The ability to produce high levels of muscular power is critical for successful performance in most contact sports such as American football and rugby league (5,27). Furthermore, it has been suggested that possessing high levels of maximal strength is the most important factor influencing power production (8,35,38). Although maximum strength and power tests are not measures of sporting ability, they are believed to represent performance characteristics of playing potential in many sports (1).
Since the introduction of professionalism in rugby union in 1995, rugby players have become bigger and stronger (33,36). Indeed, in just a short period from 2004 to 2007, players had an average increase in strength of 3–5% for upper-body and 5–15% for the lower-body (36). Additionally, the southern hemisphere super rugby competition, which consisted of 10 teams in 1995, has now expanded to a 15-team competition. As a consequence, players are competing in a greater number of games throughout the calendar year. Because of the greater number of teams and increased competition demands, a greater pool of players is required. Recently, it has been suggested that younger players are being selected to fill the void (36).
Levels of strength and power have been used to effectively discriminate between different levels of competition in a range of sports including American Football (22), rugby league (4,7,11), volleyball (34), kayaking (23), and ice hockey (15). Fry and Kraemer (22) have evaluated physical performance characteristics of 19 American football collegiate programs (981 participants) across 3 different levels of competition National Collegiate Athletic Association divisions I–III). Bench press performance was significantly different between all levels of play, revealing that division I athletes were 6 and 11% stronger than were division II and III athletes, respectively. Additionally, vertical jump performance was significantly greater in division I than in division II and III athletes. Interestingly, back squat performance did not clearly differentiate between levels of competition. These findings are supported by those of Baker (4,6,7), who reported significant differences in bench press strength, and upper- and lower-body power between different levels of competition in rugby league athletes in Australia. However, in contrast to the findings of Fry and Kraemer (22), Baker and Newton (11) also reported significantly greater lower-body strength in higher level athletes.
Correlations between the change in strength and the change in power have been reported to reduce as players become more elite. For example, Baker (5) reported that the relationship between the change in strength and power was r = 0.73 and r = 0.39 in state-level and national-level rugby league athletes, respectively. These findings suggest that as players become more highly trained, improving one aspect of performance may not transfer to improvements in the other performance measure. Determining the relationships between strength and power between different levels of competition may provide an insight into what training methods may be more effective for different playing levels. Indeed, if relationships between strength and power are weak in professional athletes, these findings may suggest that more specific power-oriented training methods may be of greater benefit. If the opposite is true, and there is a large transfer of training (large correlations), traditional strength training methods may be equally beneficial for developing power.
There is currently limited literature reporting differences in physical performance between separate levels of competition in rugby union athletes. If indeed younger players are being selected as a result of greater competition requirements, a better understanding of strength and power across different levels of competition in rugby union is required. These findings will provide normative data for coaches and conditioners who are responsible for developing younger players. Normative data may provide clearer direction when allocating training time to focus on individual needs, allowing them to effectively prepare athletes for transition through to the next level of performance. However, a better understanding of the relationship between strength and power may provide a guideline as to which training methods may be more beneficial for improving performance on an individual basis. Therefore, the aim of this investigation was to characterize differences and determine the relationship between strength and power in athletes across different levels of competition in rugby union. We hypothesize that athletes who compete at a higher level will produce greater levels of strength and power that that of athletes at lower levels.
Experimental Approach to the Problem
To characterize strength and power across different levels of play in rugby union athletes, 112 participants from 4 distinct levels of competition (professional, semiprofessionals, academy, and high school first XV) volunteered to participate in this investigation. All the players were tested on 2 separate occasions to determine individual strength and power. The first occasion participants were tested for upper- and lower-body strength (bench press and box squat, respectively). On the second occasion, the players were tested for upper- and lower-body power (bench throw and jump squat, respectively). All the players had been performing these exercises in their regular resistance training sessions. The players were given verbal encouragement throughout all strength and power assessments. All the players completed testing during their in-season phase of competition. The exercises were selected because of their common usage in power training programs and research studies along with their ability to represent upper- and lower-body power (3,10,14,19). Peak power was selected as the dependent measure because it has been reported to have the greatest association with athletic performance (21). All testing took place between 8:00 and 10:00 AM. Additionally, the players were instructed to maintain a high level of hydration and nutritional intake in the 24 hours leading up to each testing occasion. The players were instructed to abstain from caffeine 12 hours before each testing session.
A total of 112 rugby union players including 43 professionals competing in an international and provincial competition full time; 19 semiprofessionals competing in the provincial competition (and who have not played in the professional level) for 6 months of the year; 32 academy level players competing in either age group provincial level or B-level provincial competition; and 18 high school (secondary school) level players competing in a regional high school competition were involved. Subject characteristics are presented in Table 1. The players were informed of the experimental risks and signed an informed consent document before the investigation. This investigation was approved by an Institutional Review Board for the use of human subjects. Because of injury from training or competition before assessment, 8 professional and 10 academy players did not take part in any of the lower-body testing. Additionally, because of their limited training history, no high school players performed the jump squat.
Bench Press and Box Squat
Maximal strength was assessed using the bench press and box squat exercises using methods previously described (3). Briefly, the players were required to perform 3 sets (50, 70, 90%) of submaximal (4–6 repetitions) bench press or box squat followed by 1 maximal set (100%) of 1–4 repetitions. For the bench press, the players used a self-selected hand position and were required to lower the bar to approximately 90° angle at the elbows and then pressed the bar in a vertical movement so that the arms were fully extended. During the box squat, the players used a self-selected foot position and were required to lower themselves to a sitting position briefly on the box and then return to a standing position. The box height was adjusted for each athlete to allow the top of the thighs to be parallel to the floor while in the seated position. A 3-minute rest period separated all sets. Each maximal set was used to predict each player's 1 repetition maximum (RM) bench press (r = 0.993) and box squat (r = 0.969) using the after equation (29,30):
Upper-body peak power was assessed using a bench throw exercise performed in a Smith Machine. The players warmed up with 2 sets of 4 repetitions of bench press at 50% of their 1RM. The players then completed 2 sets of 4 repetitions of bench throw at 50 and 60% of 1RM (3,9). The players used a self-selected hand position and lowered the bar to a self-selected depth (∼90° angle at the elbow). The players were then required to propel (throw) the bar vertically as explosively as possible. A 3-minute rest period separated all the sets.
Lower-body peak power was assessed using a jump squat exercise performed in a Smith Machine. The players warmed up with 2 sets of 4 repetitions of 90° squat at 55% of their 1RM. The players then completed 2 sets of 4 repetitions of jump squat at 55 and 60% of 1RM (3,10). The players used a self-selected foot position and lowered the bar to a self-selected depth (∼90–100° angle at the knee). The players were then required to jump as explosively as possible. A 3-minute rest period separated all the sets.
A GymAware® optical encoder (50-Hz sample period with no data smoothing or filtering; Kinetic Performance Technology, Canberra, Australia) was used to collect peak power for each repetition of bench throw and jump squat using the methods described elsewhere (20). Briefly, GymAware® consists of a spring-powered retractable cord that passes around a pulley mechanically coupled to an optical encoder. The retractable cord is then attached to the barbell, and velocity and distance are calculated from the spinning movement of the pulley upon movement of the barbell. The encoder gives 1 pulse approximately every 3 ml of load displacement, with each displacement value time stamped with a 1-millisecond resolution. The mass of the bar (as entered into a personal digital assistant), the entire displacement (millimeters) of the barbell, and time (milliseconds) for the movement are used to calculate peak values for power (20).
All data were log transformed to reduce nonuniformity of error, with effects derived by back transformation as percent changes (28). Standardized changes in the mean of each measure were used to assess magnitudes of effects by dividing the changes by the appropriate between-subject SD. Standardized changes of 0.00–0.19; 0.20–0.59; 0.60–1.19; 1.20–1.99, <2.00 were interpreted as trivial, small, moderate, large, and very large effects, respectively (37), a modification of Cohen's thresholds of 0.2, 0.5, and 0.8 (16). To make inferences about the true (large-sample) value of an effect, the uncertainty in the effect was expressed as 90% confidence limits. The effect was deemed unclear if its confidence interval overlapped the thresholds for small positive and negative effects (12).
To help explain any differences in performance, all performance data were also normalized to body mass using allometric scaling with a derived power exponent (17,18). The equation for normalizing performance to body weight was as follows: normalized performance = Y/Xb, where Y is the performance, X is the body mass, and b is the power exponent. The derived power exponent was determined by plotting performance and body mass on a log-log scale. The slope of the linear regression line was then used as the derived power exponent. Allometric scaling is generally superior to ratio scaling (performance/body mass) because ratio scaling penalizes heavier athletes.
Interclass correlation (r) and coefficient of variation (percent) for all measures have previously been assessed in our laboratory on professional rugby players were 0.900 and 5.0% (bench throw), and 0.904 and 4.8% (jump squat), 0.915 and 4.3% (bench press), and 0.915 and 4.6% (box squat), respectively. Additionally, interclass correlation and coefficient of variation were also assessed on the high school level players and were 0.860 and 6.3% (bench throw), 0.950 and 2.2% (bench press), and 0.790 and 7.0% (box squat), respectively. Validity of the Gymaware® optical encoder has been previously reported elsewhere (20).
Magnitudes of the difference between the characteristics of the player are presented in Table 2. With the exception of height, magnitudes ranged from small to very large in favor of the players in competing at a higher level of competition. Raw data (mean ± SD) for each level of competition is presented in Table 3. Correlations between strength and power are presented in Table 4.
The derived power exponents calculated for scaling to body weight were 1.073 ± 0.193 (±90% confidence limits), 1.379 ± 0.272, 1.089 ± 0.302, and 0.910 ± 0.242 for bench press, bench throw, box squat, and jump squat, respectively. The percent difference in absolute and allometrically scaled relative data between levels of competition is presented in Table 5.
The aim of this investigation was to characterize the differences in strength and power in athletes across different levels of competition in rugby union. As expected, greater absolute strength and power outputs were observed in the athletes who participated in a higher level of rugby union competition. The only measure that did not discriminate between levels of competition was box squat strength between professional and semiprofessional athletes. When performance was normalized for weight, the magnitudes of the difference were reduced for all measures and both bench press and box squat strength could not discriminate between professional and semiprofessional levels of competition.
The differences in strength and power between the athletes in different levels of competition are likely caused by maturation and body mass. As the level of competition increased, the chronological age and training age of the athletes also became larger (moderate to very large effect sizes). Maturation and training age play a large role in the ability to produce high levels of force and power. Older athletes or athletes with greater training ages will likely have developed more efficient movement patterns in the strength and power tasks assessed, have enhanced ability to activate musculature (e.g., increased synchronization of motor units, decreased antagonist coactivation), and reduced inhibitory feedback from force regulators (e.g., Golgi tendon organs) allowing for greater production of force and power (11,13,26,32).
These findings may also suggest that by the time athletes are competing at a higher level, there is less scope for improvement. Indeed, the greatest improvement in strength and power from 1 level of competition to the next was in the period from high school into an academy system. Based on our findings, by the time athletes are training in an academy system and have a training age of only 1.5 years, approximately 81% of strength and 71% of power have already been developed. Therefore, the majority of physical development appears to be attained throughout the first 1–2 years of training within a structured environment. This physical development is particularly important to emphasize because it highlights the importance of having appropriate development pathways set in place. If athletes are indeed being selected from a younger age, then attention needs to be given throughout this level of development to ensure the maximal gains are achieved.
Higher level athletes had a greater body mass than their lower level counterparts did. Although body composition was not assessed, it could be assumed that the heavier higher level athletes had a greater muscle mass than did the lower level athletes (24,25). Increased muscle mass is an important determinate of muscle strength. Indeed, Stone et al. (38) suggested that possessing greater levels of maximal strength may affect peak power output in that “(a) A given weight would represent a smaller percentage of maximal strength for a stronger person; thus, this weight would be easier to accelerate. (b) A person with greater maximum strength may have larger or greater percentage of type II muscle fibers” (38). As such, assuming skill level is equal; a larger player with greater muscle mass or a player with greater type II muscle fiber percent may be more effective in some aspects of rugby where physical domination of an opponent or maximal speed and acceleration are critical for successful performance, for example, tackling, breaking through the defensive line.
Normalizing performance to body mass reduced the magnitude of the difference between the levels of competition. These findings are in agreement with the contention that body mass contributes to performance during functional performance tests (17,18). When performance was normalized for body mass, semiprofessionals had a greater squat strength (3%) than the professionals did, although these findings were unclear trivial. However, professional athletes still possessed greater power output than the semiprofessionals. These findings suggest that although body mass and strength are important in producing power, there are other significant factors that contribute to power production. This is further highlighted by the negative correlation between lower-body strength and power in the professionals.
As athletes become more elite, increases in strength may not reflect increases in power output (4). Consequently, conditioning coaches may place more emphasis on other training methods more likely to enhance power once an ‘adequate’ strength base has been obtained (4). For example, professional athletes may complete a greater volume of modified Olympic lifts, intensified plyometrics, and advanced lifting programs such as complex and contrast training. Because rugby players have only a limited training time available, a change in emphasis would result in less training volume dedicated to improving strength and likely result in strength maintenance rather than improvement. This change in training emphasis may help explain why the professionals, although not stronger, had a greater power output than the semiprofessional players.
Similar to the findings of Fry and Kremer (22), lower-body strength values in this investigation were not substantially different between the top 2 levels of competition. Fry and Kremer (22) speculated that methodological issues (scores obtained by different researchers, discrepancies in squat depth, use of knee wraps) may have been a reason for the similar scores of each competition group. However, in this investigation, all the testing sessions were conducted by the same researcher to ensure standardized lifting technique was performed by all the athletes. As such, the lifting technique or use of lifting aids can be ruled out. It may be possible that the lack of difference in lower-body strength is because of differences in training mode and volume. Professionals typically perform a lower amount of resistance training volume throughout the year compared with semiprofessionals as a result of longer in-season training phases. The greater length of the in-season phase in the professional athletes substantially decreases the time available for off-season training phases where strength and power can be developed (2,3). Additionally, because of longer in-seasons, professionals typically perform a greater volume of nonresistance training (e.g., team training) throughout the year. This greater nonresistance training volume may attenuate improvements in strength and power because of the inability of the body to simultaneously adapt to contrasting training stress (31). Furthermore, with longer in-season phases and greater competition demands, there is an increased likelihood of injury occurring or need for increased player management, which, from an applied perspective, typically results in an unloading of lower-body training intensity and volume. All these factors are likely to limit physical development, especially in the lower body.
Upper-body correlations between strength and power ranged from 0.40 in the professionals to 0.92 in the high school athletes. The shared variance of these measures (r2 as a percentage) suggest that up to 85% of bench throw power in high school athletes can be explained by bench press strength, whereas only 16% of bench throw power in professionals can be explained by bench performance. These findings show that to improve power in professionals, other training methods separate from increasing maximal strength need to be identified and implemented. In contrast, to improve bench throw power in lower level athletes, maximal strength training may have the greatest transfer to power. It may also be suggested that greater transfer of training adaptation occurs in lower level athletes, whereas, the higher level professionals need greater specificity in training to ensure improvements are made.
Lower-body correlations between strength and power were lower than that previously reported in other rugby codes (8). The professional athletes in this investigation actually had a negative correlation between box squat and jump squat. The difference in movement patterns of the lower-body exercises selected may have influenced the relationships observed. Indeed the box squat exercise all but eliminates any contribution of the stretch shortening cycle, whereas the jump squat is performed with a countermovement, which maximizes the stretch shortening cycle.
The findings from this study suggest that both strength and power can discriminate between the higher 2 (professional and semiprofessional) and lower 2 (academy and high school) levels of competition. Notwithstanding this, the ability to produce high levels of power, rather than strength, may be a better determinate of playing ability between professional and semiprofessional athletes. Therefore, higher level athletes wanting to enhance playing potential should focus on methods to improve power. However, it must be noted that our findings do not suggest that once a certain threshold of strength has been reached that it is not longer important to keep developing it. Our findings simply show that as athletes become more elite it becomes more difficult to improve some aspects of performance (which is likely because of increased competition demands) and that other mechanisms for improving power, rather than increases in strength, are required.
Because strength and power output can discriminate between different levels of competitions, younger athletes should strive to attain greater levels of strength and power in an attempt to reach or to be physically prepared for the next level of competition. These findings also suggest that appropriate pathways that nurture physical development, such as academies or development squads, are a critical component within a professional structure to ensure player succession. Nonetheless, practitioners must be cautioned to not attempt to accelerate these physical attributes too quickly in the young untrained players and each individual should be viewed and approached differently based on individual training history, playing position, injury history and physical maturity.
The Waikato Rugby Union and the Tertiary Education Commission provided financial support by the way of scholarship for the primary author. The results of this study do not constitute endorsement of the product by the authors or by the National Strength and Conditioning Association.
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