Rozenson: The knuckle curve, how it behaves, and why it’s becoming so popular

From Dan Rozenson at Baseball Prospectus on May 7, 2014, with mention of SABR members Harry Pavlidis and Alan Nathan:

Would you be surprised if I told you that over a quarter of the curveballs delivered by major league pitchers in 2013 were thrown with a knuckle curve grip? I certainly was. This was one of the results of the first comprehensive study of knuckle curveballs in the major leagues, which I have conducted after months of data collection. It is limited to PITCHf/x data, but the large population sizes give us a good first look at what knuckle curveballs behave like relative to standard curves.

What is a knuckle curveball?
First, let me define clearly what this study entails. A curveball is a pitch that, when thrown from a conventional arm angle, will drop from its normal trajectory due to topspin. It is also typically thrown slower than other pitches, so it drops significantly from the effect of gravity, as well. The two most important fingers in the curveball grip are usually the middle finger and the thumb. The middle finger and thumb rest on or beside seams on opposite sides of the ball, as in this image. At release, the thumb and middle finger spin the ball forward, giving it the desired topspin. The index finger is not needed at all to throw the pitch, and a handful of pitchers even leave it off the ball entirely.

What I am collectively calling the knuckle curve is really two related grips (although for the sake of convenience I will use the terms interchangeably). The first involves tucking the first knuckle of the index finger up against the ball, like the Cards’ Sam Freeman does. The second, more common variant, is often known as the “spike” grip. Here’s Craig Kimbrel’s spike grip. These two versions of knuckle curveball are thrown by about 75 active major leaguers, according to my count, yet the perception still seems to be that the knuckle curve is something of a novelty pitch.

The perception may have been reinforced by how rarely the “knuckle curve” pitch tag was used in PITCHf/x data. Until last season, only A.J. Burnett, Vin Mazzaro, Chad Gaudin, and Nathan Adcock had pitches classified automatically as knuckle curveballs—and, to my knowledge, only Burnett actually threw one in that bunch. To MLBAM’s credit, they began adding the knuckle curve tag (“KC”) to a number of pitchers in 2013 after asking to see the list I was compiling. They now have about two dozen pitchers whose curveballs are classified as KC. Future analyses of knuckle curveballs using MLBAM’s automatic classifications will thus be much easier.

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Originally published: May 7, 2014. Last Updated: May 7, 2014.