I measured the rotational traction of couchgrass in comparison with perennial ryegrass on replicated plots, through a full year.
Couchgrass mostly had a higher average peak torque than perennial ryegrass, but when comparing their rotational stiffness, perennial ryegrass was consistently higher than couchgrass (see Figure 3). Given the argument that rotational stiffness is more relevant to the ACL injury mechanism than peak torque, the conclusion was that there was no reason to recommend against the use of couchgrass on football fields for reasons of excessive rotational traction.
Figure 3: Comparison of mean rotational stiffness of perennial ryegrass
Several other strategies to avoid excessive rotational traction were tested. Oversowing couchgrass with perennial ryegrass, Poa trivialis or Poa annua had inconsistent results. It reduced rotational traction early in the season, when the oversown plants were young, but led to an increase in rotational traction later in the season. The combination of couchgrass and Poa annua had a significantly higher rotational traction than pure couchgrass by mid-winter and through spring.
Thatch reduction also had inconsistent results – in some cases, reducing thatch reduced traction, in other cases it increased it. The only strategy that consistently reduced rotational traction was frequent sand topdressing (six applications x 3mm sand depth). This strategy also reduced surface hardness on a droughted, clay-based field. So if a turf manager wanted to reduce hardness and rotational traction on a clay field, then ‘dusting’ could do it – although its effect on surface drainage hasn’t yet been tested. Dusting on a clay field could reduce surface runoff and be counter-productive.
In any case, several other factors had a much greater influence on rotational traction than grass species or these other strategies. Grass coverage had a major influence. Rotational stiffness on a particular field might vary from 7 Nm/deg on a well-grassed area, down to 0.1 Nm/deg on a denuded area. Another factor that caused great variation in traction was boot type. A boot with 13mm blades had a rotational stiffness of 4.9 Nm/deg, significantly lower than a boot with 15mm studs (6.6 Nm/deg). Maybe all boots should come with a traction rating on the box?
My conclusion from the traction research was that uniform grass coverage over the whole ground and through the whole season was the most important strategy in avoiding excessively high rotational traction. Sound familiar? That was the conclusion from the hardness research as well. Consider the ground I mentioned in the previous paragraph – rotational stiffness was 7 Nm/deg on the well-grassed flanks and wings, but only 0.1 Nm/deg in the denuded centre and goalsquare. A player would want to wear boots with long studs in order to have some grip in the denuded areas, but that means he or she will experience extremely high traction when running and dodging on the well-grassed areas. This hazard would disappear if the ground was uniformly well-grassed; the player could select boots with shorter studs, and consequently avoid situations of excessive traction.
Once again, it is the denuded areas of the ground that are the weak link that compromises player safety. And once again, it is clear that many ground authorities need to do more to prevent this denudation by selecting the best grass species, managing traffic, and allocating sufficient money and resources to sustain full grasscover over the whole ground and through the whole season.
But that raises another point. There appears to be great emphasis on players not slipping over. If high rotational traction is involved in ACL injury risk, then players and coaches should be aware of this when selecting boots. Ideally, boot selection would be aided by an objective and reliable measurement of match-day traction on a field, and boots would also have some sort of traction rating. Neither of these exist at the moment. Turf managers can do their best to provide a well-grassed field, but players and teams also have a responsibility in avoiding excessive traction by their boot selection – don’t wear boots with traction that is much higher than is needed on that day.
Finally, I’d like to throw two cats among the pigeons regarding ACL injury, not derived from my data but from my reading on the topic. First, it is possible that ‘good’ grounds actually contribute to ACL injury risk, not by excessive rotational traction but by allowing faster running and more exaggerated athletic manoeuvres, including a more rapid deceleration and a wider foot placement in a sidestep manoeuvre. Look at the dramatic improvement in turf quality on elite-level football grounds over the past ten years – yet ACL injury rates are still at near-record levels.
Second, all the discussion about grass type and ground quality concerns risk factors that are extrinsic, outside the body. But most of the ACL injury research focuses on the many intrinsic risk factors, such as a player’s landing technique, hamstring to quadriceps strength, family history and so on. Possibly, a certain percentage of players are intrinsically susceptible to ACL injury; the injury is just waiting to happen. Some footballers have had repeated ACL ruptures, which points to an intrinsic susceptibility. A strategy that virtually halved ACL injury in the American NFL through the 1980s was screening and rejecting draftees who had some of these intrinsic ACL risk factors (Nicholas et al, 1988).
There is no simple answer to the ACL injury problem, and anyone who advocates some simple remedy is almost certainly going to be proved wrong. Reducing ACL injury requires progress in many small areas. Turf managers should do what they’re always done, prepare the best quality ground they can, and continue to push their managers for the tools to do this – the best grass species, a manageable traffic load, and sufficient money and resources to sustain full grasscover over the whole ground and through the whole season.
Acknowledgements: Phillip Ford would like to thank PGG Wrightson Turf (Australia), Strathayr and Rocla Quarries for their material assistance in this project. (Other references upon request).