How Do We Generate Spin?

With increasing accessibility to tools such as Rapsodo, Trackman, and Flightscope at all levels of the game, spin rate and spin axis have largely dominated much of the recent conversation revolving around pitch design and repertoire development. This greater emphasis should come as no surprise given the relatively strong initial links between spin rate and descriptive measures of performance.

However, despite the large amounts of energy and attention being invested into looking at spin rate by coaches and athletes alike, many questions still linger in the public realm regarding the most fundamental aspects of spin rate.

Given that we now have four full years of Statcast data and a handful of recently published papers to analyze, we can start tackling some of the most basic questions about generating spin that were left unanswered just a few years ago.

How Do We Spin the Baseball?

Dating to as far back as Stevenson (1985), researchers have been interested in examining finger kinematics and kinetics during the release sequence of overhand throws. Most recently, a paper by Kinoshita et al. (2017) found that pitchers impart three “peaks” of force on the baseball during the delivery corresponding with max external rotation, the ball-rolling phase, and what we’ll describe as spin creation.

Essentially, as the arm accelerates, the ball produces more force on the fingers, which if unmatched, would likely cause the ball to fling backwards at any point during the delivery (Matsuo et al., 2017). Since we do not want this to occur, we subconsciously balance this force with our fingers by imparting a normal force on the ball in the opposite direction (Hore and Watts, 2011).

As we get to the end of this process, the thumb slips off the baseball roughly 6-10ms before release (Matsuo et al., 2017), allowing the ball to roll up our fingertips so that we can accelerate the ball towards the target and impart “shear force” (or tangential force) on the baseball about 3-5ms before release. It is hypothesized that this production of tangential force on the baseball just before release is both how and when spin is generated (Kinoshita et al., 2017).

Picture from Matsuo et al

How Can I Increase Spin Rate?

It might surprise you to learn that there are actually a few straightforward ways to increase your spin rate.

Increasing Velocity

As mentioned on this blog, velocity is linearly related to spin rate from an individual perspective, so increasing your velocity is a surefire way to increase spin.

However, the reasons as to why spin and velocity are correlated in such a manner are a bit less clear. According to Kanosue et al. (2014), “the angle at which the fingertips reached forward over the ball during the top-spin phase (arm acceleration) was highly correlated with ball spin.” The researchers speculate that because higher-velocity pitches need to have a lower vertical release angle to reach home plate as a strike (all else equal), the palm of the throwing hand would have to be angled farther downward at release, thus causing the fingertips to flex farther over the ball. Indeed, when we plot vertical release angle by spin rate once controlling for a pitcher’s individual average using 2018 MLB data, we do see some evidence of this effect.

Alternatively, since shear force was found to closely mirror resultant forces (the forces imparted on the ball to keep it in our hands), it could also be the case that as velocity increases, so too does the force of the baseball on the pitcher’s fingers. As a result, forces generated by the pitcher’s fingers are also magnified, thus increasing the ability of the pitcher to impart greater shear force before release. An illustration of this hypothesis is shown below.

In all likelihood, both of these theories have validity in explaining the link between velocity and spin rate and are at least somewhat interrelated with one another.

Adjusting Spin Axis

Beyond simply increasing velocity, we can also impart more relative cut on the baseball to increase our spin rate. This holds true not only for fastballs, but also across most pitch types as well.

The reasons as to why this occurs are a bit unknown given that forearm and wrist kinematics are difficult to measure and have often been overlooked in biomechanical research. We do know that pitch types with increased amounts of cut, such as curveballs, generally have greater supination of the forearm, ulnar deviation, and wrist flexion when compared to pitches with natural run or fade (Solomito et al., 2014). Perhaps one, two, or all three of these traits can explain why more cut equates to more spin, but more research is necessary to identify potential root causes.

Less Certain Ways of Potentially Increasing Spin

Finger Strength

If generating shear force is likely the main producer of spin rate, then the role of finger strength in generating shear force (particularly of the index and middle finger) should not be overlooked. Within the Kinoshita study, it was found that pitchers impart finger forces on the ball close to their strength limit (>80%) while pitching at max intent. From a perspective of purely force production, one could reasonably make the connection that increasing finger strength could have the potential to help an athlete generate more shear force on the baseball.  

Despite this fairly straightforward link, to our knowledge there has been only one study which has investigated finger strength and release spin (Woods, Spaniol, & Bonnette 2018). Perhaps counterintuitively, the authors found a negative correlation with spin and finger strength, which can likely be attributed to the fact that grip and intent did not seem to be controlled for and that pitchers were asked to throw curveballs rather than fastballs.

More work is needed to investigate the correlation between spin rate and finger strength, which is a subject we look forward to researching in greater detail moving forward.

Friction

Given that we know pitchers have only a handful of milliseconds to impart shear force on the baseball, any additional adhesive properties or friction between the fingers and the ball is likely going to be valuable in helping generate spin (Kinoshita et al., 2017).  

But achieving maximum levels friction and adhesion ultimately depend on anatomical and biological factors that are typically outside of an athlete’s control (Spinner, Wiechert, & Gorb, 2016). Attributes such as finger length, age, and sex (though not fingerprints) ultimately become key components in being able to impart spin on a baseball. As a result, it is easy to see why there is a common belief that all pitchers have their own inherent spin rate that is difficult to alter.

Complicating matters further, friction is also impacted by skin hydration levels, which are ever changing and often a function of the surrounding environment (Adams et al., 2013). For example, as air becomes drier, pitchers intuitively know to blow on their hands to apply moisture. Vice versa, as moisture is in the air and athletes begin to sweat, they know to apply rosin to minimize moisture.

It has been found that all athletes have their own optimal moisture level on their fingers to maximize their friction coefficient, meaning that there is no “one size fits all” formula with foreign substances to impart more spin (Adams, Briscoe, & Johnson, 2007). Instead, pitchers should find the right combination of legal substances available to them at any given time to obtain the ideal moisture levels to feel comfortable and impart spin on the baseball.

Can I Decrease My Spin Rate?

Changing Grips

Besides taking something off a pitch, it is widely known that using a grip that either splits your index and middle fingers or that incorporates your ring finger likely helps decrease your spin rate, even when controlling for velocity. This is something that we have tested in the past, finding that both a split finger grip and three-finger grip decreased Units when compared to more traditional fastball grips.

While the splitfinger effect is a bit easier to understand (try applying force to the palm of your opposite hand using both a regular and splitfinger grip), the three-finger finding is perhaps a little more peculiar given that more fingers should equate to more friction, and thus more spin.

Fortunately, we can look back to the individual forces produced by each finger during arm acceleration provided within the Kinoshita paper to understand why using the ring finger mitigates spin.

Looking at the illustration below, we find that the ring finger, when measured against the index finger, middle finger, and thumb, imparts about half as much shear force onto the baseball, on average, compared to the index and middle finger. The ring finger also has no “peak” of shear force just before ball release, which has been hypothesized as the main link in generating spin.  

So, in both cases, it seems as though each grip constrains our ability to impart absolute shear force on the ball in a unique way, thus limiting our ability to impart spin while holding intent levels equal. This lends credence to the idea that there is no grip that works best for everybody, as each pitcher will have different measures of finger strengths, finger lengths, and frictional forces that interact with the ball in a specific way.

Where Does This Leave Us Today?

Although we now know more about generating spin than we did a few years prior, there is a decent amount of work still needed to be done if we want to fully understand spin and how to develop it. Moving forward, investigating interactions between friction, finger strength, and grips and how they relate to changing spin should provide us with a better understanding of how we can improve the pitching quality of individual athletes.

The future is bright for pitch design, and it seems like we’re only scratching the surface.

This article was written by Dan Aucoin