Head injuries part 2: What makes a tackle risky?

31 Jul 2017 Posted by

In Part 1, I explained the principle behind trying to understand how head injuries happen in rugby, as well as the first important findings of our study, which were a) that the tackle is both the most numerous and riskiest match activity in rugby, and b) that the tackler has a higher propensity for head injury than the ball carrier.  That installment is basically a summary and translation of this paper, published two weeks ago in the British Journal of Sports Medicine.

That leads us into a more detailed examination of the tackle, which is the focus of Part 2.  This installment summarizes parts of a second paper, also published in BJSM, which you can read here.

So let’s get straight into it. 

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Detailed investigation of the tackle

Let’s take a look at a couple of examples of tackles, so that I can talk you how they are coded.  Here’s an incident where you’ll see the Yellow 11 shoot out of his defensive line to tackle Black 13.

His head makes contact with the ball carrier’s shoulder, and he comes off second best.  This case would have been coded as a front-on active shoulder tackle, with the tackler accelerating and at high speed, while the ball carrier is at high speed.  Both players are upright (though there is of course some subjective component to all these calls).

Then here’s another one, where you’ll see the Fiji (white) 6 run into space, with the Australian (gold) 17 chasing back.  The Australian attempts to make a tackle that wraps the ball up and prevents the Fijian from offloading the ball in the tackle (as is part of the plan, an instruction from coaches, I suspect), and you’ll see a head to head contact.  In this instance, it’s a smother tackle, from behind, both players are upright in the tackle, both are at high speed, and neither is accelerating.

So this process of coding or describing in detail the characteristics of the tackle was done 464 times on the HIA cases, and then Ben did another 3,160 tackles that did not cause an HIA so that we could work out the propensity in HIAs per 1000 events, and off we went with the analysis.  Here’s what we found.

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Primary Level analysis

Bear in mind that it is possible to combine each of the coded variables any way we want to.  This could get crazy – I could tell you, for instance, the risk of a front-on active shoulder tackle where the ball carrier is at high speed and the tackler is static and both players are upright.  Or we can calculate the risk of a passive shoulder tackle from the side, with players at low speed and when the tackler is bent at the waist while the ball carrier is diving.  Every combination is possible, but not all are particularly helpful.

So let us not lose ourselves in the minutiae just yet.  Let’s start by addressing the primary outcomes, which are the risks of the various categories of tackle.  For the sake of simplicity, I’ll visualize these risks on a spectrum, from low risk to high risk, which shows the propensity of a specific situation to cause an HIA.

Here’s the risk of different types of tackles:

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Tackle type

So three types of tackles make up 99% of all legal tackles (tap tackles and legal lifts are the other 1%).  Of these, the active shoulder tackle carries the greatest HIA risk.  They cause an HIA once every 335 tackles (propensity of 2.98 HIAs per 1000 active shoulder tackles).

That’s 2.1 times higher than both the passive shoulder (1 in 693) and smother (1 in 714) tackles.  This 2.1 value, by the way, is the IRR or incident rate (or risk) ratio, and if you read the academic paper, you’ll see most of the results are compared and discussed in this way.  I’m sparing you the statistical discussion to focus more on concepts for this article.

Anyway, the higher risk of the active shoulder tackle is not surprising – active shoulder tackles are by definition more “forceful”, with the tackler attempting to dominate the tackle by going “through” the ball carrier.

An interesting clue here, at least with respects to our later discussion, was the finding that “illegal tackles”, as ruled by the referee during matches, are 36-times more likely to cause a head injury than legal tackles.  Of the illegal tackles, the high tackle was by far the most dangerous, with a propensity of 237 HIAs per 1000 high tackles.  That means that a high tackle caused an HIA once every 4.21 high tackles.  Remember our ‘baseline’ is 1 in 516, and the active shoulder tackle risk is 1 in 335.  So high tackles, ruled illegal, are 80-times more likely to cause an HIA.

What’s really interesting is that illegal tackles (high tackles and tip tackles) were the only tackle type where the ball carrier has more risk than the tackler.  Remember I told you above that our first interesting finding was that the tackler is more at risk?  The only exceptions are these illegal tackles.  Nevertheless, the fact that (illegally) high tackles are so much riskier is important, as we will return to later this week in Part 3.

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Tackle direction

Front on tackles are between 1.65 and 2.02 times more likely to cause an HIA than other tackle directions.  Remember our baseline risk is set at 1.94 HIAs per 1000 tackles, and is shown on the spectrum above.  Front on tackles are the ones that have a higher risk than this.  This is also not surprising, given that energy transfer influences injury risk and the transfer of energy is greatest when you have two players entering a tackle in opposite directions.

To emphasize a point that I have made a couple of times, the risk here lies mostly with the tackler.  In fact, 67% of head injuries during front-on tackles happen to the tackler.

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Tackler and ball carrier acceleration

Next, acceleration, where the highest risk exists when the tackler accelerates into the tackle.  The HIA risk is between 2.3 and 3.1 times higher than for any other situation.  Interestingly, the risk when the ball carrier accelerates, or even when both players do, is lower than our baseline risk.

Who gets injured?  When the tackler accelerates into the tackle, 64% of HIAs happen to that tackler (I showed you one example of this in the first video case above), and only 36% happen to the ball carrier.  This suggests that the acceleration exposes the tackler to more risk, possibly because they have less control or some degree of compromised technique.

This result also gives us a “teachable moment”:  Look at the figure above, and you’ll see that in blue text, I’ve shown you how many HIAs happened because of an accelerating ball carrier (221 HIAs) compared to an accelerating tackler (116 HIAs).

See how the number of HIAs is far higher when the ball carrier accelerates, even though the risk of an HIA is higher for an accelerating tackler?

That is because the highest risk situation – a tackler accelerating – happens infrequently, in only 11% of all tackles.  Far more common is for the ball carrier to accelerate into the tackle – this happens 59% of the time.  So even though the risk of a specific situation is lower, the fact that it happens so often means that it causes a high number of events.

This becomes important when you start thinking about solutions to the problem.  Do you target the behaviour that causes more HIAs (ball carrier accelerating)?  Or do you target the behaviour that has the highest risk (tackler accelerating)?  The answer is that if you can do both, then you should.

However, if there is a danger of “substitution” of one behaviour for another, then you must focus on the high RISK behaviour, and not the high number one, because you might end up shifting behaviour from left to right on that spectrum, which is just what you don’t want.

Just imagine, for instance, that you come up with a strategy to reduce the number of times the ball carrier accelerates, but a side effect of your plan is that the tackler accelerates more.  Imagine you reversed the numbers, so that tacklers accelerated into the tackle 59% of the time, and ball carriers only 11% of the time.  The overall effect of this ‘swap’ would be to increase the total number of HIAs by 336 (you can do the sums if you want).  In other words, you’ll have almost doubled the head injury risk from tackles.

This is why it’s so important to have the control group and to understand every implication of what you do.

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Tackler and ball carrier speeds

Next up, something quite interesting, and important for understanding the complexity of the tackle.  Having seen how active shoulder tackles, front-on tackles and accelerating tacklers increase the risk, you’d be forgiven for thinking that speed will follow a predictable pattern.  That is, high speed tackles will have the highest risk.

And you’d be only partly correct.  Take a look at the following figure, which groups the risk of head injury into three scenarios.

First, the green symbols in the left column on the graph show the risk when the tackler is static, and as the ball carrier’s speed increases.  See how the risk (propensity) increases as the ball carrier goes from “static” to “low” to “high”?  That’s what you’d expect – more speed, more kinetic energy, more energy transfer, more risk.

You see the same in that middle column with blue symbols, which show the risk when the tackler is at “low” speed – the risk of a head injury increases as the ball carrier speeds up.

But now look at the red symbols in the far right column, which shows the risk when the tackler is at high speed.  First point, overall HIA risk is higher for a high speed tackler – it is 3.9-times higher than it is when the tackler is static, and 2.4 times higher than when the tackler is slow.  So speed of the tackler clearly influences the risk, as you’d expect.

However, you’ll also notice that the pattern as relates to the ball carrier speed is reversed – the risk of an HIA actually decreases as the ball carrier speeds up.  The highest risk exists for a high speed tackler and a static ball-carrier, and then it drops as the ball carrier runs faster.

Why might this be?  It’s because these variables do not exist in isolation, and there is interplay between them, though I have not yet shown this data (it’s for a future publication.  And trust me, you have enough data to take in as it is!)

For instance, when the tackler and ball carrier are both at high speed, the tackle direction is much more likely to be from the side or back.  Remember, those are tackle directions that have the lowest risk.  You can quite easily imagine the scenario – a player makes a line break, and is tackled by a chasing player.  Both are at high speed, but the direction of the tackle (from behind) reduces the risk.

Also, the type of tackle tends to be different as speed changes – more passive shoulder tackles and far fewer active shoulder tackles are made when both players are at high speed.  This too has the effect of reducing the risk, as you can see in the risk spectrum on Tackle Type above.

If you want some figures – when both players are at high speed, only 7% of tackles are from the front, compared to 85% from the back (45%) or side (40%).  In other words, when both players are at high speed, the tackle is 13 times more likely to be from the safer directions of side or back, and that’s why the risk drops off.  In contrast, when you have a static ball carrier and a high speed tackler, 83% of the tackles are front-on, compared to only 2% of tackles from the side or back.

Similarly, only 10% of both player at high speed tackles are active shoulder tackles, while 53% are passive shoulder tackles.  The latter have a risk that is half that of active shoulder tackles, and so even the higher speeds, which do have an overall influence on HIA risk, are offset by this interplay between factors.

This is quite an important concept, because it shows how a) the speed of the players has a contextualizing or moderating effect on other factors and behaviours within the tackle, and that b) speed may influence the decisions, techniques and types of tackles executed.  This is something we plan to look at more closely in future studies.

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Why the tackler?

There is of course an obvious reason why the tackler risk is higher – his head is always at risk, whereas the ball carrier’s isn’t.  For instance, if a tackle is made at the level of the hips of the ball carrier, then realistically, only one head is “viable” for injury.  Unless the ball carrier is hit with enough force to cause whiplash, or they hit their head on the ground (both of which do happen, incidentally), the injury is happening to the tackler.

So there’s that.  But what’s interesting is that even head-to-head impacts are far more likely to injure the tackler (this data will be explained on Wednesday in Part 3).  For reasons of coding and stats, we don’t have the propensity for ball carrier HIAs when head-to-head contact occurs, but we do know that when there is head-to-head contact, the tackler is injured 78% of the time.  Similarly, the risk to the tackler is disproportionately higher in every situation with the exception of those illegal tackles shown above.

Part of the reason for this may be that the tackler in a sense ‘controls the risk’ through their decision-making, and that the ball carrier controls the risk through their behaviour in response to an imminent tackle.  The ball carrier, in other words, can ‘brace’ for contact, and we have data (you’re being spared data overload here) showing that a head injury is far more likely to happen when the ball carrier is unbalanced or unsighted, compared to one who is sighted, and ‘protected’ in the sense that they are bent at the waist and braced for impact.  That is, if they cannot anticipate or adjust for the contact, their risk is increased.

Applying this logic to the tackler, I would hypothesize that a tackler is who at higher speeds, or who is accelerating, and whose purpose is not to ‘brace’ but rather to create a collision probably lacks the ability to ‘manage’ or control the impact.  Yes, they can anticipate contact (they’re about to create it), but are perhaps not thinking about self-preservation as much as a ball carrier.  They are also, be definition, more “open” in contact (the rules determine this), and thus more vulnerable to the injury outcome of the tackle.

So the key point from this section of results is that the tackler seems, through their own behaviour, decisions and tackle execution, to be more influential in injury risk.  Thus, if you want to reduce risk, you have to look at ways to manage the decisions and execution, or the behaviour, of the tackler.  How do you do that?  Think about that ahead of next time.

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Conclusion

Let’s leave it there for now – that’s a lot of data, and there are two more very important pieces of “evidence” that I want to share with you.  They were so important for the expert coaching, player and official group, that they warrant a discussion all of their own, and so rather than mix them up with the above, I’m going to address them in part 3, later this week.

Thanks for following (I hope!), and let’s wrap this up with the key evidence and conclusions next time.

Ross

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