As the average fastball in Major League Baseball (MLB) has climbed to superhuman heights. Teams have looked for new measurements to define the value of a pitcher. The spin rate has most recently become the popular metric. Baseball has worked hard to connect spin rates to game statistics like swing strike percentage and fly and ground ball percentage but has yet to investigate the link of average to elite baseball spin rates to the biomechanics of the pitching delivery. Therefore, the purpose of this review is to discover the biomechanics of the pitching delivery linked to the spin rate of each pitch. Specifically, the study proposed in this literature review will seek to determine correlation to spin rate of the pitch in the dominate or throwing arm internal rotation angle at pitch release (PR), peak linear wrist speed from stride foot contact (SFC) to PR, front leg extension angle at PR, front leg shin angle at PR, trunk distance traveled from SFC to PR, peak linear trunk speeds from SFC to PR, trunk angle at PR or front leg force production from SFC to PR.

There are many different ways to measure ball flight kinematics. Here are some common measurements; speed, spin rate, elevation, azimuth, four-seam rotation index, and release variability (Whiteside, McGinnis, Deneweth, Zernicke, & Goulet, 2016). Once the ball is thrown its motion depends only on gravity, its velocity, and its spin. Ball velocity or simply ball speed and spin rate have been the most widely used statistic in the MLB ever since the capability to record the measurements existed due to advancements in Doppler radars and high-speed cameras. In engineering notation, these pitch characteristics are described, respectively, by a linear velocity vector and an angular velocity vector, each with magnitude and direction. The magnitude of the linear velocity vector is called ball speed and the magnitude of the angular velocity vector is called the spin rate (Bahill, & Baldwin, 2007). Ball speed is important because it dictates the time the hitter must react and hit the pitch and spin rate is important because it determines the movement or deflection of the pitch. Ball speed on average loses about 5% of its initial velocity from pitch release to the catcher’s glove which causes the flight path to drop on the vertical plane as it moves closer to the target (Alaways, Mish, & Hubbard, 2001). A spin rate can influence the loss of speed which can create the infamous rising fastball where the fastball isn’t rising but not dropping on the vertical plane as much as the hitter would perceive based on its ball speed (McBeath, 1989). How the ball spin creates lift with a fastball is when the ball is spinning clockwise to create a faster speed of air movement on top of the ball and a lower speed of air movement on the bottom. The lift force can deflect a ball vertically or laterally, and orientation of the spin axis is the main factor determining the direction of the lift force which would define each different pitch type (Nagami, Higuchi, & Kanosue, 2013). This lift force created by the spin of a pitched baseball is called the Magnus force because Gustav Magnus was one of the first scientists to study this effect (Barkla, & Auchterlonie, 1971). Two models explain the origin of this Magnus force, one based on conservation of momentum and the other based on Bernoulli’s principle (Bahill, & Karnavas, 1993; Watts, & Bahill, 2000). Each pitch thrown in the game of baseball is either with a four-seam spin or a two-seam spin based on the design of the regulation baseball which is around 5 ounces. The lift on four-seam pitches can be as much as three times the lift on two-seam pitches (Alaways & Hubbard, 2001). The direction of deflection of the ball is perpendicular to the cross product of the spin axis and the direction of motion (Bahill, & Baldwin, 2007).

## The Effects of Baseball Spin Rates

In the MLB, there is a positive correlation of pitching velocity to strikeouts where r is .479, which is why ball speed is such a popular metric for scouting in baseball (Cameron, 2009). Spin rate is also another valuable metric because it determines ball movement or vertical to lateral deflection. In the MLB, there is a positive connection between spin rate, swing strike percentage and fly ball percentage which is proof why the spin rate is becoming another popular metric for scouting today (Petriello, 2017). Swing strike percentage and fly ball percentage is a good indicator of a pitcher’s effectiveness. Swing strike percentage equals swings and misses divided by total pitches. Fly ball percentage is the percentage of balls hit into the field of play that is characterized as fly balls. What is interesting is there is a positive correlation of initial ball speed to spin rate where r is .477, therefore, there is a good chance that pitchers in the MLB who have good pitching velocity have a good spin rate (Nagami, Morohoshi, Higuchi, Nakata, Naito, & Kanosue, 2011). The chart below does a great job illustrating the effectiveness of ball speed and spin rate working together in the MLB. This chart is showing a positive correlation of spin rate and velocity to swinging strike rate which is the same as swing strike percentage:

The focus of this study is to look more in-depth into the spin rate. Especially at what influences spin rate within the biomechanics of the elite pitcher. I have been unable to find any studies on what kinematic or kinetic variable or variables are influencing spin-rate. There are many studies on what kinematic and kinetic variable or variables is influencing ball speed or pitching velocity that ultimately are also influencing spin rate due to the positive relationship of initial ball speed to spin rate. The only studies showing influences of ball flight kinematics are the spin axis. For example, the spin axis is determined by the position of the hand and fingers of the dominate arm at pitch release (Jinji, & Sakurai, 2006). The list of independent variables I have chosen to find the correlation to spin rate are some of those variables that case studies have linked to increases in pitching velocity, therefore, they all have a very good chance of establishing the positive correlation. I am more intrigued to find which ones have a stronger correlation than the others (Fortenbaugh, Fleisig, & Andrews, 2009; Stodden, Fleisig, McLean, & Andrews, 2005).

In conclusion, baseball has some new statistics in the game that have become very popular with the advancement of new technology. The spin rate which defines the movement of the pitch once it leaves the pitchers hands is currently one of the most popular along with pitch velocity which they both share a positive correlation to each other. I propose, the critical importance of spin rate and its relationship to pitching velocity, swing strike percentage and fly ball percentage must be explored. I have yet to find any evidence that links to the source of baseball spin rates in the biomechanics of the pitching delivery. I hypothesize that due to the positive correlation of initial ball speed to spin rate that the kinematic and kinetic links in the pitching delivery to pitching velocity may hold the same links to spin rate. I will not be looking at all these kinematic and kinetic variables linking to pitching velocity but would like to encourage more studies to target these variables and their links to spin rate in the future. I am encouraged to make this first step to find the source of baseball spin rates in the pitching delivery because once the biomechanics of spin rate has been discovered and mapped out, then strength and conditioning specialists along with pitching coaches can program these improvements into their training.

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