Active Spin

Definition

Statcast refers to the spin that contributes to movement as Active Spin.

Not all spin actually contributes to a pitch's movement. For a football, or a bullet, none of the spin contributes to movement, and so, a football or a bullet has 100% Inactive Spin. In order for a baseball to have Inactive Spin, you would twist your arm slightly so that the baseball is thrown more like a football. The more the baseball is thrown like a football, the more the spin of a baseball will go from Active Spin to Inactive Spin.

When thrown with pure topspin or backspin, then 100% of the spin contributes to movement. You can also throw a pitch with some side-action so that some of the deflection will go up or down, and some will go side to side. All of this is Active Spin.

In 2019, Rich Hill had 100% Active Spin for his curveball -- he was getting the most out of his spin. For four-seam fastballs, Justin Verlander led with 98.5% Active Spin, meaning he was almost entirely optimizing his spin.