Head injury risk in rugby: U20 World Champs case study

23 Aug 2019 Posted by

Last week, I posted a lengthy but detailed article explaining how the current initiatives to sanction high tackles in rugby arose. It was an article that described a process, start to finish, that led experts to advise that head injury risk for both players might be reduced if high tackle sanctions were more frequently and consistently applied. In it, I said I’d use the recent U20 World Champs as a case study to further explain why some actions and behaviours are more risky or likely to cause head injuries than others.

This is that piece! So if you’re interested, do read on and hopefully it clears up the issue around risk or propensity, and why simply counting and “believing your eyes” might not be the best way to understand risk!

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Background – the rationale

First, the rationale, which is really important and thus worth re-iterating:

The strategy to harshly sanction dangerous high tackles is based on evidence gathered by analyzing when head injuries occur, and then recognizing that there are certain factors or behaviours that increase the risk.  Once those were identified, World Rugby consulted with expert groups that included players and coaches, and they proposed and refined the initiatives that are now in place, including the Sanction Framework, whose intention it is to:

  • Improve consistency in decision making
  • Act as a disincentive to what is recognized as the more high-risk tackle situation, namely one where the tackler is upright, and is making higher contact with the ball carrier, such that his head is in close proximity to the ball carrier’s head or shoulder.

What World Rugby is NOT trying to do is dictate what height the tackle should be (if they wished to do this, they would have lowered the height of a legal tackle to the armpit, sternum or even the waist.  This is currently being explored in two trials – one in Stellenbosch for armpit height, another in a few competitions for waist height, after a French Rugby proposal to trial this. Once those trials are completed, then based on evidence, this situation can be revisited).

Instead, what World Rugby ARE trying to do is to get players and coaches to take ownership of tackle technique and execution to reduce the risk.  Effectively, the mandate is to make adjustments in technique in order to reduce the risk.

The specific adjustments that should be made are up to the coaches and players, for they know best, but the ultimate intention is to reduce head contacts to both the tackler and the ball carrier in the sport.  To appreciate this, we can begin, conceptually, by asking when the dangerous head contacts happen?  And we know that the answer is:

  1. For the ball carrier, head injuries can only really happen when their head is struck by the opponent. This happens more often in illegal high tackles
  2. For the tackler, there is always a risk of head contact, but it is HIGHER when the tackler is upright, and when the tackler’s head is in close proximity to the ball carrier’s head or shoulder. You can read all about the detail behind these points in the article I wrote last week

What the sanction framework is doing is disincentivizing the highest risk outcome of the tackle – head contact to either player, and in particular higher risk contact to the tackler’s head.  How the coach achieves this is very much up to them, but sanctions are intended to draw attention to the risk, and to compel those in the sport to act for the protection of both players – tackler and ball carrier.

With that in mind, below is a brief analysis of the U20 championship head injury set.  This uses identical methods to the analysis that was undertaken on three years of global rugby to arrive at the initiatives that were proposed by the various expert working groups.

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The process at U20s

In order to understand risk of head injuries, you need to know two things:

  • How many injury events occurred?
  • How many total events occurred?

The latter is basically asking “How many could there have been?”. This allows you to work out the relative likelihood of a given event to cause an injury.  In effect, you’re able to to identify “Per 1000 events of each type, how many injuries happened?”

So, we begin with the head injury cases.  World Rugby has access to the full medical record of the tournament in CSx, and can thus determine the precise number of HIA 1 entries – these are players removed after a head impact, and then they are either cleared of concusison, or diagnosed as concussed as part of the entire HIA process. The table below presents all the tournament’s HIA 1s, in random order:

In total, there were 24 HIA 1 cases documented in the CSx system.  Of these, 20 occurred in tackles, two in open play (players diving onto a loose ball), one at a ruck, and one occurred where no video footage exists of the injury.

Note that these proportions – 20 out of 24 HIAs in tackles (83%) – are consistent with our global study, where 75% of head injuries happened in tackles.

Next, we can analyze the 20 cases that happened during tackles, and determine various numbers of events with known risk factors.  In the table above, we have identified:

  • Which player was injured in the tackle?
  • What was the tackler’s body position in the tackle?
  • What was the ball carrier’s body position in the tackle?
  • Where was the head contact with the opponent?
  • Was the tackle deemed foul play?

The summary of these variables is shown in the following tables:

First, we can look at an overall summary, which then sub-divides HIAs into those occurring to the tackler and those to the ball carrier.

Here we see that of the 20 tackle HIAs:

  • 15 were to the tackler (75%)
  • 5 were to the ball carrier (25%)

Note that this too is the same as the finding from the global study, where the split was 72% to tacklers and 28% to ball carriers.

When we look at HIAs to the tackler, we see:

  • Of the 15, 4 occurred when the tackler was upright – 27% of the total
  • 10 happened to a bent at the waist tackler – 67% of the total
  • 1 happened when a tackler was diving – 7% of the total

When we look at HIAs to the ball carrier, we see:

  • 2 of the 5 happened when the tackler was upright – 40%
  • 3 of the 5 happened when the tackler was bent at the waist – 60%

In order to further explore the risk of HIAs when tacklers are either upright or bent, we can combine the above analysis and produce the tables below:

Here we see that six HIAs happened when tacklers were upright (4 to the tackler and 2 to the ball carrier), and that 13 HIAs happened when tacklers were bent at the waist.

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The risk issue – counting and believing your eyes is not enough

At this point, one might look at the following statistics:

  • Six HIAs happen when tacklers are upright
  • 13 HIAs happen when tacklers are bent at the waist

and conclude that bent at the waist tacklers are more dangerous, because more HIAs happen in this scenario.

However, this would be incorrect, because it would not be factoring in the important concept that you can only know the risk of something if you know how many total events of that thing occurred.

For instance, we know that many more people die in car accidents than motorbike accidents.  But this does not mean that cars are more dangerous than motorbikes, because we also know that many, many more people drive cars than bikes.  And so when we correct for the total number of cars, then we would realize that cars are actually far less likely to kill than bikes, and so if we were asked to recommend what people use for safer transport, we would suggest cars.

So what we should be doing is understanding that in risk management, if you cannot totally remove the risk event (tackling), then you have to ask one simple question:

Of two options, which is LEAST DANGEROUS?  In the case of tackles, we are asking:  Is it more dangerous to tackle upright/higher, or is it more dangerous to tackle bent/lower?

In order to answer this, we must know:

  • How many times is a tackler bent at the waist?
  • How many times is a tackler upright?

World Rugby does have these numbers, because its analysts have been coding them ever since the height of a tackle and body position were recognized to be risk factors.

In the U20 tournament, we know that in the 30 matches, there were:

  • A total of 5820 tackles (average 194 completed tackle events per match)
  • A total of 1185 UPRIGHT TACKLES (average 39.5 per match)
  • A total of 4074 BENT AT THE WAIST tackles (70% of all tackles)

Now, we can use these numbers to actually work out how likely a head injury is when a tackler is upright or bent:

So, we can see from the table above that a tackle where the tackler is upright is actually more likely to result in a head injury than a tackle where the ball carrier is bent at the waist.

In the U20 Championship, an upright tackle caused a head injury every 197.5 situations.  A bent at the waist tackle caused a head injury every 313 events.

The conclusion is that the relative risk of being upright is 1.59 times higher than being bent at the waist.

Note that in the global 3-year analysis from 2013 to 2015, we found a very similar number – it was 1.44 times more dangerous for the tackler to be upright than bent at the waist.  The U20 championships consolidates that finding, though I can’t stress enough that it’s a small cohort, but I use it to illustrate the point.

Put differently, what we are seeing here is the following:

  • There ARE more head injuries when a tackler is bent at the waist than when the tackler is upright (just like there are more car-related deaths than motorbike deaths).  In fact, we can quantify this for the case tournament – 13 Bent HIAs and 6 Upright HIAs, a ratio of 2.2 to 1
  • However, there are MUCH more tackle events where the tackler is bent at the waist.  This ratio is 3.4:1, because we had 4074 bent tacklers and 1185 upright tacklers.
  • Basically, upright tacklers are like motor bikes, and bent tacklers are like cars
  • As a result of this combination, we can work out that one of the players involved in a tackle is 59% more likely to experience a head injury when the tackler is upright than when they’re bent at the waist

Note that this does not distinguish between whether the head injury would be to the tackler or the ball carrier when the tackler is upright.  This can be done (if you’re interested, see Appendix A at the end of this article). 

However, for the global purposes of risk management, it is the total risk of injury that is important, and it is clear from the above that risk is reduced for a bent tackler compared to an upright tackler.

Therefore, returning to this important question:

Of two options, which is LEAST DANGEROUS?  In the case of tackles, we are asking:  Is it more dangerous to tackle upright/higher, or is it more dangerous to tackle bent/lower?

The answer is that it is relatively safer to tackle bent at the waist, even though you will see more cases from this action.  Bent at the waist is the car. Upright is the motorbike. And so given that you are going to swap one behaviour for another, the number of incidents does not matter, only the risk.

Note again, and I emphasize this in closing, the main priority here is not to force the player to go lower by making it obligatory

There remains a choice, and it is clear that some low tackles are more dangerous than they may have been if the tackler remained upright and went higher.  The specific situation will influence the risk, but when one steps back from the detail, we see that it is upright tackles that currently pose a higher risk to players.  Therefore, you can reasonably conclude that if technique is to blame, it is the technique of upright tackles that is inferior than bent at the waist tackles.  Or you could suggest that many upright tackles will be safer if they are lowered in height.

The message is for coaches to prioritize safe technique that avoids contact to the head of either player, irrespective of whether they’re in upright tackles or not, because as the very first table suggests, when the tackler is upright, the danger exists from a head to head or head to shoulder impact.

Thus, the priority in terms of player welfare is to identify and then execute the technique that is LEAST LIKELY to result in head contact for either player.

If this means a lower tackle, then that is a choice that the coach and player may make.  If it means a higher tackle, then the same is true. The point is avoidance of head, through better technique, regardless of method. But if method changes that risk, then it must be factored in.

And high tackle sanctions are simply intended to draw attention to the intention.

Ross

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Appendix – more detail on interaction of behaviour and risk

For those interested in a second-level of analysis, below is a brief summary of the relative risk to the players involved in the tackle, and how their body positions combine to create risk.

What this shows is the following:

  • When the tackler is upright, the risk to the tackler is twice as great as to the ball carrier.  This is in agreement with our previous global study result
  • When the tackler is bent at the waist, the risk to the tackler is 2.45 times greater than to the ball carrier
  • However, when we correct for the total number of cases, we see that:
    • The greatest risk exists for an upright tackler, to himself
    • Next largest risk is for a tackler who is bent at the waist, to himself
    • Third is for an upright tackler, but the BC is injured
    • The lowest risk exists for a ball carrier when the tackler is bent.  This is obvious because a bent tackler has almost no chance of striking the head of a ball carrier, unless the ball carrier also bends into contact.  This does happen, as the data show, but is very rare overall – 0.74 Head injuries per 1000 such instances, a risk of 1 in 1358.
  • When combined, as per the previous tables in the main analysis, the risk of a head injury when the tackler is upright is 1.59 times greater than when the tackler is bent.  

Next, let’s consider how the risk changes for different combinations of tackler and ball carrier body position.

Unfortunately, for the U20 Championships, this analysis is basically ineffective, because there are too few cases of HIAs for comparisons. This is a table showing HIA 1s for different tackler and ball carrier positions:

So you have half the combinations with zero cases, and thus no meaningful calculations. We also don’t have a control or denominator for these combinations, and as I explained above, that is non-negotiable if you’re trying to understand risk. The 6 and 13 HIA 1s for upright and bent tacklers, respectively, is what you saw in the main part of this article, and is used to show that an upright tackler has higher risk per 1000 such events than a bent tackler.

Given the thin data from this short tournament, we have to go back to the big study for the data on this. The table below shows the data. It is a little busy (apologies), but here you can see the risk per 1000 tackles for every combination of tackler and ball carrier body position.  I have highlighted the key cells with light red and green shading, and will describe them below:

So, for “Tackler Upright”:

  • When the ball carrier is also upright, we had 131 HIAs, with a risk of 2.8 HIAs per 1000 such tackles.
  • If the ball carrier is bent at the waist, the HIA risk drops to 2.0 HIAs per 1000 tackles.  Also note that this is quite a rare event, and happens only 9.3 times per match on average

For “Tackler bent at the waist”:

  • When the ball carrier is upright, there are 133 HIAs, with an overall risk of 2.6 HIAs per 1000 such tackles
  • When the ball carrier is also bent at the waist, there are 83 HIAs, with a risk of 1.1 HIAs per 1000 tackles.  Note that this is the most common situation of the four looked at here – 48 times per match.

The conclusions from this perspective of analysis is that irrespective of the tackler’s body position – upright or bent – the risk is lower when the ball carrier is bent.

The same is true in the other direction, though, and we can show that irrespective of the ball carrier’s body position, the risk is lower when the tackler is bent at the waist.  This is perhaps even more important a concept.  

Look at the comparison between the Columns labeled A – these two columns compare the risk when the ball carrier is upright.  You can see that if the ball carrier is upright, you have a risk of 2.8 if the tackler is upright (A1) and 2.6 if the tackler is bent at the waist (A2).  Thus, for an upright ball carrier, we generally would want the tackler to be bent at the waist where the relative risk is lower.

Now look at Columns labeled B – if the ball carrier is bent at the waist, the risk is either 2 HIAs per 1000 tackles (B1) for an upright tackler, or it is 1.1 HIAs per 1000 tackles for a bent at the waist tackler (B2). 

So here again, the risk is lower when the tackler is bent at the waist.

Point is, and this was in fact the key conclusion of a scientific paper on the interaction between the players body positions and risk, irrespective of the ball carrier body position, the risk is higher when the tackler is upright, and lower when the tackler is bent at the waist.

Of course, there is an interaction between them.

So we can rank the risk from highest to lowest:

  1. Greatest risk – both players upright
  2. Tackler bent, ball carrier upright
  3. Tackler upright, ball carrier bent
  4. Lowest risk – Both players bent
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