In 2009 with the Houston Astros, Jose Valverde held opposing batters to a 72.5% contact rate on his fastball with a K-rate of 20.4%. In 2010 pitching for the Detroit Tigers, batters made contact on the pitch 83% of the time, and struck out only 12.9%. So his fastball was obviously not as effective last season, right?
While batters made better contact on the pitch and struck out less, Valverde's fastball was even more productive in his first season with the Tigers. In 2009, batters had a .379 slugging percentage on his fastball; this dropped to .284 in 2010. Check out the change in where he located the pitch, accompanied by opposing batters' results.
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So Valverde was locating his fastball further down in the zone last season, and while batters were making contact more, they weren't producing the same results as in 2009. One possible reason for this is Valverde's increased use of his splitter. In 2009, he relied heavily on his fastball, using his splitter sparingly. In 2010, he threw both pitches equally. A splitter is typically thrown low in the zone and resembles a regular fastball out of the hand. Batters would have found it even tougher to distinguish Valverde's splitter from his fastball given that he was locating both pitches similarly. And while the result was an increase in contact on his fastball, keeping batters guessing meant less well hit balls overall.