What is Arm-Side Run? Definition, Formula, and Example

What is Arm-Side Run?

Arm-side run is the horizontal movement of a pitched baseball toward the pitcher's throwing-arm side. For a right-handed pitcher, arm-side run moves the ball toward a right-handed hitter (inside) or away from a left-handed hitter. This inward tailing action is the primary horizontal movement component of fastballs and changeups. It is the opposite of glove-side break, which is the horizontal movement toward the pitcher's glove side. Arm-side run is created by a combination of lateral Magnus force and forward gravity, causing the ball to deviate from a straight-line trajectory as it approaches home plate.

How it's Calculated / Measured

Arm-side run is measured by Statcast and Hawk-Eye tracking systems as Horizontal Break (or "HB"), reported in inches. The metric specifically isolates the movement of the baseball from the release point to home plate, excluding the effect of gravity.

The calculation relies on the spin axis of the baseball. A pitch with backspin and a tilted spin axis will generate a Magnus effect pushing the ball laterally. If the spin axis is tilted such that the top of the baseball is rotating toward the pitcher's arm side, the resulting Magnus force pushes the ball in that same direction. Statcast measures this in two ways:

1. Observed Horizontal Break: The actual physical deviation of the pitch from its initial straight-line path.

2. Induced Horizontal Break: The horizontal movement attributed solely to the pitch's spin, stripping out the 9.8 inches of horizontal deviation caused by gravity over 60 feet, 6 inches.

Negative horizontal break values (in Statcast's coordinate system) indicate arm-side run for right-handed pitchers, while positive values indicate arm-side run for left-handed pitchers.

Worked Example

Logan Webb, a right-handed pitcher for the San Francisco Giants, relies heavily on arm-side run to generate weak contact and called strikes. In 2023, Webb's primary pitch was his sinker, which averaged 93.1 mph. Because of his lower arm slot and specific spin axis, his sinker generated an average of 16.5 inches of arm-side run (measured as -16.5 inches of induced horizontal break).

When Webb throws his sinker to a right-handed hitter on the outer edge of the plate, the ball does not travel in a straight line. Over the final 15 feet of its flight, the 16.5 inches of arm-side run causes the ball to tail back toward the middle of the plate or even inside, frequently jamming the hitter or inducing weak rollover groundballs.

Why it Matters

Arm-side run is critical for pitch design and sequencing. Front offices evaluate amateur and minor league pitchers based on their ability to generate horizontal movement, as pitches with heavy arm-side run (like sinkers and splitters) correlate strongly with high ground-ball rates. For fantasy baseball and DFS players, identifying pitchers with extreme arm-side run can help predict double-play induction and home-run suppression, particularly in ground-ball-friendly stadiums. Pitchers use arm-side run to "catch the plate" on the inner edge, establishing dominance inside without risking hit-by-pitches.

Limitations / Common Misconceptions

Arm-side run is often confused with vertical drop, but the two are distinct. A pitch can have heavy arm-side run (horizontal) while maintaining high vertical "rise" (induced vertical break). Additionally, arm-side run does not guarantee success; if a pitch has too much arm-side run and poor vertical approach angle, it can leak back over the heart of the plate, resulting in hard-hit balls. Furthermore, arm-side run is highly dependent on the pitcher's arm slot. High-over-the-top pitchers naturally struggle to generate arm-side run compared to lower-slot pitchers.

Related Terms

• What is Horizontal Break?
• What is Induced Vertical Break?
• What is Spin Axis?
• What is a Sinker?
• What is Arm Slot?

In Legends Deck: Arm-side run directly influences the horizontal movement ratings on pitcher cards. A high arm-side sinker will boost a Legends Deck pitcher's "Movement" attribute, increasing the likelihood of inducing ground balls and soft contact in simulated at-bats, making it a vital rating for ground-ball specialists.