This study examined the relationship between heart rate variability (HRV) and pitching performance in 10 professional baseball pitchers. HRV was measured via root mean square of the standard deviation (RMSSD) from resting heart rates. Pitching performance was assessed by total batters faced and a new metric called BFPIP. Results showed that pitchers with lower RMSSD at rest faced more batters per game and had higher in-game heart rates, suggesting greater sympathetic influence reduces pitching efficiency. Additionally, batters faced was associated with lower RMSSD and higher heart rates. Thus, pitchers with a larger increase in heart rate from rest to game may perform more efficiently. Future research should explore how preparation and psychology impact this relationship