# Changes in Sprint Momentum in Elite Rugby Union Players over a Three-Season Period
## 1 Introduction  
Rugby union is a team sport that involves intermittent physical efforts, with frequent physical contacts and wrestling phases [1]. Achieving a high-level performance in rugby union requires the ability to sprint, as well as to accelerate and decelerate quickly [2, 3, 4]. The ability to accelerate, or the rate of change in speed, is considered more fundamental to success in team sports than maximum velocity [5]. [PERSON] et al. [6] found significant correlations between players' sprinting speed and technical/tactical performance indicators such as linebreaks and tries scored. A high sprinting speed allows players to be more difficult to tackle by opponents, while providing an advantage in making a tackle in the defensive phase [7]. Generally, backs are faster than forwards, particularly over distances of 30-40 m [8], and, between the 15 th and 35 th meter of a sprint, they can reach sprinting speeds comparable to those of competitive sprinters [4]. A large body size has been identified as a crucial factor for success in rugby union [9; 10]. Over the past 35 years, the physical characteristics of rugby union players have shown an evident trend to increase [1; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. Previous evidence suggests that the most successful teams in Rugby World Cups from 1987 to 2007 had the heaviest forwards [10] and the highest average team body mass [9]. Moreover, higher body mass in forwards correlates with increased force production during scrums [20; 21] and tackles [22]. Forwards are also the players more involved in contacts in elite-level rugby, thanks to their greater body mass as compared to backs [4; 23; 24]. Another important physical characteristic of rugby players, especially in relation to tackles and collisions, is sprint momentum, that is, the product of a player's sprinting velocity and body mass [25]. Momentum was observed to discriminate rugby union players of different competitive levels [16; 19; 26; 27; 28; 29; 30]. Forwards, despite exhibiting lower sprinting speeds than backs, show higher momentum thanks to greater body mass [17]. Previous authors also reported relationships between momentum and technical/tactical performance. [PERSON] et al. [31] analyzed the correlations between physical test results and individual players' key performance indicators measured during rugby union matches, observing large correlations between momentum and both dominant collisions (\(r\) = 0.862) and offloads (\(r\) = 0.776) performed by backs. Furthermore, in international female rugby union players, [PERSON] et al. [32] observed that momentum was associated with carries and tackles relative to time in forwards. Since high body mass might be counterproductive to achieving a high running speed in a rugby union player, an optimal combination of body mass and sprinting speed is likely a key factor to maximize the momentum. At present, the interrelationships between short- and long-term changes in sprinting speed, body mass, and momentum, and the impact of these changes on a competitive performance in elite rugby union, have not been fully elucidated. Some authors have examined variations of fitness characteristics, including momentum, in rugby union across one season, reporting changes in sprint speed and momentum during the season in young players [31; 32; 33]. Instead, little has been investigated about longer term changes in momentum. To our knowledge, the only study performed over multiple seasons was conducted by [PERSON] et al. [34], observing an increase of approximately 4.8% in body mass over a two-year period in junior players transitioning to the senior category. Noticeably, this change did not negatively affect the sprinting speed, leading therefore to an overall increased momentum [34]. There are no previous studies examining how long-term changes in momentum affect the individual technical/tactical match performance in rugby union players.