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In the second half of the 00s, Zach Duke was a part of the awful Pittsburgh Pirates teams that never won more than 68 games. He was a member of a once-promising rotation that also featured Paul Maholm and Ian Snell, but none of the three turned into anything special. Snell has washed out of professional baseball, and Maholm is struggling in the Dodger bullpen.

Duke, though, is flourishing with Milwaukee. He is a dynamite reliever with a 33.7 percent strikeout rate and a 5.3 percent walk rate whom manager Ron Roenicke has trusted to appear in high-leverage situations. So what has changed? What is it that has turned a below-average starter into a fantastic reliever?

Let’s begin with velocity. Shockingly, he is throwing his fastball just as hard (or, to be more precise, not hard) as he was when he was a starter. Despite the fact that he now throws fewer pitches, his velocity is still primarily in the high 80s.

2014 fastball velocity on the left, 2010 fastball velocity on the right

Since velocity has been ruled out as the cause of the increased effectiveness, the next step is to look at the physical pitches themselves. He has, in fact, changed his repertoire in the intervening years. In 2010—his final year as a full-time starter, he threw his changeup 14 percent of the time and his slider just eight percent. Now, though, that has changed: he has a heavier reliance on his slider (24 percent) and he has thrown just one changeup.

That alteration was precipitated by the changing qualities of his pitches. As a starter, the spin on his slider was nearly 200 rpm slower than the left-handed average. It has now climbed to 981 rpm this season, which is a 400 rpm jump and is now 200 rpm faster than the average. Therefore, it makes sense that he is now relying on it more.

His newly-effective slider is in direct contrast to his changeup. Even at its best in 2009, its spin was right around league-average for lefties. At its worst, though, it was well below that.

In fact, we can see the change in his slider in the actual results he’s put up. He is generating far more swings and misses from his slider than he did in the past, and hitters are making far less solid contact. In 2010, Duke’s slider generated whiffs 14.7 percent of the time and batters hit line drives 4.3 percent of the time. But, in 2014, those numbers have improved to 17.9 percent and 1.5 percent, respectively.

Normally, a logical expectation for a starter becoming an excellent reliever would be to see a velocity jump. However, that is not the case for Duke. Instead, we are seeing a repertoire shift that is possible because relievers do not need to turn a lineup over. While starters need to pace themselves, relief pitchers can simply throw their two best pitches for an inning at a time. In 2014, Duke’s best secondary pitch is his slider, so it has replaced his changeup and enabled him to succeed in his new role.

NOTE: All statistics accurate as of 6/04/14

By Seth Victor
AriBall.com

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