Matt Thornton’s Fantastic Fastball

In 2009, only 41 pitchers managed to compile 10.0 runs above replacement in relief. Chicago White Sox left hander Matt Thornton, through the first two months of the season, is one of six pitchers to have accomplished this feat in 2010. Thornton has a spectacular 10.95 strikeout/walk differential (SO-BB), which is fifth in the major leagues among pitchers with at least 10 innings pitched. Thanks to this utter dominance, Thornton is running a spectacular 1.59 ERA, 1.07 FIP, and 2.09 xFIP through the first two months of the season.

This emergence, remarkably enough, comes with Thornton at the ripe old age of 33. Thornton struggled in his first two seasons with Seattle, particularly in 2005 when he posted a -0.9 WAR season. The Mariners then traded him to the Chicago White Sox for Joe Borchard. Thornton blossomed in Chicago, putting up two straight 1.0 WAR seasons. Thornton leaned heavily on his mid-90s fastball and rode an 8.00+ K/9 to a 3.70 FIP.

Over the last three seasons, however, Thornton has emerged as one of the most dominant pitchers in the game. He’s consistently recorded well over a strikeout per inning and has severely reduced his walk rate. Over his last 162.3 innings, Thornton has recorded a whopping 4.5 WAR.

The apparent cause of this success is his blazing fastball. Thornton is extremely reliant on the number one, using it a whopping 90.6% of the time – over 3% more than the next highest qualified reliever. Hitters have had an extremely difficult time with it. The 17.4% whiff rate is nearly three times the league average, and even when hitters do manage to make contact, it’s much more likely to wind up as a foul ball (24.6%) as opposed to in play (12.8%). Even when it’s in play, hitters haven’t been doing much with it. As the final nail in the coffin, hitters can’t simply take and expect a ball – Thornton’s fastball has a 70% strike rate, about 6 points above the league average. As such, Thornton’s fastball has been worth over 10 runs according to pitch type linear weights, the most valuable pitch to any reliever in baseball, eclipsing Carlos Marmol’s slider by over a run.

It appears that a huge part of Thornton’s development as a pitcher has been figuring out how good his fastball really is. There’s a direct relationship between how often Thornton throws his fastball and how successful he has been over the course of his career.

I don’t need to tell this audience that correlation doesn’t imply causation, but personally, I doubt that a lower FIP has caused Thornton to throw his fastball more often. Thornton’s fastball is a fantastic pitch, and his secondary stuff, although good, is dependent on his fastball, as he allows his slider to be put in play at an above average rate. Until batters show that they can consistently make good contact against his fastball, he needs to keep going to it. If he can keep drawing whiffs on it as he has this season, he will undoubtedly continue to thrive.