Maintaining Mass in Motion: The Longitudinal Tracking of Acceleration, Momentum and Body Mass in School Rugby Union Through a 15 Week In Season Phase

Introduction
High speed running has been identified as an important physical characteristic for performance in Rugby Union (RU; Duthie, 2006; Gamble, 2004). Indeed, authors have proposed that the combination of sprint velocity, sprint momentum and body mass is able to discriminate between playing levels and age categories (Barr et al., 2014; Till et al., 2014). However, research suggests there is a need to describe seasonal changes in such physical characteristics to provide perspective on variation in performance across playing positions and age groups (Darrell-Jones et al., 2015). Given the importance of these physical qualities in youth RU and the paucity of longitudinal data on such performance markers, the aim of this study was to explore the change in acceleration, momentum and body mass over a 15-week in season period within a school RU season. In addition, the study aimed to assess the difference in these physical qualities between age categories and playing positions. It is hoped the findings of this study provides an insight into the potential for development in acceleration performance and body mass during a school RU season, as well as highlighting the differences between divergent categories of players to aid physical training practitioners in supporting the development of acceleration, momentum and body mass in school RU across age groups and playing positions.


Methodology
34 school RU players participated in the study. Participants ranged from 16-18 years of age and were assigned as year 12 or 13 based on their academic year (Yr) and forwards or backs based on their most common playing position. All testing was completed at the same time point in the training week and all testing was undertaken by an accrediated strength and conditioning coach (UKSCA). All players engaged in a physical training programme consisting of strength, power and speed development, as well as rugby training and matches. Body mass was measured to the nearest 0.1kg using Seca alpha (model 813) scales. Acceleration was measured at 10m using timing gates (Brower Timing Systems, IR Emit, USA). This distance was chosen to enable assessment of initial sprint velocity and momentum as used by Barr et al. (2014). Each sprint was started 0.5m behind the first timing gate. Players were instructed to set off in their own time and to run maximally through the second gate. Times were measured to the nearest 0.01s. The average of three trials was used for analysis of acceleration and momentum.

Results
In order to compare the change in performance in acceleration, momentum and body mass over 15 weeks, a single factor anova was conducted. An analysis of variance showed that the effect of weeks was significant for acceleration, F (14, 183) = 2.05, P = 0.02. However, the only significant difference between consecutive weeks was for weeks 10 and 11 (p < 0.05). The effect of weeks on momentum and body mass was not significant and there were no significant differences between consecutive weeks. In order to compare performance between age categories and playing positions an independent samples t-test was conducted. For 10m acceleration, the test was not found to be statistically significant between Yr12 vs Yr13 but was statistically significant between forwards vs backs (p < 0.05), with the backs performing better with a small practical significance. For momentum, the test was found to be statistically significant for both Yr12 vs Yr13 and forwards vs backs (p < 0.05), with forwards and Yr13 performing better with a large practical significance. For body mass, the test was found to be statistically significant for both Yr12 vs Yr13 and forwards vs backs (p < 0.05), with forwards and Yr13 performing better with a large practical significance.

Practical Applications These results provide an insight into the changes in acceleration, momentum and body mass over a 15-week in season period, as well as the difference between age categories and playing positions in school RU. These results suggest that, with the regular inclusion of weekly strength, power and speed development sessions, acceleration, momentum and body mass can be maintained through an in-season period in school RU players despite the fatigue associated to regular training and playing load (Oliver et al., 2015), in addition to academic and pastoral commitments. As such, the consistent exposure to such physical training qualities should be encouraged during the competition phase of a school RU season. Given the greater mass in motion of older forwards, physical training practitioners may consider appropriate training formats to drive further adaptations within particular age categories and playing positions, i.e. younger forwards, to prepare for the physical demands of more senior RU as outlined by in the literature (Darrell-Jones et al., 2015).

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