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May 7, 2014
The Knuckle Curveball
How it Behaves, And Why it's Becoming So Popular
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?
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.
Player cards on BrooksBaseball.net also now feature grip information for many pitchers, which should increase awareness of the prevalence of the spike/knuckle curve grip.
The population of pitchers who use knuckle and spike curveball grips was generated through a variety of methods. It involved searching for news reports of people with such grips, watching game video, and browsing through thousands of pictures of pitchers’ grips. I have identified over 100 pitchers, active or retired, with such a grip; 84 of them threw a curveball in the 2013 season. In almost every case, I was able to find photographic evidence. Other evidence that I treated with equal weight was a pitcher’s verbal description of his own grip.
The population of the “other” group consists of every pitcher who threw even a single curveball in 2013, but for whom I could not find evidence of a specialty grip. I was able to identify a non-knuckle curveball grip for a majority of the pitchers in this group. There may well be a handful of “false negatives,” but they are disproportionately likely to be pitchers who threw very few curveballs (and thus had fewer available photos of their grip). I am very confident that whatever remaining knuckle curves exist in this population would not substantially change the findings presented.
What the data says
The most obvious difference in the two groups is that knuckle curves are thrown, on average, more than 2 mph faster than other curves. (This difference is highly significant, with a t value of 6. The vertical movement + gravity variable—likely as a result of the speed differential—was also significant.) Thoughtful readers might ask whether knuckle curve throwers simply throw harder in general. As it turns out, the aggregate fastball speed of the knuckle group was 92.75 mph, and the other group averaged 93.09 mph. That suggests that the knuckle grip itself has an effect of adding 2.5 mph to a pitcher’s curveball. Additionally, the spin data suggests that rotation rate on knuckle curves is roughly equivalent. The spike and knuckle grip group was therefore able to add speed to the pitch without sacrificing spin deflection.
A sabermetric theory for the knuckle curve’s appeal
To test this, I had the ever-helpful Harry Pavlidis draw up a diagram showing the average pitch trajectories of the fastballs and curveballs thrown by right-handed pitchers to right-handed batters in the 2013 season, separated by whether they were knuckle curve throwers or non-knuckle curve throwers. The curveball characteristics of RHP to RHH mirror the overall trends: on average, the knuckle curves were thrown 2.4 mph faster, and their drop due to gravity, drag, and spin deflection combined was slightly less.
To begin with, here is the side view of the fastballs. Knuckle-curve throwers are in orange, with non-knuckle throwers in black. The scale is shortened somewhat to make differences in the flight paths more visible.
No real difference. Now for the curveballs.
Two things jump out. The first is that the orange dots are spaced farther apart, which reflects the faster speed of the knuckle curves. The second is that the balls are released noticeably farther apart in height and maintain separation throughout the time that hitters would be trying to pick up the ball’s pitch type, though that gap closes as the ball arrives at home plate. In other words, it appears that pitchers can release curves with the knuckle grip from a height and angle that’s closer to their other pitches than those throwing a conventional curveball can. The combination of faster speed and subtler trajectory could create a more effective offering for some pitchers.
I tested both the overall “break” in trajectory for the two types of curveballs and the overall distance (on the x and z axes, with y set at 55 feet from home plate) in release points between fastballs and curveballs for both groups of pitchers. Both tests showed small but statistically significant differences—about half an inch of extra “hump” for non-spike curves and a release point a half-inch closer to the fastball release point.
Now let’s look at how knuckle curves and regular curves fared against hitters last season, at the aggregate level.
There are a couple of trends in the data presented above. There is a significant tendency toward ground balls and away from pop-ups among knuckle curve throwers, which perhaps explains the higher BABIP. (The GB rate difference was significant at the 99 percent level). However, there may also be a tendency to swing and miss more frequently (which fell just short of being significant at the 95 percent level). These trends do not prove, but are consistent with, poorer pitch recognition by the batter (on average) when a knuckle curveball is thrown. PITCHf/x data cannot verify that the impaired pitch recognition explanation is true, so future research will have to address it. Another factor beyond the reach of PITCHf/x that may explain the ability of pitchers to disguise their knuckle curveballs is arm speed more consistent with a fastball. I encourage statheads and scouts to collaborate on investigating possible links in that area, too.
A pitcher’s explanation
The first reason, naturally, is that a pitcher must have a “feel” for a pitch he’s throwing. Bannister said, “Some guys also like the feel of having a finger on top of the ball, like they’re not going to lose it. It’s kind of a confidence thing.” That makes sense.
In terms of pitching theory, Bannister described a philosophical approach to the curveball that he said he and Rick Peterson share, among others. I’ll let Bannister explain:
For someone like Zack Greinke, who throws a very slow curveball relative to his fastball, the aim is to disrupt timing and change the eye level of the hitter. But the knuckle curve grips are natural choices for pitchers who want to live on the higher end of the curve spectrum. There is, after all, appeal in throwing a tighter, faster curveball. Bannister thinks it may be a result of the changing strike zone:
That could explain why a grip that used to be associated mostly with power pitchers has been adopted by a wider variety of pitchers—even Jamie Moyer! A curveball that acts a little more like a slider will not break as significantly from the time it crosses the plate to the time it is caught by the catcher. That means umpires and hitters have less reaction time, but also less movement to adjust to. Greinke himself said recently, “My curve is so slow, that they might say ball, and if not, they can still react to it. … [Justin] Verlander’s is so sharp that by the time they realize it’s a strike, they can’t swing at it.”
The success of any non-fastball pitch usually depends on the ability of pitchers to make it look like their fastball for as long as possible. My working theory for the knuckle curveball is that its faster speed and lower release cause batters to hit more ground balls and miss more often because they are less able to tell it apart from the pitcher’s fastball. Pitchers who tip their curveballs with slower arm speed or a higher release point might try the knuckle grip to make the pitch more effective. The grip may also be useful for pitchers stuck with looping curveballs that they find hard to control or get consistent strike calls on, or pitchers who want a pitch similar to a slider but without the associated elbow risk.
I owe special thanks to Harry Pavlidis, Dan Brooks, and Alan Nathan for their help with this project.