Direct arm angle versus release-based proxies in Major League Baseball pitchers: a large-scale validation study

Area of Science:

• Sports Biomechanics
• Baseball Analytics
• Injury Prevention

Background:

• Pitching arm slot influences biomechanics and injury risk.
• Previous large-scale studies used indirect release proxies, leading to conflicting results.
• Direct arm angle measurement was previously limited.

Purpose of the Study:

• To evaluate the accuracy of surrogate release-based metrics in tracking true pitching arm angle.
• To assess how well common proxies correlate with direct arm angle measurements.
• To highlight the implications for baseball biomechanics and injury research.

Main Methods:

• Utilized Statcast's Hawk-Eye system for precise arm angle data.
• Analyzed two datasets: year-over-year pitcher comparisons and pitch-level analyses.
• Included 336 pitchers (season-over-season) and 10 high-volume pitchers (pitch-level).

Main Results:

• Vertical release position showed the strongest correlation with arm angle changes (r=0.74).
• Geometric proxies had moderate correlation (r=0.47), while horizontal release was inversely correlated (r=-0.38).
• Vertical release consistently tracked short-term arm angle variations (r=0.35-0.71).

Conclusions:

• Release-based proxies only partially reflect true pitching arm orientation.
• The accuracy of vertical release as a proxy varies significantly between pitchers and timeframes.
• Direct arm angle measurement is essential for future biomechanical and injury risk research in baseball.