How the knuckle-curve works & why it's become so popular
MAY 08, 2014 9:59a ET
Would it surprise you if over a quarter of the curveballs delivered by major-league pitchers in 2013 were thrown with a knuckle-curve grip? It certainly surprised me.
This was one of the results of the first comprehensive study of knuckle-curveballs in the major leagues. It is limited to PITCHf/x data, but the large population sizes give a good first look at what knuckle-curveballs behave like relative to standard curves.
What is a knuckle-curveball?
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.
The knuckle-curve is really two related grips. The first involves tucking the first knuckle of the index finger up against the ball, like the Cardinals’ Sam Freeman does. The second, more common variant, is often known as the “spike” grip. Here’s Atlanta closer Craig Kimbrel’s spike grip. These two versions of knuckle-curveball are thrown by about 75 active major leaguers, 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 Philly’s A.J. Burnett, Pittsburgh’s Vin Mazzaro and free-agents Chad Gaudin and Nathan Adcock had pitches classified automatically as knuckle-curveballs — 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 some of Baseball Prospectus’ data. 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.
All of the tracked pitch data is from the PITCHf/x system. The pitches labeled “curveball” in this study were manually tagged by Harry Pavlidis. This data (including release point, which will be important later on in this study) has also been corrected for calibration errors in the PITCHf/x system, thanks to hard work by Dan Brooks and Alan Nathan. This is an impressive reduction in systematic error that should lead the reader to interpret the results below with minimal concerns about measurement accuracy.
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. More than 100 pitchers, active or retired, have been identified with such a grip; 84 of them threw a curveball last season. In almost every case, there was 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 Baseball Prospectus (BP) couldn’t find evidence of a specialty grip. BP 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).
What the data says
This study will allow some insight for the first time on what has made this pitch grip so popular and what its characteristics are compared to other curveballs. BP compared curveballs thrown with either of the knuckle grip variants vs. anything else Pitch Info has classified as a curveball in the 2013 season.
Here are the physical characteristics of the pitches as BP might typically display on the Brooks Baseball site. The main number in each cell is an aggregate result, and the number in parentheses is the average of each pitcher’s average — the basis for significance testing.
Dragless V-mov + gravity
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 has an effect of adding 2½ 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
A pitch thrown faster has less time to drop due to gravity and drag. For a pitch that travels slowly, as most curveballs do, that means that pitchers need to begin the arc of the pitch higher than they do for their other pitches in order to have the ball cross the plate in the strike zone. Astute batters are often able to spot curveballs directly out of a pitcher’s hand for this reason. It stands to reason that using the knuckle grip can negate some of this advantage for hitters, if a curveball with similar spin deflection can be thrown faster.
To test this, Pavlidis helped draw up a diagram showing the average pitch trajectories of the fastballs and curveballs thrown by right-handed pitchers to right-handed batters in 2013, 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.
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.
Testing 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 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.
Here’s data for how knuckle-curves and regular curves fared against hitters last season, at the aggregate level.