Force production during the sustained phase of Rugby scrums: a systematic literature review

Martin and Beckham BMC Sports Science, Medicine and Rehabilitation (2020) 12:33 https://doi.org/10.1186/s13102-020-00174-z RESEARCH ARTICLE Open Access Force production during the sustained phase of Rugby scrums: a systematic literature review Eric Martin* and George Beckham Abstract Background: Since World Rugby changed the laws regarding scrums in the 2013–2014 season, the sustained push phase of the scrum has increased in tactical importance. Therefore, the purpose of this systematic literature review was to examine the biomechanical demands during the sustained push phase of individual, unit, and full pack scrummaging. Methods: Pubmed, EBSCO (specifically and simultaneously searching Academic Search Premier, CINAHL, and SPORTDiscus), and Google Scholar were searched for any research that presented force production in a live or simulated rugby scrum. Study quality was appraised using the National Institute of Health’s Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies. Recorded scrum forces, positioning of players including joint angles, and testing procedures were extracted and narratively synthesized. Results: Twenty six studies were included in the review. 50% of included studies were rated good, 31% fair, and 19% poor. Major limitations included not reporting any effect size, statistical power, or reliability. Reported group mean values for average sustained forces against a machine generally ranged from 1000 to 2000 N in individual scrums and 4000–8000 N for full packs of male rugby players older than high school age. Individuals seem to optimize their force generation when their shoulders are set against scrum machine pads at approximately 40% of body height, with feet parallel, and with knee and hip angles around 120°. A 10% difference in pack force seems to be necessary for one pack to drive another back in the scrum, but little data exist to quantify differences in force production between winning and losing packs during live scrums. Data collection within studies was not standardized, making comparisons difficult. There is a lack of data in live scrums, and the current research indicates that machine scrums may not replicate many of the demands of live scrums. There is a lack of data for female rugby players. Conclusions: This review indicates an optimal individual body position for players to strive to achieve during scrummaging, consisting of a low body height (40% of stature) and large extended hip and knee angles (120° each). Keywords: Rugby union, Scrummaging, Biomechanics * Correspondence: Kinesiology Department, California State University Monterey Bay, 100 Campus Center, Seaside, California 93933, USA © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Martin and Beckham BMC Sports Science, Medicine and Rehabilitation (2020) 12:33 Background In rugby (including union, league, and 7 s), when play is restarted after a dead-ball infringement such as a knockon or other stoppage, the two teams contest for the ball with a scrum [1, 2]. For example, in the fifteen-a-side variant of rugby union (the setting in which most research has been conducted), eight players from each team bind together to form a pack, which then opposes the other team’s pack, giving each team an opportunity to gain possession of the ball. During the scrum, each pack attempts to push forward with more force than the other team to gain ball possession and territory and to disrupt the other team from successfully handling the ball. While winning many scrums does not necessarily mean a team will win the game [3], a successful scrum (whether the team is putting the ball in or manages to steal the put in from the other team) can provide a strong platform for scoring tries [4]. Thus, the ability to apply greater force against the ground and against the opposing pack during the scrum may be of great interest to players and coaches, in order for them to gain a tactical advantage in the game. Under the old laws of rugby union (prior to the 2013– 14 season), opposing front rows started further apart, resulting in greater peak impact forces upon engagement. A pack that could generate greater peak impact forces than the opposing pack during the engagement phase (often defined as the moment from initial contact until 1 s after peak force occurs [5, 6]) had a tactical advantage. However, high impact forces were related to injury rates in the scrums [7], especially the catastrophic injuries [8]. Thus, over the years, World Rugby has changed the rugby union laws about scrummaging with the specific goal of decreasing the impact forces and thus decreasing injury rates. One law modification tried was a staggered scrum engagement, in which the opposing front rows engaged each other before the rest of the pack bound on. While this resulted in lower impact force, it also created greater scrum instability and therefore still an unsafe scrum [9, 10]. To better understand the risk of injury and effect of potential scrum rule modifications pertaining to the process of pack engagement, World Rugby commissioned a series of studies [5, 6, 11]. The findings from these studies led World Rugby to adopt the “crouch, bind, set” method of engaging a scrum, which reduces the peak forces on engagement while still providing good scrum stability [5, 11]. Due to the law changes about scrum engagement, the engagement phase of the scrum has not only become safer but of less tactical importance. The sustained push phase of the scrum (from the moment the ball is put in until the scrum has ended), which has received less research attention, has now become of greater tactical importance. Furthermore, the rule changes regarding the scrum have resulted in scrums lasting longer, from an average of 7.5 s prior to the law change to 10.8 s by the 2016 season [12]. Additionally, since this law change, there has been a significant increase in the number of scrums performed during Englis (...truncated)