International Journal of Sports Physiology and Performance, 2015, 10, 695 -702 http://dx.doi.org/10.1123/ijspp.2014-0151 © 2015 Human Kinetics, Inc. ORIGINAL INVESTIGATION Mechanical Properties of Sprinting in Elite Rugby Union and Rugby League Matt R. Cross, Matt Brughelli, Scott R. Brown, Pierre Samozino, Nicholas D. Gill, John B. Cronin, and Jean-Benoît Morin Purpose: To compare mechanical properties of overground sprint running in elite rugby union and rugby league athletes. Meth- ods: Thirty elite rugby code (15 rugby union and 15 rugby league) athletes participated in this cross-sectional analysis. Radar was used to measure maximal overground sprint performance over 20 or 30 m (forwards and backs, respectively). In addition to time at 2, 5, 10, 20, and 30 m, velocity-time signals were analyzed to derive external horizontal force–velocity relationships with a recently validated method. From this relationship, the maximal theoretical velocity, external relative and absolute hori- zontal force, horizontal power, and optimal horizontal force for peak power production were determined. Results: While dif- ferences in maximal velocity were unclear between codes, rugby union backs produced moderately faster split times, with the most substantial differences occurring at 2 and 5 m (ES 0.95 and 0.86, respectively). In addition, rugby union backs produced moderately larger relative horizontal force, optimal force, and peak power capabilities than rugby league backs (ES 0.73–0.77). Rugby union forwards had a higher absolute force (ES 0.77) despite having ~12% more body weight than rugby league forwards. Conclusions: In this elite sample, rugby union athletes typically displayed greater short-distance sprint performance, which may be linked to an ability to generate high levels of horizontal force and power. The acceleration characteristics presented in this study could be a result of the individual movement and positional demands of each code. Keywords: acceleration, sprint kinetics, radar gun, power, body orientation Rugby union and rugby league are collision sports that require In addition to between-codes differences, there are noted differences intermittent bouts of maximal sprint running.1,2 Although the in physiological demands between the forward and back positions majority of sprinting efforts occur over relatively short distances of both rugby union1 and rugby league.4,5 For example, positional (ie, <30 m), athletes will repeatedly achieve maximum acceleration differences have been identified in anthropometric characteristics, and velocity during competition.3,4 Thus a successful rugby code short-distance sprint performance, and horizontal force output athlete at an international (elite) level must be proficient over the between forwards and backs in professional rugby league athletes.5 different phases of sprint running, including initial acceleration and During sprint acceleration, the orientation of ground-reaction maximum velocity. force (GRF) has been shown to be a stronger indicator of per- There are distinctions in physiological profiles between rugby formance than overall magnitude.6,7 Thus, athletes who train union and rugby league owing to the differing training and competi- with differing body orientations, and therefore force-application tion demands of each sport. Rugby league features fewer on-field techniques,6 may display different acceleration capabilities. With players than rugby union (13 vs 15), resulting in greater distance noted differences in physiology and strength profiles between rugby between positions, therefore allowing athletes to obtain higher codes,8 it could be hypothesized that each would display differ- sprinting velocities. Moreover, rules regarding engagement around a ent capacities for force orientation at lower or higher velocities. ruck or tackle result in a disparity in space between codes. Namely, This could occur from code-specific training, on-field competition rugby union rules allow athletes to contest for ball after tackle, which demands, or a combination of the 2. results in lower-orientation, force-dominant movements. Competi- Linear force–velocity (F–v) and parabolic power–velocity tion play is punctuated with short maximal accelerations with ath- (P–v) relationships have been used to profile the macroscopic exter- letes driving over the ball for possession and resisting attacks from nal mechanical capabilities of the neuromuscular system during a opposing players. Rugby league athletes, however, must back-pedal host of multijoint movements.9–14 Researchers have reported these 10 m toward their half, initiating more open space to sprint, which capabilities during overground sprinting.15 Essentially, the F–v rela- gives credence to higher hits to keep the ball from going to ground. tionship can indicate an athlete’s ability to produce net horizontal GRF with increasing velocity and is summarized by the following variables: theoretical maximum force (F0), theoretical maximum velocity (v0), and maximum power (Pmax) produced during the 12,16 Cross, Brughelli, Brown, Gill, and Cronin are with Sports Performance given movement. Graphically, F0 and v0 represent the extreme Research Inst New Zealand (SPRINZ), Auckland University of Technol- ends of the linear F–v relationship, and Pmax represents the apex ogy, Auckland, NZ. Samozino is with the Laboratory of Exercise Physiol- of the parabolic P–v curve (see Figure 1).14 As the relationship ogy, University of Savoy, Le Bourget-du-Lac, France. Morin is with the between these variables encompasses the entire capability of the Laboratory of Human Motricity, Education Sport and Health (LAMHESS), neuromuscular system, it is inclusive of mechanical properties of University of Nice Sophia Antipolis, Nice, France. Address author cor- individual muscles, morphological features, and neural mechanisms respondence to Matt Cross at [email protected]. underpinning motor-unit drive. Moreover, these variables integrate 695 696 Cross et al an athlete’s ability to orient the total (ie, resultant) force developed measurement technique.15,17 Given that previous authors have in a forward horizontal direction. Hence, F0 and v0 also depend on reported differences in sprinting kinematics and kinetics between athlete ability to orient force effectively onto the ground at low and forwards and backs in both codes,5,18 we separated the athletes by high running velocity, respectively. The addition of theoretical and position. Although the current study design does not allow for inves- maximal outputs allows for a more comprehensive analysis of the tigation into the cause and effect of long-term training, it provides properties underlying sprint performance.14 a useful model for comparisons between similar athletes who train The purpose of this study was to determine and compare and compete with different body orientations. Such information mechanical properties of overground sprinting between elite rugby could potentially be used to monitor athletic performance and inform union and rugby league athletes using a recently validated field individualized training strategies.15,19,20 Methods Subjects Thirty elite rugby code (15 rugby union and 15 rugby league) athletes volunteered as participants in this study (see Table 1). Athletes were placed in a participant pool respective of their code and position, classifying them as rugby union or rugby league and forwards or backs. Fifteen rugby union athletes were grouped into 8 forwards and 7 backs, based on their regular playing posi- tion. All athletes represented New Zealand and were involved in the subsequent 2013 Investec Rugby Championship campaign. Fifteen rugby league athletes were grouped into 6 forwards and 9 backs, similarly based on their regular playing position. This elite group of athletes was drawn from a larger pool of data collected on a professional rugby league team, based on their participation at international level. Athletes represented New Zealand (n = 7), Tonga (n = 5), Australia (n = 1), Cook Islands (n = 1), and Samoa (n = 1). All participating athletes were free of any lower-extremity musculoskeletal or neuromuscular injuries that would have affected their ability to perform the required sprinting task at a maximal effort. Ethics approval for this study was granted by the Auckland University of Technology Ethics Committee (12/332). Figure 1 — A graphical representation of the force-velocity-power pro- files (and associated optimal force and velocity levels for power produc- Design tion) for 2 athletes from each code (light gray [rugby league player] vs dark gray [rugby union player]) over a 30-m maximal overground sprint. This cross-sectional study sought to investigate code and positional Abbreviations: Pmax, maximum power; F0, theoretical maximum force; differences in time-to-distance splits and power-force-velocity char- Fopt, optimum force. acteristics through a recently validated method using spatiotemporal Table 1 Athlete Characteristics Rugby Union Rugby League N = 15; 8 forwards, 7 backs N = 15; 6 forwards, 9 backs ES (CI) Inference Age (y) forwards 28 ± 5 25 ± 3 0.59 (–0.30;1.48) Unclear backs 24 ± 3 24 ± 2 0.29 (–1.17;0.58) Unclear Height (m) forwards 1.90 ± 0.1 1.87 ± 0.1 0.38 (–0.51;1.27) Unclear backs 1.82 ± 0.1 1.80 ± 0.1 0.09 (–0.76;0.95) Unclear Mass (kg) forwards 114.55 ± 6.3 107.13 ± 7.3 1.01 (–0.09;1.94) Moderate** backs 92.64 ± 4.9 94.57 ± 11.5 0.16 (–0.97;0.66) Unclear Note: Values are mean ± SD or effect size (ES) ± 90% confidence interval (CI). **Likely, 75–94%. IJSPP