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Original Research

Comparison of Lower Body Strength, Power, Acceleration, Speed, Agility, and Sprint Momentum to Describe and Compare Playing Rank among Professional Rugby League Players

Baker, Daniel G; Newton, Robert U

Author Information
Journal of Strength and Conditioning Research: January 2008 - Volume 22 - Issue 1 - p 153-158
doi: 10.1519/JSC.0b013e31815f9519
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Abstract

Introduction

Success in rugby league football seems heavily reliant on the players possessing an adequate degree of various physical fitness qualities, such as strength, power, speed, agility, and endurance, as well as individual skills and team tactical abilities (9,16,17). Previous research has indicated that upper body maximal strength and power can clearly distinguish among players from different ranks, ranging from high school to elite professionals (2,4). The data concerning the contribution of the lower body seems less informative. In particular, lower body strength, power, acceleration, speed, agility, and endurance, which are of importance to rugby league players because of the large amount of running, changing of direction, and tackling that occur during a game, have attracted minimal research attention.

Although lower body endurance must be such that an elite rugby league player can cover 10 km in an 80-minute game (15), the nature of the more anaerobic aspects of muscular performance are also of interest. Despite the large aerobic running volume, there is also a considerable amount of sprinting and fast striding, almost limitless changes of direction entailing acceleration and deceleration of the player's body mass, and tackles being made by or on the player (which require a great degree of leg strength and power). A number of studies over the years have attempted to provide normative data in a number of tests for rugby league players. Some have addressed combinations of sprinting tests of acceleration, maximal speed, and agility (1,11,12) or tests of strength and power (3,4), whereas other studies have attempted a more holistic test battery (9,12,16,17).

Some lower body strength data illustrate strength differences between national rugby league (NRL) and third-division players and younger high school and college-aged players but fewer differences between NRL and state rugby league (SRL) players (3,4). Other strength test data have been reported for rugby league players, with equivocal results (15-17), and a paucity of data exists concerning lower body power levels. Tests of running speed and agility have been performed in a bid to establish normative data or to compare the abilities of players participating in different leagues (e.g., NRL, SRL and intra-city leagues [CRL]) (1,11,12). These data have so far revealed mixed results concerning acceleration (sprinting farther than 10 m) or maximal speed (sprinting farther than 40 m) that may exist between rugby players in different leagues. Much younger and some third-division or lower-level players do tend to exhibit much slower sprint scores than those reported for professional players, with differences of more than 25% for sprint times over 40 m (1,11,12). Differences in running agility can also sometimes discriminate between players at national league and third-division levels (1). However, some studies reveal little difference between national league and second- or third-division players from the same football club undergoing similar training (1,11). Thus, more study is required to determine if any speed parameters differentiate between national league first-division and second-division rugby league players.

Despite the studies cited above, no conclusive study of a large number of different lower body tests and how these tests differentiate among differently ranked teams from the same club who have undergone similar training has been performed.

The purpose of this study is to present data on the lower body strength, power, acceleration, maximal speed, agility, and sprint momentum capabilities of selected rugby league players participating in the elite, national (Australia) first-division (NRL) or the state-based second division (SRL). These data will be compared to determine the relative importance of these qualities to level of participation in these leagues. If differences exist, then players currently involved at SRL level or even lower grade or age levels, but who aspire to be participants in the elite NRL, would be able to identify the physical improvements they would need to make to succeed. This basis for testing has been used extensively in American football at the national combine camp, where potential participants in the National Football League (NFL) draft undergo a large battery of tests that may help to determine their position in the draft and thus future earnings as professional football players (14).

Methods

Experimental Approach to the Problem

Tests of lower body strength, power, acceleration, maximal speed, and agility were measured in rugby league players participating in two different leagues (NRL and SRL competitions). Scores in these tests were analyzed to determine whether there were differences between the squads. This type of analysis would provide data pertaining to the relative importance of each of these physical qualities to attaining NRL level.

Subjects

Forty rugby league players, comprising 20 full-time professionals participating in the elite first-division professional NRL competition and 20 semi-professionals participating in a second-division SRL, served as subjects in this investigation. All were members of the same football club and performed similar training for strength, power, speed, and endurance relative to their different playing positions, individual capacities, and training experience. However, the full-time professional NRL players also performed approximately three to five additional training sessions (fitness, skills, tactics) per week. All subjects were informed of the methods and nature of the testing and voluntarily participated in the testing sessions, which were a regular part of their testing and conditioning regimen. All of the athletes had performed a pre-season training cycle immediately before testing, establishing them in peak condition at the time of testing. The mean (standard deviation) height, body mass, and age of the subjects was 185.2 (4.6) cm, 98.2 (9.9) kg, and 25.3 (3.4) years versus 184.4 (4.6) cm, 91.3 (8.4) kg, and 22.6 (3.6) years, respectively, for the NRL versus SRL groups.

Testing

Tests were chosen to measure the strength, power, acceleration, maximal speed, agility, and sprint momentum of the lower body musculature.

Strength Testing

Maximal lower body strength was assessed by the 1 repetition maximum full squat (1 RM SQ) using free weights and according to methods previously outlined (3,8). Briefly, this entailed the athletes warming up using a light resistance (60 kg), then lifting progressively heavier resistances until 1 RM was achieved. The depth of squat was determined as top of thigh below parallel to the floor, which was visually determined by the researcher. Six (three NRL, three SRL) of the 40 players were returning to play after surgery, and their surgeon(s) requested that they refrain from 1RM testing for the time being. For these players, a repetitions to fatigue test (RTF) was performed, which was similar in nature to the NFL 225 bench press test performed at college campuses and the NFL combine draft camp (10,14), except the players performed the squat exercise with a similar resistance (100 kg) and their 1RM was predicted according to published regression equations (10). As each of the extrapolated 1RM SQ values was lower than previous actual 1 RM SQ values for these six athletes (performed 1-2 years previously) and they were evenly distributed at three per group, it was thought that this would not adversely affect the general outcome of the study.

Power Testing

The test of lower body maximal power was performed 2 days later and entailed the use of the Plyometric Power System (PPS; Fitness Technology, Adelaide, Australia), which has been described previously (6,7). Briefly, the PPS is a device whereby the displacement of the barbell is limited to the vertical plane, as in a Smith weight-training machine. The linear bearings that are attached to each end of the barbell allow the barbell to slide up and down two hardened steel shafts with a minimum of friction. A rotary encoder attached to the machine produces pulses indicating the displacement of the barbell. The number of pulses, denoting barbell displacement, and the time of the barbell movement were measured by a counter timer board installed in the computer. The PPS software calculated the average mechanical power output of the jump squat based on the displacement, time, and mass data. The athletes performed three repetitions with resistances of 40, 60, 80, and 100 kg using methods previously described (3,5,6). The highest single power output, which could occur with any of the resistances, was deemed to be the maximal power (JS Pmax). Five of the athletes (two NRL, three SRL) refrained from this test for medical reasons. Test-retest reliability of r = 0.92 has previously been established with a group of 12 subjects.

Sprinting and Agility

Sprint testing was performed in the month before the maximal strength and power testing, when the athletes were in a relatively rested state (4 days of rest). This was 1 week before the athletes were to commence practice games. We thought it best to test speed qualities at this time as game demands and minor injuries may have precluded a full speed testing battery for all the athletes if it was scheduled 1 month later, when strength and power were to be tested. Sprint ability was assessed over two distances, 10 and 40 m, to reflect the somewhat separate entities of acceleration (0-230 m) and maximal speed (approximately 40 m) (6,18). Previous studies have shown sprint capabilities longer than 10 m do not always correlate highly to 40-m tests (1, 6). These capabilities can be defined as separate entities to a degree, and it has been recommended that athletes be assessed over these two differing distances (18). Agility can be defined as the ability to cover distances of sport-specific running patterns in the least amount of time. Typically, sport-specific agility tests entail changes of direction or the inclusion of some sport skill (18). It has been shown that the more changes of direction involved in an agility test, the less correlation is exhibited between agility and maximal speed (1,18). Therefore, although acceleration, maximal speed, and agility are related, they also entail unique features requiring separate testing.

Straight-line Sprint Testing

Straight-line running speed was assessed by the 10-m (designating acceleration ability) and 40-m sprint (designating maximal speed ability) times using electronic light gates and methods previously described (6). Briefly, all the running tests were performed on a turf track, with athletes wearing special running shoes designed for sprinting on turf. After an intensive warm-up, the athletes performed a minimum of two trials sprinting longer than 40 m, with light gates set at the 10-m and 40-m marks. The best result for each distance was recorded as the test score. The intratrial reliability of the above procedures has been previously established at r = 0.86 and r = 0.98 for the 10-m and 40-m sprints, respectively (6).

Agility Testing

The agility test was a novel test that requires explanation. After completing the trials of the sprint testing, the players were assessed for agility. Standing on the goal line, they sprinted 10 m at a 45° angle, turned around a marker pole, and sprinted at 45° (thus a 90° turn) for a further 10 m back to the goal line. Here they made a sharp 135° turn around another marker pole and sprinted 20 m in a straight line perpendicular to the goal line. Therefore, this was a 40-m sprint (or slightly more depending on the athlete's ability to corner the marker poles) with one 90° turn and one 135° turn. It was thought that this pattern entailing pronounced sharp turns would best mimic and test the agility requirements of rugby league players as it tended to mimic some aspects of movement when the team is defending.

Sprint Momentum

The athletes'sprinting momentum was determined by multiplying their body mass by the average velocity from the 10-m test. The 10-m sprint was chosen because this is the distance typically run in rugby league before coming into contact with an opposing player (16).

Statistical Analyses

Factorial analyses of variance was used to determine whether differences existed between the groups overall in 1RM SQ, JS Pmax, 10 m, 40 m, agility testing, sprint momentum, and anthropometric data. In the event of a significant F-ratio, Fisher PLSD post hoc comparisons were used to determine where those differences existed. The criterion for significance of any differences in the measured variables between the two squads was set at an α level of P ≤ 0.05.

Results

The NRL players were significantly heavier (8.9%) and older than their SRL counterparts. Further the NRL players were stronger (17.0%) and more powerful (11.5%) and produced greater momentum (7.0%). Acceleration, maximal speed, and agility were not significantly different between the two groups (Table 1).

Table 1
Table 1:
Comparison of lower body strength, power, sprint momentum, acceleration, speed, and agility between national league (NRL) and state league (SRL) rugby players.

Discussion

In this study, we compared elite rugby league players in the top national grade with those in the grade below; that is, state level. The purpose was to determine aspects of lower limb performance that separate the quality of these athletes with a view to understanding the factors that have the greatest impact on success. Of all the variables measured in this investigation, maximal strength and power seemed to be the best descriptors of which athletes were in the NRL or SRL squads. Maximal leg strength and power differentiated the two squads by approximately 17% and 12%, respectively. It could be postulated that increased leg strength and power would act favorably for players in all facets of the rugby league. Increased leg strength and power, facilitating increased leg drive, would aid in tackling opposing players when in defense and in helping to break tackles when in attack. These are the most fundamental tasks for all rugby league players, irrespective of playing position. Such is the theoretical importance of leg drive to all facets of the rugby league, and it is almost inconceivable that maximal strength and power would not differentiate between players of different ranking. Although there are few data concerning 1RM SQ and JS Pmax testing of rugby league players, the current finding is in concert with some previous research on lower body power distinguishing among rugby league players of different ranking (3,4).

The significantly heavier body mass of the NRL squad would certainly favor them in the expression of maximal strength and power. Previous comparisons of players of similar ranking have not always displayed differences in body mass (3). However, the 9% difference in body mass in this study between NRL and SRL players does not fully account for the differences in strength (1RM/kg of body mass was 1.78 and 1.64 for the NRL and SRL squads, respectively; P ≤ 0.05). Therefore, despite the increased body mass, the NRL squad was still significantly stronger per kilogram of body mass. As athletes become heavier, they do not become inherently stronger per kilogram of body mass. Consequently, various neural, tissue/morphological, or maturation (the NRL group were older) adaptations must explain this result. It has been shown that increased neural activity occurs in muscles, perhaps because of increased rate coding and signal intensity, in the first 8-12 weeks of strength training (13), although the extensive training history of all the athletes in this study would preclude this phenomenon from accounting for the differences in absolute and relative strength and power. It has been postulated that other neural adaptations occur with long-term periodized strength and power training. These include more efficient neural patterning of the skill of the strength exercises, diminished levels of unwarranted antagonist co-contraction, synchronous firing of motor units (especially during the initial concentric phases of ballistic power exercises), and reduced inhibitory feedback from force receptors/regulators such as the Golgi tendon organs and the Renshaw cells (13). To what extent these adaptations occur and the time frame for their occurrence has yet to be fully determined. Qualitative muscle tissue adaptations, such as changes to the fiber type and myosin heavy chain expression, could also presumably occur with increased training experience. Further discussion of the type, extent, and nature of these adaptations is beyond the scope of this article but has been reviewed extensively elsewhere (13).

In this investigation, the 10-m sprint was assessed as part of the 40-m sprint, as has been done previously (1,6). It is conceivable that athletes adopt slightly different sprinting mechanics when sprinting 10 m as an end point versus 10 m as part of a longer 40-m sprint. This may make comparisons with the results of other studies in which rugby league players sprinted only 10 m difficult to interpret, but it would not change the basic result of this study, as both groups performed the sprint tests at the same time. It could be expected that they adopted similar sprinting tactics.

The results of this investigation also illustrate that not all the measured variables effectively discriminate between NRL and SRL rugby league players. There was no difference between the NRL and SRL squads in acceleration, maximal speed, or agility. A number of researchers have measured acceleration and maximal speed in rugby league players (11,16,17). The most recent of these studies (11) also attempted to differentiate between NRL and SRL players and reported no difference in sprint ability using these two distances, confirming earlier research (1). Thus, it must be stated that once rugby league players attain SRL level, speed qualities seem to be markedly similar to NRL players. In this study, they performed essentially the same sprint and agility training alongside each other.

This does not preclude the fact that lower-level rugby league players (third-division and lower) may not be as fast as the NRL and SRL players in this investigation and that speed may indeed differentiate among players from different rankings or age groups (e.g., 6, 11 vs. 12). It is just that acceleration and maximal speed did not differentiate between the NRL and SRL squads in this investigation. Indeed, the 10-m and 40-m sprint times for the squads in this investigation are slightly faster than those previously reported for NRL players (11,16) and much faster than reported for other professional (17) or third- or fourth-division, amateur, or high school players (12). Thus, although speed did not differentiate between NRL and SRL squads overall in this study, it must be noted that the SRL players in this investigation exhibited faster sprint times than those previously reported for NRL (11) and other professional players (12). This may reflect the genetic abilities, recruitment, and training of these players.

Although almost all running in the rugby league entails some changing of direction and studies have shown that speed tests with many changes of direction do not correlate highly to straight-line sprint tests (6,17), very few data exist concerning agility in rugby league players. One study reported that NRL players were significantly more agile than third-division rugby league players (6). It was theorized that the NRL players in that study were superior to the third-division players in agility because of their greater strength, power, abdominal control, and agility training experience. Therefore, although agility testing did not differentiate between the NRL and SRL players in this study, it is plausible that both of these groups may again be superior to lower-level rugby league players.

Whereas it would seem that maximal strength, power, and body mass, rather than speed, best discriminate between NRL and SRL players, there was one speed-related variable that did display significant differences between the groups. This study is the first to calculate sprint momentum based on average 10-m sprint velocity and body mass. Sprint momentum, which can be defined as body mass multiplied by the sprint velocity, is of considerable interest in collision-oriented sports such as rugby. Coaches often state the importance of building “momentum” and the “go forward,” especially of the heavier forward players, in driving the game down field and setting up play for the more highly skilled ball distributors or the faster running back-line players. Thus, heavier players who possess good running speed would crash into opposing defenders with greater momentum to drive the defenders backward and thus theoretically push their own team further down field. Therefore, this ability to generate greater sprint momentum over short sprints (typically 10 m) is of considerable importance. The NRL players exhibited the same velocity as the SRL players, but their greater body mass allowed them to generate greater sprint momentum. Players in rugby leagues typically sprint approximately 10 m before encountering opposing players. Thus, although speed, acceleration, and agility per se do not discriminate between NRL and SRL players, the combination of a large body mass with any speed quality leads to a physical advantage in sprint momentum. This increased sprint momentum would give NRL players an advantage over SRL and other lower ranked players given the intense physical collision nature of rugby league football. This is an important and original finding from this study that has considerable relevance to athletes and coaches in such sports.

One point that requires some explanation is the month's difference between speed and strength/power testing. Because practice games were set to commence, the speed testing was performed 1 month before strength and power testing so the leg fatigue and minor injuries associated with rugby league games would not adversely affect the speed results. Thus, the athletes were in peak condition, having completed their pre-season speed training cycle and having 4 days'rest immediately before speed testing. The strength/power testing occurred 1 month later at the completion of the pre-season strength/power training cycle, which established the players in peak condition.

The basic result of the squad comparison was that increased leg strength and power, facilitating increased leg drive, and sprint momentum seem to be of crucial importance to rugby league players. However, it is also critical to remember that all these tests were performed in a fresh, rested state, but that rugby league players cover distances of 10 km or more in each 80-minute game (16). Therefore, apart from the initial moments of a game, the players are never in a rested state but are battling cumulative fatigue for 80 minutes. How the fatigue associated with such a high running volume would affect any of the above strength, power, and sprint factors is not known and would be difficult to quantify. It may be that players'speed, strength, power, agility, and acceleration after at least 30 minutes of play, when a distance of 3-4 km has been completed, is more important. Although endurance was not tested in this study, some distinct interaction or interference between endurance capabilities and the above anaerobic factors of strength, power, acceleration, maximal speed, and agility must be considered.

Practical Applications

Of all the lower body tests assessed in this study, maximal strength, irrespective of whether it is analyzed in absolute terms or relative to body mass, seems the most highly related to success in rugby league. Maximal power and sprint momentum were also strongly descriptive of whether a player had NRL or SRL status. Based on these data, rugby league players should strive to increase lower body maximal strength and power to increase leg drive. Increased body mass also plays a role in enhancing sprint momentum, which is of utmost importance from a rugby coach's standpoint. Thus, rugby league players should participate in a periodized resistance training program aimed at developing high levels of strength, power, and body mass. Acceleration, maximal speed, and agility did not differentiate between NRL and SRL players, but this finding does not preclude their importance in the physical preparation of rugby league players. Previous research (1,3,4) has indicated that both NRL and SRL players are similar in speed qualities and superior to lesser ranked players in these qualities.

The undeniable important of endurance was not investigated in this study, but its interaction with the other performance factors warrants further study. Specifically, whether the ability to tolerate fatigue and lessen its deleterious effect on strength, power, acceleration, maximal speed, and agility would also distinguish NRL players from lower ranked players is of considerable interest.

References

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Keywords:

squat; jump squat; running; performance

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