With a huge assist from ESPN Stats & Information, we've attempted to track the frequency of ACL injuries in the NFL this season. If you recall, a significant spike was evident near midseason, and while the pace has since slowed, the total still exceeds the number of confirmed injuries in each of the past two seasons.

Based on research from Rachel Eldridge, using information verified by StatsPass.com, at least 40 players have been placed on injured reserve because of ACL injuries. (There likely have been more, but these are diagnoses we could confirm.) Using the same parameters, there were 32 such injuries in 2012 and 25 in 2011.

Browns QB Brian Hoyer suffered a torn ACL in his right knee Oct. 3 while playing on a grass field. AP Photo/David Richard

The natural inclination is to wonder if the NFL's expanded rules on hits to the helmet have prompted more shots to the knee area, but an analysis this week by The MMQB.com cast data-based doubt on that possibility. Earlier this year, NFL commissioner Roger Goodell suggested an alternate theory: Some players are choosing shoe styles that emphasize speed over support and safety.

This week, I wanted to look at another explanation that often surfaces during ACL discussions: Playing surface.

Many of you are probably aware that a study published last year in the American Journal of Sports Medicine found the rate of ACL and ankle sprains were 67 percent higher on artificial turf than grass between 2002-09. But for reasons that aren't immediately clear, most of the ACL injuries we've confirmed this season have occurred on grass.

Of the 40 we've documented, one occurred in college. So among the remaining 39, we counted 34 to have happened on grass and five on some version of artificial turf. Twenty-three occurred in games and 16 in practice, including training camp. All of the turf injuries came in stadiums during games rather than on practice fields.

As we discussed in the ESPN.com Hot Read last month, the NFL's competition committee will compile all of this information after the season and analyze it for trends. This season's compilation will be of particular note considering the new rules, but I don't think we'll see anything to suggest that they are responsible for an increase in ACL injuries. The rate of injury on grass will certainly merit further inspection.

The increase in ACL injuries, now that the pace has slowed, generally reflects what we found in the Hot Read: Overall injuries, and resulting missed starts, are up this season. But in terms of raw data, the frequency is not as dramatic as it appeared anecdotally earlier this season.