Thanks for posting this. I am not a fan of air vests, and I do not have the full text (I will request it through my University), but there appears to be an issue in the analysis. They say that they used a chi-square test. Since they do not mention adjusting for potential confounders through stratification, I think that they did not adjust for competition level. If the riders who wore air vests were also competing at a higher level, the association between air vest use and injury severity could be confounded by level. That is, worse outcomes were at the higher levels at which the riders were more likely to be wearing air vests.