A couple of years ago, I got to wondering how to define “ground-ball pitcher” ... and while I won’t make you go back and read through the whole process, I will tell you that I came up with a conclusion: Get ground balls more than half the time and you’re a ground-ball pitcher; get ground balls more at least 55 percent of the time and you’re an extreme ground-ball pitcher.

Which is obviously a strict dichotomy that doesn’t necessarily mean anything in real life. I mean, really: 54 percent isn’t extreme, but 56 percent is? We do enjoy our categories, though, and these are the categories I came up with.

Last year, there were only three extreme ground-ball pitchers: Justin Masterson (58 percent), A.J. Burnett (56.5) and Rick Porcello (55.3). Among the 81 pitchers with enough innings to qualify for the ERA title, only 10 more have ground-ball rates higher than 50 percent. Which makes a total of 13 ground-ball pitchers last season.

Which seems low, since there were 21 in 2012 and 21 in 2011. Just one of those things, probably.

This year, though? There have been a TON of ground-ball pitchers, as 31 of 100 qualifiers have ground-ball rates 50 percent or higher. I think this is just a random fluctuation, due in part to a high number of qualifiers. A fair number of starting pitchers who currently qualify will get hurt and won’t end up qualifying, and their replacements also won’t qualify.

But there are a TON of extreme ground-ball pitchers this season, too; eight of them, including Bronson Arroyo (55.1 percent) ... which is weird because he’s never been anything like a ground-ball pitcher. My definition of a fly-ball pitcher is a ground-ball percentage lower than 40 percent, and Arroyo almost qualifies with a 41-percent career mark. So what’s he doing differently this season? You got me. He seems to be throwing the same pitches this season as last season. Well, except he’s throwing them even softer than before. Maybe his fastballs are so slow they're just sinking more?

In Arroyo’s case, maybe it’s just Ordinary Baseball Weirdness and his ground-ball rate will regress hard. Which would be a problem for his ERA. But this isn’t about Bronson Arroyo. This is about the new King of Ground Ball Pitchers.

Or the presumptive King, anyway. It’s early still.  But Houston’s Dallas Keuchel leads the majors with a 66.5 ground-ball percentage. How high is that? It’s six points higher than Tim Hudson, who’s next on the list.

Among starting pitchers with at least 750 career innings, the top guys are all the famous sinker-ballers: Brandon Webb, Derek Lowe, Chien-Ming Wang, Jake Westbrook, Tim Hudson, Roberto Hernandez (née Fausto Carmona), Aaron Cook, and Justin Masterson. You know, the freaks. Webb leads the group with a 64.2 career ground-ball percentage, while Masterson sits at 56.6 percent.

This season, Keuchel’s at 66.5 percent.

Which might be historic, at least for this era. Webb topped out at 66.3 percent in 2006. Wang never reached 64 percent. Derek Lowe did get to 67 percent in 2006. And our data goes back only so far, but I would guess that pitchers in the 1970s and ‘80s were in this range, too. Just looking at Dan Quisenberry’s late-career grounder/fly ratios, for example, I will guess that his career ground-ball percentage was in the same range as Webb’s, maybe even a touch higher. Mel Stottlemyre must have had an exceptionally high grounder rate, too.

But just in terms of the last 15 or 20 years, Keuchel’s performance is exceptionally notable. How’s he doing it? Thursday, FanGraphs’ Brad Johnson penned a breakdown of Keuchel’s pitches and results. Highlights:

He throws three fastballs, with the sinker getting the most work. Left-handed sinker ballers are pretty rare. Keuchel’s ground ball rate matches the elites – Derek Lowe and Brandon Webb. Unless I missed a name, the only other extreme left-handed groundball pitcher since 2002 is Jaime Garcia. Hitters see a lot of sinkers when they’re ahead in the count, which is the perfect time to throw a well controlled, moving fastball. His slider comes out to play once he’s ahead in the count. As we’ll see in a moment, it’s a doozy. His change gets a lot of action against right-handed hitters, although lefties never see it.

... Notice his slider and change generate a very high rate of whiffs per swing. Moreover, when batters swing, his slider is whiffed or fouled away over 77 percent of the time! We’re working with small sample sizes, but that’s still mighty impressive. His sinker and change are both fantastic at drawing ground balls when they’re put in play.

Can he keep doing this? Probably not. Most extraordinary things aren’t sustainable. I do think it’s safe to classify Keuchel as an extreme ground-ball pitcher, considering his rate was nearly 56 percent last season, in 154 innings. And if you believe PITCHf/x, he’s throwing significantly more two-seam (sinking) fastballs this season, which could logically lead from ground-ball pitcher to extreme ground-ball pitcher.

Is Keuchel heading toward a long and productive career? Derek Lowe aside, there’s at least some doubt about extreme ground-ball pitchers’ ability to stay healthy for a great number of years. But that’s not really the Astros’ problem, is it? I’m sure they would be thrilled to have Brandon Webb for the next four or five years. This season, Keuchel’s been one of the better pitchers in the American League. Which is extremely good.

Senior Baseball Editor Rob Neyer's Twitter feed is extremely annoying if you're annoyed by such things.