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Performance Changes Following Training in Junior Rugby League Players

Gabbett, Tim J1; Johns, James2; Riemann, Matt3

Journal of Strength and Conditioning Research: May 2008 - Volume 22 - Issue 3 - p 910-917
doi: 10.1519/JSC.0b013e31816a5fa5
Original Research
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The purpose of this study was to investigate the time course of adaptations to training in young (i.e., <15 years) and older (i.e., <18 years) junior rugby league players. Fourteen young (14.1 ± 0.2 years) and 21 older (16.9 ± 0.3 years) junior rugby league players participated in a 10-week preseason strength, conditioning, and skills program that included 3 sessions each week. Subjects performed measurements of standard anthropometry (i.e., height, body mass, and sum of 7 skinfolds), muscular power (i.e., vertical jump), speed (i.e., 10-m, 20-m, and 40-m sprint), agility (505 test), and estimated maximal aerobic power (i.e., multistage fitness test) before and after training. In addition, players underwent a smaller battery of fitness tests every 3 weeks to assess the time course of adaptation to the prescribed training stimulus. During the triweekly testing sessions, players completed assessments of upper-body (i.e., 60-second push-up, sit-up, and chin-up test) and lower-body (i.e., multiple-effort vertical jump test) muscular endurance. Improvements in maximal aerobic power and muscular endurance were observed in both the young and the older junior players following training. The improvements in speed, muscular power, maximal aerobic power, and upper-body muscular endurance were greatest in the young junior players, while improvements in lower-body muscular endurance were greatest in the older junior players. These findings demonstrate that young (i.e., <15 years) and older (i.e., <18 years) junior rugby league players adapt differently to a given training stimulus and that training programs should be modified to accommodate differences in maturational and training age. In addition, the results of this study provide conditioning coaches with realistic performance improvements following a 10-week preseason strength and conditioning program in junior rugby league players.

1Brisbane Broncos Rugby League Club, Brisbane, Australia; 2Easts Tigers Rugby League Football Club, Queensland, Australia; 3University of South Australia, Adelaide, Australia

Address correspondence to Dr. Tim Gabbett, timg@broncos.com.au.

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Introduction

Rugby league is a collision sport played by junior and senior competitors at amateur, semiprofessional, and professional levels (16). A typical senior rugby league match is 80 minutes in duration, with frequent intense bouts of running and tackling, interspersed with short bouts of recovery (21). As a result of the physical demands of the game, the physiological qualities of players are highly developed, with players requiring high levels of aerobic fitness, speed, muscular power, and agility (16).

Several studies have documented the physiological capacities of rugby league players, with the fitness of players typically increasing with increases in playing level (11,16,22,24). More recently, studies have documented changes in fitness over a season in both junior (15) and senior (14) players. Interestingly, due to high match loads and low training loads in the latter phase of the season, the aerobic fitness and muscular power of senior players tends to decline over the course of a season (14). Conversely, junior players are able to maintain high levels of fitness throughout an entire playing season (15). A subsequent study demonstrated that junior and senior players adapted differently to training, with junior players experiencing greater improvements in maximal aerobic power and muscular power than senior players in response to a given training stimulus (17). Collectively, these findings demonstrate that the adaptive responses to training differ in junior and senior rugby league players and that training loads should be modified to accommodate differences in training age (6,8,17).

Studies of the physiological and anthropometric characteristics of junior rugby league players are limited, with most (11,13,19), but not all (15,17) studies restricted to physiological profiles of different playing levels and positions. Studies of junior (i.e., 13-17 years) rugby league players have reported a significant effect of age and playing level on body mass, muscular power, speed, agility, and maximal aerobic power, with the physiological capacities of players progressively improving as the playing level increased (11,19). While these studies have provided important performance standards and normative data for junior rugby league players, no information was provided on the adaptive responses of junior rugby league players competing at different playing levels. In addition, to date, no study has investigated the time course of adaptations to a game-specific training program in junior rugby league players. Due to differences in maturation and training age, it is possible that younger and older junior rugby league players may tolerate an absolute training stimulus differently and, therefore, adapt differently to that training stimulus. Therefore, the purpose of this study was to investigate the time course of adaptations to training in young (i.e., <15 years) and older (i.e., <18 years) junior rugby league players.

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Methods

Experimental Approach to the Problem

This study used a repeated-measures experimental design to track the physiological and anthropometric characteristics of under-15 and under-18 junior rugby league players over a 10-week preseason strength and conditioning program. It was hypothesized that under-18 players would have superior physiological capacities to the under-15 players, but the rate of adaptation to training would be similar between groups.

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Subjects

Thirty-five junior rugby league players participated in this study. All players were scholarship holders with the South Australian Rugby League elite development program and were competing in the under-15 (n = 14) or under-18 (n = 21) divisional age groups. The mean ± SEM age, height, and body mass of the under-15 and under-18 players were 14.1 ± 0.2 years, 169.5 ± 2.1 cm, and 65.9 ± 2.7 kg, and 16.9 ± 0.3 years, 179.7 ± 1.3 cm, and 80.1 ± 2.3 kg, respectively. The preseason training period lasted for 10 weeks. All players underwent fitness testing before and after the preseason training program. In addition, players performed a smaller battery of fitness tests every 3 weeks to assess the time course of adaptation to the prescribed training stimulus. All subjects received a clear explanation of the study, including the risks and benefits of participation, and written parental or guardian consent was obtained.

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Training

Players underwent 10 weeks of training, consisting of strength and conditioning and rugby league-specific skills training. None of the players had any specific strength training or rugby league experience. Players performed 3 sessions each week. Strength sessions typically progressed from high-volume, low-intensity training in the initial 6 weeks of the program (e.g., using 2 or 3 sets of 12 to 15 repetitions) to lower-volume, higher-intensity training (e.g., using 3 or 4 sets of 8 to 12 repetitions) in the final 4 weeks of the program. Players completed upper-body (e.g., bench press, shoulder press, wide grip pull-down, and chin-ups) and lower-body (e.g., leg press) exercises designed to train muscle groups commonly used in rugby league (1,4). Due to the relative training inexperience of players and available resources, most strength training exercises were performed by using machine weights and submaximal loads (7). Conditioning sessions progressed from continuous aerobic training in the initial 6 weeks of the program, followed by higher-intensity, anaerobic training in the final 4 weeks of the program. Conditioning sessions typically consisted of sets of 10- to 40-m sprints performed on a 15- to 30-second cycle to improve repeated sprint ability, with programs overloaded by manipulating the number of sets, repetitions, or distance or by decreasing the recovery time between sets and repetitions. In addition, given that a set of 6 tackles lasts on average between 45 and 60 seconds, and players may be required to perform repeated sets of tackles with minimal recovery, short-interval training lasting approximately 45 to 90 seconds, with a 1:1 and 2:1 work-to-rest ratio were also employed. Skills sessions were designed to improve basic passing and catching skills, evasion skills (e.g. 2 vs. 1), tackling and defensive skills, and offensive skills (e.g., hit and spin, play the ball).

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Fitness Testing Battery

Training adaptations were assessed by using 2 different approaches. First, the physiological and anthropometric characteristics of players were assessed before and after the 10-week training period. Standard anthropometry (i.e., height, body mass, and sum of 7 skinfolds) (23), muscular power (i.e., vertical jump) (9,10), speed (10-m, 20-m, and 40-m sprint) (9), agility (505 test) (5), and maximal aerobic power (i.e., multistage fitness test) (25) were the fitness tests selected. Second, players underwent a smaller battery of fitness tests every 3 weeks to assess the time course of adaptation to the prescribed training stimulus. Players completed assessments of upper-body and lower-body muscular power and endurance. The number of push-ups, sit-ups, and chin-ups completed in a 20-second and 60-second period were used as indices of upper-body muscular power and endurance. A multiple-effort vertical jump test was used to test lower-body muscular power and endurance. Subjects were instructed to refrain from strenuous exercise for at least 48 hours prior to the fitness testing sessions and consume their normal pretraining diet prior to the testing sessions.

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Training Phase Testing

Anthropometry

As an estimate of adiposity, skinfold thickness was measured at 7 sites by using a Harpenden skinfold caliper. The biceps, triceps, subscapular, supraspinale, abdomen, thigh, and calf on the right side were the 7 sites selected. The exact positioning of each skinfold measurement was in accordance with procedures described by Norton et al. (23). Height was measured with a stadiometer, and body mass was measured with 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 skinfolds measurements were 0.99, 0.99, and 0.99, and 0.2%, 0.8%, and 3.0%, respectively.

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Muscular Power

Lower body muscular power was estimated by means of the vertical jump test (9) using a Yardstick vertical jump device (Swift Performance Equipment, Lismore, New South Wales, Australia). Players were requested to stand with their feet flat on the ground, extend their arm and hand, and mark the standing reach height. After assuming a crouched 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 centimeter. 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.

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Speed

The running speed of players was evaluated with a 10-m, 20-m, and 40-m sprint effort (9) by using dual-beam electronic timing gates (Swift Performance Equipment). The timing gates were positioned 10 m, 20 m, and 40 m crosswind from a predetermined starting point. Players were instructed to run as quickly as possible along the 40-m distance from a standing start (9). Speed was measured to the nearest 0.01 second. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the 10-m, 20-m, and 40-m sprint tests were 0.95, 0.97, and 0.97, and 1.8%, 1.3%, and 1.2%, respectively.

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Agility

The agility of players was evaluated by using the 505 test (5) using dual-beam electronic timing gates (Swift Performance Equipment). Two timing gates were placed 5 m from a designated turning point. The 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 through the timing gates, pivot on the 15-m line, and return as quickly as possible through the timing gates. Agility times were measured to the nearest 0.01 second. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the 505 test was 0.84 and 1.9%, respectively.

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Maximal Aerobic Power

Maximal aerobic power was estimated by using the multistage fitness test (25). Players were required to run back and forth (i.e., shuttle run) along a 20-m track and keep in time with a series of signals on a compact disc. The frequency of the audible signals and, hence, running speed was progressively increased until subjects reached volitional exhaustion. Maximal aerobic power (i.e., V̇o2max) was estimated by using regression equations described by Ramsbottom et al. (25). The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the multistage fitness test were 0.90 and 3.1%, respectively.

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Triweekly Testing

Sixty-Second Push-Up Test

Athletes assumed a prone starting position on the floor. The body was lowered until the elbows reached 90°, followed by a return to the starting position with arms fully extended. The push-up action was continuous, with a single rest of no more than 2 seconds permitted between repetitions. Athletes were required to perform the maximum number of push-ups as fast as possible in 60 seconds. The number of full repetitions completed at the 20-second mark was recorded as an assessment of muscular power. The number of full repetitions completed at the 60-second mark was recorded as a relative assessment of muscular endurance. If the athlete failed to complete the full 60 seconds due to fatigue, this failure was recorded, together with the number of full repetitions performed and the time of fatigue. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the 60-second push-up test were 0.94 and 7.3%, respectively.

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Sixty-Second Sit-Up Test

Athletes were instructed to assume a supine position with knees bent to 90°. Feet were flat on the floor and held by a partner. Arms were folded across the chest. For a successful sit-up to occur, the body was raised to a position of 90° to the ground (i.e., vertical) and then returned to the starting position. The sit-up action was to be continuous, with a single rest of no more than 2 seconds allowed between repetitions.

Athletes were required to perform the maximum number of sit-ups as fast as possible in 60 seconds. The number of full repetitions completed at the 20-second mark was recorded as an assessment of muscular power. The number of full repetitions completed at the 60-second mark was recorded as a relative assessment of muscular endurance. If the athlete failed to complete the full 60 seconds due to fatigue, this failure was recorded, together with the number of full repetitions performed and the time of fatigue. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the 60-second sit-up test were 0.90 and 7.9%, respectively.

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Sixty-Second Chin-Up Test

Athletes assumed a hanging starting position on the bar. Hands were shoulder width apart with a supinated grip on the bar. Arms were to be fully extended with hips to be kept stationary at all times (i.e., no deliberate swinging). The body was raised until the chin touched the top of the bar with the head in a neutral position. This was followed by a return to the starting position, with arms fully extended. The chin-up action was continuous, with a single rest of no more than 2 seconds allowed between repetitions. Athletes were required to perform the maximum number of chin-ups as fast as possible in 60 seconds. The number of full repetitions completed at the 20-second mark was recorded as an assessment of muscular power. The number of full repetitions completed at the 60-second mark was recorded as a relative assessment of muscular endurance. If the athlete failed to complete the full 60 seconds due to fatigue, this failure was recorded, together with the number of full repetitions performed and the time of fatigue. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the 60-second chin-up test were 0.98 and 6.4%, respectively.

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Multiple-Effort Vertical Jump Test

The multiple-effort vertical jump test was performed by using a Yardstick vertical jump device (Swift Performance Equipment). Players were requested to stand with their feet flat on the ground, extend their arm and hand, and mark the standing reach height. After assuming a crouched position, each subject was instructed to spring upward and touch the Yardstick device at the highest possible point. However, rather than a single maximal effort, athletes were required to perform 10 vertical jumps on a 5-second cycle (i.e., 1 vertical jump every 5 seconds). 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 total height reached during the multiple-effort vertical jump test was measured to the nearest centimeter. The intraclass correlation coefficient for test-retest reliability and typical error of measurement for the multiple effort vertical jump test were 0.89 and 4.5%, respectively.

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Statistical Analyses

Differences in physiological and anthropometric qualities between the under-15 and under-18 players and over the training period were compared by using conventional statistical significance testing and by using a practical approach based on the real-world relevance of the results (2,20). First, differences between younger and older players and across time were evaluated by using a 2-way analysis of variance with repeated measures. When required, comparisons of group means were performed by using a Tukey honestly significant difference post hoc test. The level of significance was set at p < 0.05, and all data are reported as mean ± SEM. Second, given the relatively small sample sizes and the practical nature of the study, differences between younger and older players and over the training period were also analyzed by using the Cohen effect size (ES) statistic (3). Effect sizes of 0.2, 0.5, and 0.8 were considered small, moderate, and large, respectively (2,20).

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Results

Physiological and Anthropometric Characteristics

The physiological and anthropometric characteristics of the under-15 and under-18 players are shown in Table 1. Under-15 players were significantly lighter (p < 0.05) than under-18 players prior to training. No significant differences (p > 0.05) were detected between under-15 and under-18 players for skinfold thickness, 10-m, 20-m, or 40-m sprint times, agility, vertical jump height, or estimated V̇o2max prior to training. While performance tended to improve in most tests, the improvements were typically nonsignificant and small in magnitude. Training elicited increases in body mass (p = 0.46; ES = 0.4) and V̇o2max (p = 0.049; ES = 1.2) in the under-15 players and reductions in body mass (p = 0.57; ES = 0.2) and skinfold thickness (p = 0.44; ES = 0.3) in the under-18 players. The improvement in V̇o2max was greater in under-15 players than in under-18 players (p = 0.01; ES = 0.8).

Table 1

Table 1

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Time Course of Adaptations

No significant differences (p > 0.05) were detected between under-15 and under-18 players over the course of the training period for push-up and sit-up muscular endurance. However, under-15 players had a greater rate of adaptation for chin-up muscular endurance (p = 0.04; ES = 0.54-0.66), while under-18 players had a greater rate of adaptation for performance on the multiple-effort vertical jump test (p = 0.29; ES = 0.51-1.11) (Table 2, Figure 1).

Table 2

Table 2

Figure 1

Figure 1

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Discussion

This study is the first to investigate the training adaptive responses of junior rugby league players competing at different playing levels. In addition, no study has investigated the time course of adaptations to a strength and conditioning program in junior rugby league players. The results of this study demonstrate that young (i.e., <15 years) junior rugby league players have greater improvements in speed, muscular power, and maximal aerobic power than older (i.e., <18 years) junior rugby league players in response to a given training stimulus. In addition, under-15 players had greater improvements in upper-body muscular endurance than under-18 players. However, under-18 players had greater improvements in lower-body muscular endurance than under-15 players. These results are in partial agreement with previous studies (17) that have found different rates of training adaptation in junior (i.e., <18 years) and senior (i.e., >25 years) rugby league players. Furthermore, these findings suggest that young and older junior rugby league players adapt differently to a given training stimulus and that training programs should be modified to accommodate differences in maturational and training age.

The present study found greater improvements in speed, muscular power, and estimated V̇o2max in under-15 players compared to under-18 players. However, the differences in muscular and aerobic adaptations are unlikely to be due to differences in training experience or higher pretraining physiological capacities, as there were no significant differences between the younger and older players prior to training. Indeed, the finding of similar physiological capacities prior to training, coupled with greater improvements in speed, muscular power, and V̇o2max in response to the same training program, suggests that training stimulated appropriate neuromuscular and cardiovascular adaptive mechanisms in the under-15 players that were not stimulated to the same extent in the under-18 players.

While training resulted in no significant differences between groups for improvements in the maximum number of repetitions performed in the push-up and sit-up tests, the magnitude of improvement in the chin-up muscular endurance test was greater in the under-15 players. These results coincided with a small, but meaningful increase in body mass and stable skinfold thickness in the under-15 training group, whereas training elicited reductions in body mass and skinfold thickness in the under-18 players. While the under-18 players reduced body fat over the training period, it is also possible that the reductions in body mass were accompanied by reductions in lean body mass, thereby reducing strength in these players. It is well documented that a significant relationship exists between strength and endurance, with stronger muscles requiring a smaller percentage of its maximal strength in performing a given task (26,27). It is likely that the maintenance of lean muscle mass in the under-15 players facilitated a greater maintenance of muscular strength, thereby delaying the onset of fatigue in the chin-up muscular endurance test.

While under-15 players had greater improvements in chin-up performance in response to training, these superior improvements in upper-body muscular endurance did not extend to lower-body muscular endurance qualities, with under-18 players having better performance on the multiple-effort vertical jump test, both before and after training. While the exact mechanism for the differential responses is unknown, the higher single vertical jump results suggest that a higher lower-body maximal strength may contribute to the greater lower-body muscular endurance in older junior players (26,27).

This study found an estimated V̇o2max of 48.8 mL·kg−1·min−1 and 45.2 mL·kg−1·min−1 following a 10-week progressively overloaded training program in young (i.e., <15 years) and older (i.e., <18 years) junior rugby league players, respectively. Measurements of 10-m speed, 20-m speed, 40-m speed, and vertical jump height were approximately 1.84 seconds, 3.17 seconds, 5.72 seconds, 4.78 seconds, and 43.7 cm, respectively. These physiological capacities are considerably lower than previously reported for junior subelite (11,13) and elite (19) rugby league players competing on the eastern seaboard of Australia. Although the players were participants within an elite development program, all players were from a region of Australia (i.e., South Australia) that is not renowned for producing quality rugby league players. The lower physiological capacities in the junior rugby league players of the present study most likely reflects differences in long-term exposure to rugby league training, compared to the coaching expertise offered in other rugby league playing states (e.g., Queensland and New South Wales). Alternatively, the lower speed, muscular power, and estimated V̇o2max in the current sample of players may reflect the greater training intensity at higher playing levels.

The current findings provide the expected changes in physical fitness from a 10-week preseason strength and conditioning program in subelite under-15 and under-18 junior rugby league players. These findings demonstrate that subelite junior rugby league players undertaking a progressively overloaded strength and conditioning program may expect 16.9% to 60.7% improvement in upper-body muscular endurance, 1.83% improvement in lower-body muscular endurance, 6.4% increase in aerobic fitness and stable 10-m, 20-m, and 40-m speed. Furthermore, the improvements in physiological capacities were typically greater in the under-15 players than in the under-18 players. However, the improvements in V̇o2max are lower than the 15.8% improvement in V̇o2max previously reported for under-18 rugby league players (15). It is likely that the small improvements in performance in the present study reflect, at least in part, the necessity to enhance skill and also train all physiological fitness parameters effectively. Clearly, any future studies of the effect of game-specific training on the physical qualities of junior rugby league players should also include assessments of game-specific skills and playing ability.

A possible explanation for the greater improvements in speed, muscular power, and maximal aerobic power in the under-15 players may lie in the increased injury risk associated with higher playing levels. Indeed, it has been reported that under-17 and under-19 players have a higher injury incidence than under-15 players (12). The greater improvements in speed, muscular power, and maximal aerobic power following training in the under-15 players may be due to the lower incidence and severity of injuries sustained by this cohort, thereby allowing players to participate in a greater number of quality sessions than under-18 players.

In conclusion, this study investigated the time course of adaptations to a strength and conditioning program in young (i.e., <15 years) and older (i.e., <18 years) junior rugby league players. The results of this study demonstrate that under-15 players have greater improvements in speed, muscular power, and maximal aerobic power than under-18 players in response to a given training stimulus. In addition, under-15 players had greater improvements in upper-body muscular endurance than under-18 players. However, under-18 players had greater improvements in lower-body muscular endurance than under-15 players. These results are in partial agreement with previous studies (17) that found different rates of training adaptation in junior (i.e., <18 years) and senior (i.e., >25 years) rugby league players. Furthermore, these findings suggest that young and older junior rugby league players adapt differently to a given training stimulus and that training programs should be modified to accommodate differences in maturational and training age.

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Practical Applications

This study is the first to document changes in the physiological and anthropometric characteristics of young and older junior rugby league players in response to a progressively overloaded strength and conditioning stimulus. From a practical perspective, these findings provide conditioning coaches with realistic performance improvements following a 10-week preseason strength and conditioning program in junior rugby league players.

In response to a 10-week strength and conditioning program, under-15 players exhibited greater adaptations in speed, muscular power, V̇o2max, and upper-body muscular endurance than under-18 players. However, under-18 players had greater improvements in lower-body muscular endurance than under-15 players. These findings suggest that in comparison to older (i.e., <18 years) players, young (i.e., <15 years) junior rugby league players exhibit accelerated neuromuscular and cardiovascular adaptations in response to a given training stimulus. However, these superior improvements in speed, muscular power, maximal aerobic power, and upper-body muscular endurance do not extend to lower-body muscular endurance qualities, with under-18 players having better performance on the multiple-effort vertical jump test in response to training. While the exact mechanism for the differential responses is unknown, the higher single vertical jump results suggest that a higher lower-body maximal strength may contribute to the greater lower-body muscular endurance in older junior players (26,27).

The smaller improvements in maximal aerobic power, speed, and muscular power in under-18 rugby league players demonstrate that older players require greater than 10 weeks to develop an appropriate standard of physical fitness for optimal performance. In addition, under-18 rugby league players may require training programs with a greater emphasis on aerobic conditioning and muscular strength and power development. Interestingly, recent evidence has suggested that rugby league players who perform less than 18 weeks of preseason training are at increased risk of sustaining an injury during the competitive phase of the season (18). The results of the present and previous (18) studies suggest that under-18 rugby league players may benefit from an additional 8 weeks of preseason training (i.e., ≥18 weeks of training) to facilitate greater improvements in aerobic capacity, speed, and muscular power, while also minimizing the risk of injury.

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

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Keywords:

adaptation; fitness; maturation; youth; football

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