A couple years ago, I was introduced to how Visual Memory Index (VMI) describes the adverse effect on players moving between different air density environments by Al Melchior on The Athletic. It’s now common knowledge that we can’t simply take a player like Nolan Arenado or Trevor Story and assume that their road stats are representative of their park-neutral true talent, because playing on the road after a series in Coors Field involves an adjustment period. The general idea is that pitchers and hitters each require time to update their visual memory to account for the effect of air resistance on a pitched ball. While it’s true that sliders move less in Colorado, a big part of the problem is not being able to command the pitch. Leaving it over the plate when you’re trying to get it a few inches off the plate. Bud Black acknowledged the problem, as well as how Rockies pitchers have had more success at home than in previous seasons.

“We’ve talked about that …We realize, and the pitchers realize, that the ball moves more when we go on the road. And to be able to control that movement is a challenge … But I don’t think that’s the main cause of the disparity. I think that for whatever reason … there is a comfort (level) pitching at home and a comfort pitching on that mound and in our ballpark”

https://www.denverpost.com/2021/08/30/colorado-rockies-starters-home-road/

Maybe the pitchers have a better “comfort level”, but why are they all of a sudden more comfortable at home? There are a few ways to acclimate a player to new air densities without the benefit of practicing at the new altitude, as noted on BaseballVMI.com. One is hyperbaric batting cages, and another is substantially raising the seams on baseballs used in training. If Rockies pitchers found a way to improve command of their sliders between varying air densities, they’d benefit from throwing them more often, especially at home where they previously thought sliders simply didn’t work. And that is indeed what we saw.

Kyle Freeland was the only member of the 2021 rotation that didn’t increase his slider percentage by 7+ points from 2020. The majority of the group’s change came from increasing usage in Coors Field (21.1% to 29.3%). And despite throwing it more often, and more often at home, the group’s combined CSW% on sliders remained constant at 31% even with a drop in spin rate after the sticky substance ban. Did it help? From 2016-2020, Rockies starting pitchers averaged an ERA split of +0.81 runs higher at home vs on the road. In 2021, possibly aided by that increased slider usage (and surely some good luck), that number flipped to -1.03 runs.

It’s still Coors Field, and the ball still flies. But any improvement dealing with air density changes should theoretically help them on the road as well. With Jon Gray likely departing via free agency, there don’t exist a lot of quality innings in the Rockies rotation. But names like German Marquez and Austin Gomber may be a little less unappealing if they’ve indeed figured something out.