute of rugby league players. Players are required to have high levels of muscular power in order to effectively perform the tackling, lifting, pushing, and pulling tasks that occur during a match (31). In addition, high levels of muscular power are required to provide fast play-the-ball speed and leg drive in tackles. Lower body muscular power was estimated by means of the vertical jump test (10) using a Yardstick vertical jump device (Swift Performance Equipment, New South Wales, Australia). Players were requested to stand with feet flat on the ground, extend their arm and hand, 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. No specific instructions were given regarding the depth or speed of the countermovement. Vertical jump height was calculated as the distance from the highest point reached during standing and the highest point reached during the vertical jump. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the vertical jump test was 0.96 and 3.3%, respectively.

Speed

Rugby league players require the ability to move quickly in order to position themselves in attack and defense (31). However, time-motion studies have shown that rugby league players are rarely required to sprint distances greater than 40 m in a single bout of intense activity (30). The running speed of players was evaluated with a 10-, 20-, and 40-m sprint effort (11) using dual-beam electronic timing gates (Swift Performance Equipment, New South Wales, Australia). The timing gates were positioned 10, 20, and 40 m cross wind from a predetermined starting point. Players were instructed to run as quickly as possible along the 40-m distance from a standing start. Subjects started from a stationary, upright position with the front foot on the 0-m point, in line with the start gate. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the 10-, 20-, and 40-m sprint tests were 0.95, 0.97, and 0.97 and 1.8%, 1.3%, and 1.2%, respectively.

Agility

Rugby league players require the ability to rapidly accelerate, decelerate, and change direction (31). The agility of players was evaluated using the L run (36) using dual-beam electronic timing gates (Swift Performance Equipment). Three cones were placed 5 m apart in the shape of an L. Players were instructed to run as quickly as possible along the agility run. Agility times were measured to the nearest 0.01 second with the fastest value obtained from 3 trials used as the agility score. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the L run was 0.84 and 1.9%, respectively.

Maximal Aerobic Power

Rugby league matches last 80 minutes, with players covering up to 10 km in a match (30). Players also require high levels of aerobic fitness to aid recovery after high-intensity bouts of activity. Maximal aerobic power was estimated using the multistage fitness test (34). Players were required to run back and forth (shuttle run) along a 20-m track, keeping in time with a series of signals on a compact disk. The frequency of the audible signals (and hence running speed) was progressively increased until subjects reached volitional exhaustion. Maximal aerobic power (o2max) was estimated using regression equations described by Ramsbottom et al. (34). The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the multi-stage fitness test were 0.90 and 3.1%, respectively.

Statistical Analyses

Influence of Fatigue on Tackling Technique

Differences in repeated-effort time, heart rate, blood lactate concentration, ratings of perceived exertion, and tackling technique across the 4 conditions (rest, moderate, heavy, and very heavy fatigue) were assessed using a repeated-measures analysis of variance. When required, comparisons of group means were performed using a Tukey honestly significant difference post hoc test. In addition, the influence of fatigue on tackling technique was analyzed using Cohen's effect size (ES) statistic (3). Effect sizes of 0.2, 0.5, and 0.8 were considered small, moderate, and large, respectively. Finally, the influence of fatigue on tackling technique was analyzed by comparing the true change in tackling technique with the minimum clinically important difference in tackling technique. The minimum clinically important difference was defined as the smallest worthwhile change deemed to be practically significant to the average athlete (22). The probabilities that the true difference in tackling technique were negative, trivial, or positive were expressed as percentages, reflecting the following descriptors: <1%, almost certainly not; 1-5%, very unlikely; 5-25%, unlikely; 25-75%, possibly; 75-95%, likely; 95-99%, very likely; >99%, almost certainly (1,22).

Relationship Between Physical Fitness and Fatigue-Induced Decrements in Tackling Technique

The Pearson product moment correlation coefficient was used to determine the relationship between the physical (age), anthropometric (height, skinfold thickness, and body mass), and physiological (vertical jump, agility, speed, and estimated maximal aerobic power) characteristics of players, and decrement in tackling technique under fatigued conditions. In addition, the relationship between tackling technique under nonfatigued conditions and tackling technique under fatigued conditions was assessed using the Pearson product moment correlation coefficient.

Based on an α level of 0.05 and a sample size of 8, the β level (power) was ≥0.80 for detecting effect sizes of 5 arbitrary units in technical proficiency, and 1.0 m·s−2 in acceleration. All data are reported as mean ± SE.

Results

Physiological Responses

A progressive increase in total repeated-effort time (ES = 0.4-1.2), heart rate (ES = 7.8-11.2), blood lactate concentration (ES = 7.9-9.5), and ratings of perceived exertion (ES = 7.5-61.8) occurred throughout the repeated-effort protocol, demonstrating a progressive increase in intensity and fatigue (Table 2).

T2-41
Table 2:
Physiological responses during the repeated-effort protocol.

Tackling Technique

Fatigue resulted in progressive reductions in tackling technique/proficiency (Figure 1). The most notable changes in tackling technique were a decreased acceleration into the contact zone (ES = 4.2), a decreased ability to contact the target in the center of gravity (ES = 0.4), a failure to wrap arms around the target on contact (ES = 1.7), a decreased alignment of the body (i.e., body not square/aligned) (ES = 0.4), and a decreased leg drive on contact (ES = 2.3) (Table 3).

T3-41
Table 3:
Tackling technique before strenuous exercise and after game-specific repeated-effort exercise of progressively increasing intensities.
F1-41
Figure 1:
Influence of fatigue on tackling technique. *Different from rest. †Different from moderate fatigue. AU = arbitrary units. Data are aggregate scores from all technical criteria (excluding acceleration data) and are reported as mean ± SE.

Relationship Between Physical Fitness and Fatigue-Induced Decrements in Tackling Technique

The physiological and anthropometric characteristics of the rugby league players are shown in Table 1. Players with the best tackling technique in a nonfatigued state demonstrated the greatest decrement in tackling technique under fatigued conditions (r = 0.71-0.98) (Table 4, Figure 2). In addition, a significant association (P < 0.05) was observed between estimated o2max (r = −0.62) and agility (r = 0.68) and the decrement in tackling technique under fatigue conditions (Table 5).

T4-41
Table 4:
Relationship between tackling technique before strenuous exercise and decrement in tackling technique under fatigued conditions.
T5-41
Table 5:
Relationship between selected physiological capacities and decrement in tackling technique under fatigue conditions.
F2-41
Figure 2:
Comparison of tackling technique between two players. Good tackling technique represents the player with the best tackling technique under nonfatigued conditions, while poor tackling technique represents the player with the worst tackling technique under nonfatigued conditions. Note that while the relative decrement in tackling technique was greatest in the player with the greatest initial tackling technique, absolute tackling proficiency under fatigue was still better in the most skilled player. AU = arbitrary units. Data are aggregate scores from all technical criteria (excluding acceleration data).

Discussion

The purpose of this study was to develop an objective method of assessing tackling technique in rugby league players and to determine the effect of fatigue on tackling technique in these athletes. The results of this study demonstrate that fatigue results in progressive reductions in tackling technique, with players with the best tackling technique in a nonfatigued state experiencing the greatest decrement in tackling technique under fatigued conditions. These findings demonstrate that an effective tackling technique under nonfatigued conditions does not necessarily transfer to effective tackling technique under fatigued conditions. Clearly, any defensive drills designed to improve tackling technique should be performed before and under fatigue.

The heart rate and blood lactate concentration recorded during the repeated effort protocol were similar to those previously reported during rugby league competition (4,6,12,17,33). In addition, the repeated-effort protocol specifically simulated the most extreme demands that players were likely to experience during game-specific situations (24). Therefore, it is likely that the reductions in tackling technique with fatigue are indicative of the type of changes that would occur under match conditions. The finding that fatigue reduces tackling technique provides some explanation for the high incidence of fatigue-related tackling injuries in rugby league players (7,9,23).

The finding that fatigue reduced tackling technique is consistent with previous studies that have demonstrated a reduction in technical skill under fatigued conditions (5,25-29,35). Royal et al. (35) reported reduced technical proficiency during water polo goal shooting in the fatigued state. Interestingly, these authors also reported unchanged shooting accuracy and improved decision-making ability with progressively increasing levels of fatigue (35). In the present study, only the technical components and not the outcome (i.e., whether the tackle was “won” by the attacking or defensive player) of the tackle were assessed. In addition, perceptual skill, where defending players are required to anticipate and rapidly respond to the movements of attacking players in a one-on-one tackle situation was not assessed. Poor agility was also significantly associated with fatigue-induced decrements in tackling technique, suggesting that one-on-one tackling performance is dependent, at least in part, on the attacking abilities of the ball carrier. Indeed, from an attacking perspective, evasive skills, particularly in the one-on-one tackling contest, are the key playing skills that discriminate successful from less successful rugby league players (19). Given that fatigue elicits reductions in tackling technique (and defensive abilities), it is conceivable that a similar magnitude of fatigue could be experienced by attacking players during the course of a match, resulting in similar reductions in attacking skill. Future studies investigating the influence of fatigue on playing skill in rugby league players should also include technical assessments from an offensive perspective. In addition, studies on the outcome of the tackle contest (i.e., whether the tackle was “won” by the attacking or defensive player), and decision-making ability in the tackle are warranted.

The present study found that a high o2max was associated with smaller fatigue-related decrements in tackling technique. Previous rugby league injury studies have reported an increased risk of tackle injuries in players with a low o2max (18). The present results coupled with previous findings (18) suggest that high aerobic fitness may reduce fatigue-induced decrements in tackling technique and consequently reduce fatigue-related tackling injuries. There was also a trend toward greater fatigue-related decrements in tackling technique in players with lower muscular power. Given that muscular power is critical for providing leg drive in tackles and that forceful leg drive on contact was a key technical criterion within the tackle assessment, this trend between muscular power and fatigue-related decrements in tackling technique may be expected. Clearly, strength and conditioning programs designed to develop muscular power and endurance may reduce decrements in tackling technique with fatigue, while also facilitating the development of physical capacities that are necessary for effective playing performance. The ability to express this muscular power under game-specific fatigue also appears critical to preventing fatigue-induced decrements in tackling technique.

The finding that fatigue-induced decrements in tackling technique were greatest in players with the best tackling technique in a nonfatigued state could possibly reflect the higher initial tackling proficiency of the skilled players. Indeed, a better tackling technique in a nonfatigued state provides a greater reserve for reductions in technical skill as a result of fatigue. However, while the relative decrement in tackling technique was greatest in players with the greatest initial tackling technique, absolute tackling proficiency under fatigued conditions was still better in the most skilled players (Figure 2). Therefore, in terms of talent identification and player selection, coaches may benefit from selecting players with better tackling technique in a nonfatigued state, because (a) these players are able to demonstrate a higher tackling proficiency during the early (higher intensity) stages of matches and (b) despite having a higher relative decrement in tackling technique, their absolute tackling technique under fatigued conditions is still higher than that demonstrated in lower skilled players.

In general, there was a wide range of responses to the one-on-one tackle test. While players were able to excel on some of the technical criteria (e.g., contacting the target in the center of gravity), the same players were unable to successfully perform other technical criteria (e.g., leg drive on contact, or maintain a square/aligned body position). These findings suggest that the ability to effectively perform tackles is limited by different factors for different players. Coaches should address these factors with individual players on a case-by-case basis.

In conclusion, this study developed an objective method to assess tackling technique in rugby league players, and investigated the effect of fatigue on tackling technique in these athletes. The results of this study demonstrate that fatigue results in progressive reductions in tackling technique, with players with the best tackling technique in a nonfatigued state demonstrating the greatest decrements in tackling technique under fatigued conditions. These findings demonstrate that an effective tackling technique under nonfatigued conditions does not necessarily transfer to effective tackling technique under fatigued conditions. Clearly, any defensive drills designed to improve tackling technique should be performed before and under fatigue.

Practical Applications

There are several practical applications from this study that are relevant to the strength and conditioning coach. It is well documented that the majority of rugby league injuries occur in the tackle (8,13,15,16,20,21), and under fatigued conditions (7,9,23). If the strength and conditioning coach can minimize the fatigue associated with game-specific repeated efforts, they may also minimize fatigue-induced reductions in tackling technique and subsequently reduce the incidence of tackle-related injuries.

In the present study, a direct relationship was observed between physiological capacities and fatigue-induced decrements in tackling technique, with players with poorer physiological capacities demonstrating the greatest fatigue-induced decrements in tackling technique. Strength programs designed to improve structural stability may allow rugby league players to maintain body alignment and tolerate the impact forces associated with physical collisions. Improving endurance, muscular power, and agility may also minimize the fatigue associated with high-intensity repeated efforts, thereby improving performance in the tackle contest (e.g., improving both anticipation and leg drive in tackles) and reducing fatigue-related tackling injuries. Finally, players with the best tackling technique in a nonfatigued state experienced the greatest decrement in tackling technique under fatigued conditions. These findings demonstrate that an effective tackling technique under nonfatigued conditions does not necessarily transfer to effective tackling technique under fatigued conditions. Clearly, any defensive drills designed to improve tackling technique should be performed before and under fatigue in order to facilitate the development of skills under fatigued conditions.

Note. At the time of this study, the author was employed by the Queensland Academy of Sport, Brisbane, Australia.

References

1. Batterham, AM and Hopkins, WG. Making meaningful inferences about magnitudes. Int J Sports Physiol Perform 1: 50-57, 2005.
2. Borg, GA. The psychophysical basis of perceived exertion. Med Sci Sports Exerc 14: 377-381, 1982.
3. Cohen, J. Statistical Power Analysis for the Behavioral Sciences (2nd ed). Hillsdale, NJ: Lawrence Erlbaum, 1988.
4. Coutts, A, Reaburn, P, and Abt, G. Heart rate, blood lactate concentration and estimated energy expenditure in a semi-professional rugby league team during a match: a case study. J Sports Sci 21: 97-103, 2003.
5. Davey, PR, Thorpe, RD, and Williams, C. Fatigue decreases skilled tennis performance. J Sports Sci 20: 311-318, 2002.
6. Estell, J, Lord, P, Barnsley, L, Shenstone, B, and Kannangara, S. The physiological demands of rugby league. In: Proceedings of the Australian Conference of Science and Medicine in Sport. Canberra: Sports Medicine Australia, 1996. pp. 388-389.
7. Gabbett, TJ. Incidence, site, and nature of injuries in amateur rugby league over three consecutive seasons. Br J Sports Med 34: 98-103, 2000.
8. Gabbett, TJ. Severity and cost of injuries in amateur rugby league: a case study. J Sports Sci 19: 341-347, 2001.
9. Gabbett, TJ. Incidence of injury in amateur rugby league sevens. Br J Sports Med 36: 23-26, 2002.
10. Gabbett, TJ. Influence of physiological characteristics on selection in a semi-professional rugby league team: a case study. J Sports Sci 20: 399-405, 2002.
11. Gabbett, TJ. Physiological characteristics of junior and senior rugby league players. Br J Sports Med 36: 334-339, 2002.
12. Gabbett, TJ. Do skill-based conditioning games simulate the physiological demands of competition? Rugby League Coaching Magazine 32: 27-31, 2003.
13. Gabbett, TJ. Incidence of injury in semi-professional rugby league players. Br J Sports Med 37: 36-43, 2003.
14. Gabbett, TJ. Incidence of injury in junior and senior rugby league players. Sports Med 34: 849-859, 2004.
15. Gabbett, TJ. Influence of training and match intensity on injuries in rugby league. J Sports Sci 22: 409-417, 2004.
16. Gabbett, TJ. Influence of the limited interchange rule on injury rates in sub-elite rugby league players. J Sci Med Sport 8: 111-115, 2005.
17. Gabbett, TJ. Science of rugby league football: a review. J Sports Sci 23: 961-976, 2005.
18. Gabbett, TJ and Domrow, N. Risk factors for injury in sub-elite rugby league players. Am J Sports Med 33: 428-434, 2005.
19. Gabbett, T, Kelly, J, and Pezet, T. Relationship between physical fitness and playing ability in rugby league players. J Strength Cond Res 21: 1126-1133, 2007.
20. Gissane, C, Jennings, DC, and Standing, P. Incidence of injury in rugby league football. Physiotherapy 79: 305-310, 1993.
21. Gissane, C, Jennings, D, Kerr, K, and White, J. Injury rates in rugby league football: impact of change in playing season. Am J Sports Med 31: 954-958, 2003.
22. Hopkins, WG. Probabilities of clinical or practical significance. Sportscience 6. 2002. Available at: sportsci.org/jour/0201/wghprob.htm.
23. King, DA, Gabbett, TJ, Dreyer, C, and Gerrard, DF. Incidence of injuries in the New Zealand national rugby league sevens tournament. J Sci Med Sport 9: 110-118, 2006.
24. King, T, Jenkins, D, Gabbett, T, and Benton, D. Repeated high-intensity efforts in professional rugby league football. In: Proceedings of the Australian Institute of Sport, Team Sport Conference. Canberra: Australian Sports Commission, 2006.
25. Marriot, J, Reilly, T, and Miles, A. The effect of physiological stress on cognitive performance in simulation of soccer. In: Science and Football II. Reilly, T, Clarys, J, and Stibbe, A, eds. London: E. &. F.N. Spon, 1993. pp. 261-264.
26. McMorris, T and Graydon, J. The effect of exercise on the decision-making performance of experienced and inexperienced soccer players. Res Q Exerc Sport 67: 109-114, 1996.
27. McMorris, T and Graydon, J. Effect of exercise on soccer decision-making tasks of differing complexities. J Hum Mov Stud 30: 177-193, 1996.
28. McMorris, T and Graydon, J. The effect of exercise on the decision making performance of college soccer players. In: Science and Football III. Reilly, T, Bangsbo, J, and Hughes, M, eds. London: E. & F.N. Spon, 1997. pp. 279-284.
29. McMorris, T and Graydon, J. The effect of exercise on cognitive performance in soccer-specific tests. J Sports Sci 15: 459-468, 1997.
30. Meir, R, Colla, P, and Milligan, C. Impact of the 10-meter rule change on professional rugby league: Implications for training. Strength Cond J 23: 42-46, 2001.
31. Meir, R, Newton, R, Curtis, E, Fardell, M, and Butler B. Physical fitness qualities of professional rugby league players: determination of positional differences. J Strength Cond Res 15: 450-458, 2001.
32. Norton, K, Marfell-Jones, M, Whittingham, N, Kerr, D, Carter, L, Saddington, K, and Gore, C. Anthropometric assessment protocols. In: Physiological Tests for Elite Athletes. CJ Gore, ed. Champaign, IL: Human Kinetics, 2000. pp. 66-85.
33. O'Connor, D. Blood lactate concentrations measured during competitive games throughout a professional rugby league season. J Sports Sci 22: 556-557, 2004.
34. Ramsbottom, R, Brewer, J, and Williams, C. A progressive shuttle run test to estimate maximal oxygen uptake. Br J Sports Med 22: 141-144, 1988.
35. Royal, KA, Farrow, D, Mujika I, Halson, SL, Pyne, D, and Abernethy, B. The effects of fatigue on decision-making and shooting skill performance in water polo players. J Sports Sci 24: 807-815, 2006.
36. Webb, P and Lander, J. An economical fitness testing battery for high school and college rugby teams. Sports Coach 7: 44-46, 1983.
Keywords:

collision sport; skill; performance; pressure; injury

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