Dallas Keuchel and Pitcher Plate-Discipline Aging Curves

The first time Dallas Keuchel broke out, it was because he found a new pitch. Moving from the curve to the slider really pulled his arsenal together. The second time he broke out, it was for reasons that were both more complicated but also just as conventional — it looks like Keuchel merely threw fewer pitches inside the zone, while getting batters to reach and swing just as much. More swings on pitches outside of the zone means more misses and more strikeouts.

All of that is nice, but it’s hard to know which breakout is easier to believe. Our intuition probably tells us that the first is less delicate — he needed a breaking ball, and he found a good one, and it should remain good. But batters could adjust more easily to the second one, couldn’t they? Just lay off more of those balls?

In the context of the PITCHf/x era, Keuchel’s drop in zone percentage was extreme but not an outlier. Baseball Info Solutions had him dropping 2.5 percentage points in that category, which would have ranked him among the most extreme 27% of drops since 2002 (minimum 120 innings pitched in both seasons). PITCHf/x has the drop as 3.8 points, which would put his 2015 season in the top 6% since 2008.

Did other pitchers that dropped their zone percentage as much retain that change in zone percentage the next year? On the BIS side, pitchers that dropped as much as Keuchel or more saw their zone percentage rise 0.7 points the year after their four-point drop. Roughly 57% of them threw the ball in the zone more often the following season. By PITCHf/x, starters as extreme as Keuchel saw their zone percentage increase 1.1 points the next year, and 62% of the pitchers threw the ball in the zone more often.

So we have a more than 50% chance that Keuchel throws the ball in the zone more often next year, and ostensibly gets fewer swinging strikes as a result. How does this fit into our concept of how pitchers age?

We know that swinging strike rate is more stable for starters rather than relievers, suggesting that size of arsenal is part of the picture. But in this case, Keuchel’s second breakout was a result of fiddling with his approach to the zone. How do those zone stats age? Thanks to Jeff Zimmerman, using the delta method, we have aging curves for pitcher plate-discipline stats.

That yellow line leaps out at you, especially in Keuchel’s situation. Pitchers throw the ball in the zone less year after year, it looks like. And since we know that contact rates are worse on pitches outside of the zone, we have more of an idea of how starters stave off aging better relievers: they throw fewer fastballs, and they throw them in the zone less often. That’s a bit of conventional wisdom coming true in the numbers.

What does this mean for Keuchel in particular, though? It at least suggests that this second breakout came through means as conventional as the first. Pitchers get better by adding pitches, and then by throwing those pitches more often and outside of the zone more often. Or, at least, that’s how pitchers are able to produce roughly the same numbers even as time robs their athleticism (read: velocity). Keuchel has now done both of those things in consecutive years. He’s just reading the playbook. Everything is fine.

Except, you have to wonder about how extreme this second move was. Yes, pitchers generally nibble more as they age, but Keuchel, by one measure, did this harder than 90% of the league. Is he pushing the Old Pitcher buttons too hard?

Let’s return to the PITCHf/x list, since it’s shorter. Let’s look at only pitchers more extreme than Keuchel in the year they dropped the zone percentage, and then their follow-up year. You’ll notice that the list is, by and large, populated by pitchers older than Keuchel. (Pitchers ordered by largest decrease in zone percentage.)

Starting Pitcher Extreme Zone Percentage Droppers

Name	Start Season	Start Age	pfxZone% yr1	pfxZone% yr2	pfxZone% yr3	YR2 FIP	Yr3 FIP	Diff FIP
Wan. Rodriguez	2009	30	53.6%	46.9%	46.6%	3.50	4.15	0.65
James Shields	2011	29	51.4%	45.0%	47.7%	3.47	3.47	0.00
Kevin Millwood	2008	33	54.7%	48.7%	50.0%	4.80	4.86	0.06
Rick Porcello	2010	21	54.3%	48.7%	48.4%	4.06	3.91	-0.15
Jer. Hellickson	2011	24	49.1%	43.5%	45.9%	4.60	4.22	-0.38
Edwin Jackson	2011	27	50.1%	44.7%	46.5%	3.85	3.79	-0.06
Aaron Cook	2008	29	52.9%	48.2%	51.3%	4.60	4.54	-0.06
Zack Greinke	2008	24	52.7%	48.1%	49.0%	2.33	3.34	1.01
Derek Lowe	2009	36	42.9%	38.3%	37.1%	3.89	3.70	-0.19
John Lackey	2008	29	54.6%	50.1%	51.5%	3.85	4.71	0.86
Ricky Nolasco	2008	25	56.7%	52.3%	52.8%	3.35	3.86	0.51
Fran. Liriano	2010	26	48.0%	43.6%	43.2%	4.54	3.54	-1.00
Johnny Cueto	2010	24	49.1%	44.8%	47.4%	3.45	3.27	-0.18
Doug Fister	2010	26	57.7%	53.5%	51.8%	3.02	3.42	0.40
Jake Peavy	2011	30	53.5%	49.4%	51.4%	3.73	3.96	0.23
His. Iwakuma	2013	32	51.4%	47.4%	46.8%	3.25	3.74	0.49
Jeff Francis	2010	29	52.6%	48.6%	46.6%	4.10	4.27	0.17
Shaun Marcum	2010	28	48.8%	44.8%	46.0%	3.73	4.10	0.37
A.J. Burnett	2010	33	48.6%	44.7%	51.2%	4.77	3.52	-1.25
Jake Peavy	2013	32	51.4%	47.5%	50.6%	4.11	3.87	-0.24
Kevin Correia	2009	28	53.8%	49.9%	51.7%	4.71	4.85	0.14
Just. Masterson	2009	24	55.1%	51.2%	55.4%	3.93	3.28	-0.65
Jason Vargas	2011	28	53.5%	49.7%	47.9%	4.69	4.09	-0.60
Freddy Garcia	2010	33	50.2%	46.4%	45.0%	4.12	4.68	0.56
Cole Hamels	2011	27	52.3%	48.5%	50.3%	3.30	3.26	-0.04
Sample Average		28.3	52.0%	47.4%	48.5%	3.91	3.94	0.03
Dallas Keuchel	2014	26	44.3%	40.1%		2.91

Minimum 120 innings thrown in all three seasons, overall n = 442

If we cut the list to just the 10 pitchers who were 27 or younger — Keuchel was 26 when he started this process in 2014 — things change. Those young pitchers actually cut five points off their FIP the next year, ostensibly because they occupied a different place on the aging curve. If you cut the list further to include only excellent young pitchers the year after they cut their zone percentage, you’re talking about Zack Greinke in 2013 and Doug Fister in 2012, and good luck figuring out which one Dallas Keuchel is going to be.

But now we’re talking about predicting Keuchel’s next season off of a sample of 10 (or, even worse, two). And you might also notice that Keuchel is even more extreme than these fellow extreme droppers, since he dropped to nearly 40.1% pitches in the zone. He wasn’t around the zone anywhere league average before (47.8% was league average last season) and now he’s hitting the zone less often than every qualified pitcher not named Kyle Gibson or Francisco Liriano.

So Keuchel is not likely to throw fewer pitches out of the zone next year, he’s already fringe in that regard. Every pitcher throws fewer pitches in the zone as they age, though, so it’s fair to ask if he pushed this button too hard, to quickly. He’s just had the sixth-lowest PITCHf/x zone% of the era, and so maybe he can’t keep dropping that percentage much further. Maybe it has some sort of implications for his longevity, since he’s starting at such a low point.

But when it comes to next year, there isn’t much to suggest that he can’t do the same thing again. After all, it’s the Aging Pitcher Playbook — add pitches, throw more junk, and nibble more. It’s just weird to see that playbook used by a pitcher in his prime to become more excellent. Weird enough that we don’t know exactly how sustainable it is.