Do Innings Pitched Prior To Age 25 Affect MLB Pitcher Longevity?

OBJECTIVES: Baseball pitchers are among the most valued players in major league baseball because of their influence on the outcome of a game. Anecdotal cases suggest that pitching more innings, arbitrarily determined to be 160 innings a year, may shorten careers. Our hypothesis was that pitchers who threw more innings/year before age 25 would have a shorter career. METHODS: Inclusion criteria were baseball pitchers who entered the Major League System from 1989-1992 only including those with no missing information for the following variables: Average Innings pitched per year before and after age 25, Earned Run Average (ERA), Walks and Hits divided by Innings Pitched (WHIP), Strike to Walk ratio (SWR), Strikeouts, Walks, Length of career, starting and Retirement age. 96 pitchers met those criteria. We did not analyze career statistics since these are created after the respective player's career ends rather analyzed their statistics before age 25. The information was obtained from http://www.baseball-reference.com. We used ANOVA to compare differences between groups and a regression model to assess relationship between variables (statistics) before age 25 and career length. RESULTS: The mean number of innings pitched per year before age 25 was 66.6, Confidence Interval (CI) 12.90-120.3. In fact, only 11 of 96 pitchers threw over 148 innings/year. ERA (mean 5.51 CI: 2.50-8.52) and WHIP (mean 1.66, CI: 1.04-2.28) may be most commonly used measurements of pitcher performance. If pitchers are divided by those who throw 80 innings/year or over, 28 over and 68 under, the over 80 innings/year before age 25 group had significantly better ERA's (4.20, CI: 2.52-5.88 vs 6.03, CI: 2.66-9.40, P=.0087), WHIP's (1.35 CI: .98-1.72 vs 1.79 CI: 1.13-2.45, P=.0012), and longer careers (13.1, CI: 8.5-17.7 years vs 8.7, CI: 2.8-14.6 years, P=.0007). In fact, the regression analysis between career length and innings pitched per year before age 25 is a positive relationship with an R2 of .15, suggesting 15% of the career length variability is positively associated with the number of innings pitched per year before age 25. ERA and WHIP had weak positive relationship with career length, R2=.039 and R2=.025 respectively. CONCLUSION: Pitchers who threw over 80 innings a year before age 25, had the lowest ERA's, WHIP's, and the longest careers. Our hypothesis was disproved. In this population, there was not an advantage to restricting the number of innings pitched before age 25 for a young pitcher.