Discussion
This study evaluated the body mass of elite international players at RWCs from 1991 to 2019 (men) and 2010 to 2017 (women). Our aims were to expand on previous findings of increased body mass after the sport became professional, with specific reference to whether developing rugby nations (T2 nations) and women have followed a similar trajectory to those observed for established T1 nations.
Time trends in men
Accordingly, our first finding is that various elements of the body mass of players in men’s RWCs has stabilised in recent RWC tournaments, suggesting that an upper limit in body mass is being approached, and may even have been surpassed. This assertion is based on numerous findings, including our finding that no increases in median mass have occurred since 2011, the first documented decrease in player mass in the 2019 tournament, and our finding that the proportion of heavy players (assessed as players heavier than the top quartile from 1991) has begun to stabilise, while the proportion of lighter players (assessed as players lighter than the bottom quartile from 1991) has increased in 2019 compared with 2015.
This confirms previous research showing large increases soon after professionalism4 followed by a levelling off in body mass,4 5 but extends those previous findings to the most recent RWC in 2019, and explores the time-course of the increases at each RWC rather than over decades.4
We do acknowledge that single tournament increases may be small and insignificant, but their cumulative effect may be a significant increase in body mass over longer periods, as we find historically (figure 1). It cannot thus be discounted that the small increase in forwards’ mass between 2011 and 2019 will continue in the future, and so it would be premature to conclude definitively that an upper limit has been reached. However, 2019 was the first RWC since 1995 where neither the forwards or backs have increased in body mass. In combination with data from the club game, we suggest that elite player mass is close to reaching levels beyond which performance advantages no longer accrue, if it has not already reached that level.
Historical comparisons between 2019 and 1991 reveal striking differences. We show, for example, that the heaviest quarter of players in 1991 would be among the lightest quarter of players in 2019 (figure 3A,B). These changes, at the heavier end of the player mass spectrum, are mirrored by similar reductions in the proportion of players lighter than a reference mass corresponding to the lightest quarter of players in 1991 (figure 3A,B), the result of which is an overall increase of 10 kg and 9 kg in the median mass of T1 forwards and backs, respectively. However, these increases were largely achieved between 1991 and 2003, with much smaller cumulative and single-tournament increases since.
The reasons for the body mass increases soon after professionalisation of rugby have been discussed previously.4 5 They include both direct and indirect consequences of professional rugby. In addition to what has been discussed previously, we offer the following coach-driven insights and hypotheses for our findings.
Directly, professional rugby has enabled players to access full-time team environments to maximise and optimise training and nutrition modalities to increase muscle mass to develop strength and power required for performance. Elements of these professional environments exist from junior levels, including talent identification systems and age-grade rugby competitions that preferentially select players with desired physical attributes. Full-time environments, including academies that were established by many professional clubs in around 2000 enable an earlier start to conditioning programmes, the effects of which have been documented as increases in body mass in 18-year-old players13 that are similar in magnitude to those described here. At some point, however, the benefits of full-time conditioning and the application of expertise will be realised, and only very small increases will be possible thereafter.
Indirectly, numerous laws have been introduced that may have facilitated the evolution of larger players. Among them was a progressive increase in the number of substitutes allowed during matches, which has allowed coaches to prepare players for shorter periods of higher intensity play. This may favour the development of power and strength, enabling greater mass to be carried without the negative performance consequences of premature fatigue.4
More recently, however, changes in the sport may have counteracted these early changes. Law changes aimed at improving aesthetics of the sport have caused ball in play time, which averaged 35% in 1995, to increase to 44% in 2019,2 and has been documented as high as 50% in recent professional club and international events.3 Combined with fewer stoppages for set pieces in the game, the result has been an increase in the length of activity cycles, particularly since 2011.2 3
Coaches systematically evaluate these game demands, and then prepare players against known targets, which may have shifted conditioning paradigms towards the development of players able to withstand fatigue. A secondary consequence of this might be smaller players who carry less potentially detrimental body mass. We suggest that further increases in player size may not be possible given the game demands on players, unless future law change allows longer periods of recovery (eg, prior to set pieces, while kicks are taken, or during Television Match Official decisions) or less ball in play time. These hypotheses may be tested in future by exploring specific player involvement in various match activities, playing time by playing position and activity cycle changes over time.
Another set of changes that may create a ceiling for player size is related to coaching intent in the sport. This encompasses both attacking and defensive strategies and tactics. It includes the desire to play continuously and at a higher pace with shorter recoveries, to seek competitive advantages through speed, fitness and playing tempo, rather than a reliance on size alone. This would necessitate a greater endurance component of conditioning, to perform constant high-tempo play. The change in tactical intent is not universal to all teams and coaches, but is generally supported by the observation that the number of passes and rucks has increased by 15% since 2007, in part due to more ball in play time, but also driven by an increase in the rate of these activities,2 as well as our hypothesis that teams may regularly retain the ball through multiple phases more frequently than in the past.
Defensively, the emergence of so-called rush–defence patterns, and teams’ approaches to contesting for the ball during rucks, may also have shifted optimal size down compared with in the past. The priority of modern rugby defensive systems is to apply pressure to attacking teams by reducing space and time through aggressive, fast-rushing defensive lines. This requires that players avoid commitment to rucks, instead returning to position in the defensive line, followed by greater acceleration and speed in repeat-bouts within their defensive strategy. The ability to meet these match demands may be constrained if excessive mass is carried. Therefore, it may be that the requirement for sustained higher intensity effort has set a ceiling on size, particularly in backline players in whom mass is not required for static, high-force generating activities, and that this limit is close to being reached.
From a player welfare perspective, the increasing size of players has been named as a primary contributor to the significant injury risks documented in various injury surveillance studies in Rugby Union.1 6 7 12 14 15 Given that most injuries occur in the tackle,1 7 16 17 with rucks second most frequently responsible for injuries, the increasing body mass of players at all levels4 11 13 18 is of concern because contact injuries are the result of the transfer of kinetic energy and excessive inertial loading.7 19 20
It would appear that such concerns had specific merit from 1995 to 2011, but that given the stabilisation of size since 2011, any impact of body mass on welfare is similar in 2019 as in 2011. Thus, while the greater size of players in 2019 is arguably a welfare consideration for comparisons between the current situation and 2003 and earlier, it is less of a factor since 2011. That said, we do recognise that body mass is a crude metric with which to understand how the physical capacity of players has changed, and that body composition, and also the ability to exert force in contact, are likely contributors to injury risk. We do not currently have measures to explore these possibilities.
Time trends in women
Among women, we have insufficient data to allow such long-term evaluation, since it was possible to evaluate body mass over only three tournaments. Within this narrow range, we find variable increases in body mass from tournament-by-tournament (figure 2). Women’s rugby is still amateur and semiprofessional in most nations, with the first elements of professionalism, including full-time salaried players, access to regular high-level club competition and the application of sports science and medicine, introduced only in 2016.
In this regard, some nations in women’s international rugby are at a stage of their life cycle similar to the men’s game in 1995. While it would be premature to predict that the trend among women will follow what was observed in men between 1995 and 2003, it is enticing to project that should similar changes occur in women as occurred in men, the body mass of women forwards and backs will increase by approximately 3% and 7% over the next three RWC cycles.
Of course, many of these gains may already have been realised as a result of application of sports science and conditioning principles learnt from men’s rugby and other sports, and the inclusion of Sevens in the Olympics will have accelerated talent search and high-performance systems to produce elite women players.21 This may be especially true for nations whose Olympic programmes and professional rugby structures are mature.
Generally, however, women’s rugby is significantly smaller in size, both financially and in terms of player number, which creates possibilities for large effects should changes be made on either the talent identification or player development pathways that produce international women’s players. That is, if investment into women’s rugby improves the identification of more athletic women, or if professional environments like those that have driven men’s increases in size are applied in more countries, then we would expect that the body mass and height of players in the sport will increase significantly in future.
T1 versus T2 comparisons
Our second important finding is that differences in body mass between T1 and T2 nations, while small in absolute terms, have persisted across the period from 1991 to 2019, with some indications that the differences may be increasing. This is seen in the absolute differences in body mass between T1 and T2 players (table 1, figure 1), in the proportion of both forwards and backs heavier and lighter than a historical reference value (figure 3), and in the evaluation of which teams are heaviest at RWCs (figure 4).
While the absolute differences in median and average body mass are small (table 1), we found that only 50% of T2 forwards were heavier than a reference value of 108 kg in 2019, compared with 72% of T1 forwards (figure 3A). Of interest is that in 2007, the proportion of T1 and T2 forwards heavier than 108 kg was similar (58% for T1, 55% for T2), but the gap has increased at every tournament since, with the result that in 2019, it reached its largest levels yet (72% vs 50%). This suggests that T2 nations are not producing heavier forwards and backs as readily as T1 nations, contributing to the 5 kg difference in median mass of forwards in 2019 (table 1).
A similar phenomenon exists when assessing the proportion of backs heavier than a reference value of 87 kg (figure 3B). This again suggests a relative inability of T2 teams to develop heavier backs, even though T2 backs median mass is similar to that of T1 nations (table 1). We also find that the proportion of T2 forwards and backs who are lighter than reference masses of 98 kg and 79 kg, respectively, is greater than in T1 teams.
In effect, whether assessed relative to the median or these 1991 quartile reference points, T2 nations produce more lighter players, while T1 nations produce more heavier players (figure 3), and this disparity is emerging and increasing in 2019. PI teams feature among the heavier teams, though they are also classified as T2 nations. We chose to analyse these nations separately because it has long been perceived that their players are larger than those of other nations, which we confirm here.
We hypothesise that these differences are related to the previously described professional systems that develop rugby players. In particular, professional environments, often linked to well-developed school competitions in T1 nations, introduced many more young players to advanced training methods sooner. This increases their training age by the time they exit junior rugby, when they receive further advantages in high developed professional environments. This is expected to drive the physical development of players in ways that T2 nations have been unable to match.
Given the purported importance of size and strength to rugby performance and success,4 11 these disparities in body mass have implications for competitive parity. They may also contribute to welfare considerations for the smaller T2 players. It would be desirable to minimise the differences, and certainly to address what may be causing any body mass differences to become larger.
The same T1 versus T2 disparities are observed in women’s rugby, where the heaviest 10% of teams are exclusively T1 nations, and the lightest teams are always T2 nations. Of significance is that in the most recent women’s RWC in 2017, T1 forwards and backs were significantly heavier than T2 forwards and backs (figure 1). This is in part the result of an increase in body mass of T1 forwards since 2014, while T2 forwards and backs have stabilised or even decreased in body mass (figure 2). The proportion of T2 forwards and backs who are lighter than 74 kg and 64 kg has increased, while heavy forwards have decreased in T2 teams (figure 3C,D).
This may be of particular concern for parity between countries in the developing women’s game. Given the purported importance of size to rugby performance outcomes,11 these enlarging disparities, while early, may entrench differences between established and developing nations and increase the barriers for success for developing T2 teams. This is particularly challenging for women’s rugby, because if professional elements drive increases in player mass, among other attributes that enhance performance, as we have described above, then there is a risk that the vastly unequal timing and magnitude of introducing these elements may create performance differences that skew results dramatically in favour of early professional teams, and which will be very difficult to overcome in future.
Finally, our third important finding is to develop a set of reference ranges for men’s and women’s elite players, that may guide player identification, including at the talent identification stage, and the selection and management of players in elite performance systems.
Limitations
This study has some limitations. We are reliant on the body mass of players as reported by teams when their World Cup squads are announced and registered. There is a risk of both under-reporting and over-reporting, and we cannot independently verify the accuracy. However, given the large size of the cohorts, a strength of this study, and its longitudinal nature, we believe that any random inaccuracies in this regard will be minimised. As mentioned, we do not have the ability to explore whether the body mass reported is changing in terms of body composition, but acknowledge that stable body mass since 2011 does not necessarily imply that physical conditioning or composition of players is unchanging. Indeed, another possible explanation for the change in body mass of players early during professionalisation is doping of players,4 the effects of which would include increases in muscle mass and reductions in fat mass, and which may be present now. We do not have the necessary data to evaluate this possibility in the context of the aims of the present study.
We have not included height in this analysis, for the sake of clarity and to allow greater depth of analysis of body mass data, but the evolution of player height in conjunction with body mass would be worthy of future research, since it is arguably a significant contributor to the changes we describe here.
Our discussion of the findings is reliant on insights from elite coaches, who offer novel, and as yet untested theories to explain the results. These must be explored in future, with research to specifically refute or confirm the validity of the hypotheses we offer here. Finally, it must be acknowledged that analysis of players at a World Cup, held every 4 years, is itself a reflection of the wider game played at the professional and occasionally amateur levels around the world. While we analyse every tournament since 1991 and 2010 for men and women, respectively, we acknowledge that these tournaments are snapshots of a select group of players in comparison to the larger elite rugby-playing community, whose body masses may have varied differently over the analysis period.
Conclusion
In conclusion, we show here that men’s player mass increased significantly soon after professional elements were introduced, but that the magnitude of these increases are now smaller in size and suggestive of the approach of an upper limit, perhaps driven by the evolution in match demands and the nature of play required in modern rugby. We show that women’s rugby, early in its life cycle, has undergone some changes similar to those seen in men, though whether this continues into the future is uncertain. We also show that T2 nations are disadvantaged in size in both the men’s and women’s game, with lower median and mean mass of forwards and backs, and an apparently inability to produce as many players whose body mass lies above what would historically be considered heavy. This may have implications for the competitiveness of T2 nations, and requires assessment if gaps are not to become excessively large and material to performance.