When pitchers age, they lose velocity. Slowly but surely, the miles per hour shrink smaller and smaller, and the strikeouts follow suit. This is inevitable for not only major league pitchers like Yovani Gallardo, for instance, but for any human being who throws balls semi-often. However, for these pitchers, they still need to perform adeptly in their roles. To do so effectively, one solution is to stop trying to strike batters out and instead generate weak, bad, or a specific type of contact. And this varies; Miguel Gonzalez likes to induce pop-ups, but a number of others are looking for ground balls.

What if you try and induce ground balls and strike some people out, regardless of if you are 22 or 38? A lot of pitchers belong in the latter bucket now, including Felix Hernandez, Tyson Ross and Corey Kluber. These arbitrary names are in fact not arbitrary, as they all induce ground balls the same way – via the sinker. The sinker is a seemingly boring pitch; it’s thrown like a fastball, usually low in the zone, and it “sinks” or has arm-side movement. There really isn’t that much to it. A non-baseball person could just as easily tell you what a sinker is going to do.

Trevor Cahill throws a sinker, and he throws it quite a bit. He threw 883 of them in a partial season last year (58.1%), per Pitch F/X. He threw 1298 of them in 2013 (51.8%), and 1707 of them in 2012 (52.5%). Cahill doesn’t throw his sinker particularly hard, as it averages between 89-90 MPH every year, mostly because he does not have great velocity in general. His sinker has significant horizontal movement, averaging 9.21 inches in 2012 and up to 9.70 in 2013 and 2014. For perspective, that horizontal movement is more than Rick Porcello, Dallas Keuchel and Andrew Cashner, all of whom are ground ball gurus. On the other axis, the Cahill sinker’s vertical delta ranks among the elite including Derek Lowe and Timmy Hudson, though it is worth noting that it has gradually lost downward movement each year. Here is a clip the sinker in action:

As you can see, his ability to get ground balls has diminished significantly. Not surprisingly, his aforementioned ground ball weapon has been the root of the problem, since the sinker’s lack of ground ball generating ability has resulted in more runs allowed and worse performance.

Quite trivially, the less ground balls Cahill’s sinker generated, the higher his ERA was in that season. The curves in this instance are showing considerable negative correlation. Sure, there is some general volatility that can happen year to year, especially since we are measuring only balls in play for a pitcher who is often on the disabled list. Nevertheless, the drop in 2014 and spike in ERA are almost certainly not a coincidence.

I pondered the possibility of this being a change in approach. For example, in 2014 Cahill went from being a 6-7 K/9 pitcher into a 8.5 K/9 pitcher in the span of a season. Additionally, his zone rate fell severely, raising the question of if he was trying to have batters chase more. A lot of this can be written up to a transition to the bullpen part-time in 2014; he had made only one relief appearance from 2009 to 2013. We know that in the bullpen, pitchers can throw harder for shorter stints and as a result induce more strikeouts. For that reason, the strikeout rate spike seems less material on the surface.

When broken down to starts and relief appearances, his strikeout rate in 2014 was in fact 8.07 K/9. Thus, independent of the relief appearances, he strikeout rate did jump. And to dig deeper on the zone rate, this is how his pitch location has changed from 2012-2013 to 2014:

The zone rate change holds up, as there is a lot more penetration down in the zone, and in fact, out of it. In the short term, it seems as though this distribution has led to an increase in strikeouts, but concurrently, his walk rate has risen as well.

I mentioned the concept of aging pitchers who are slowly losing velocity and effectiveness, see their strikeout ability diminish, and subsequently being to practice the ground ball approach. Cahill was never one of those strikeout pitchers, but on the other hand, nor has he lost any velocity. Trevor Cahill is going in the opposite direction. Yes, Cahill has had an up and down career, though in the peaks, he could, more often than not, make any sinker he threw result in a ground ball. In the one small sample season (2014) we can surmise that the “going for strikeouts” thing was not effective. Cahill can bounce back just like any other pitcher, but to do so, I suggest he revert back to his most transcendent skill: getting baseballs on the ground.