OBJECTIVE: Examine the impact of rest days on musculoskeletal (MSK) injury incidence in Major League Baseball (MLB) starting pitchers. DESIGN: Descriptive epidemiological study. SETTING: MLB pitching and injury reports. PARTICIPANTS: MLB starting pitchers. METHODS: Data (2022-2023) were obtained from Baseball-Reference.com and fangraphs.com. Teams were grouped by average rest days between starts (Group A <5 rest days, Group B >5 rest days). Poisson regression was used to compare main outcome measure incidence rate ratios (IRRs). INDEPENDENT VARIABLES: Number of rest days between starts. MAIN OUTCOME MEASURES: Pitches per start, MSK injury incidence, injured list (IL) days for MSK injury. RESULTS: From 2022 to 2023, the average MLB rest days between starts was 4.80. Group A had 54 teams. Group B had 6 teams. The range of average rest days per start by team was 4.56 to 5.73 with a mean of 4.80. Group B pitchers spent 10.7 days on IL for MSK injuries per 1000 pitches, while Group A spent 13.6 days on IL (IRR = 0.78 [95% CI, 0.72-0.85]; P < 0.0001). Group B averaged 0.93 more pitches per start than Group A (P = 0.0164). Group B had a lower number of IL assignments for MSK injuries per 1000 pitches (0.245 vs 0.351, IRR = 0.70 [95% CI, 0.38-1.18]; P = 0.1715). CONCLUSIONS: Starting pitchers on MLB teams averaging > 5 rest days between starts spent less time on the IL for MSK injuries than MLB teams averaging < 5 rest days from 2022 to 2023. There was no clinically significant difference in pitch count and no significant difference in the number of IL assignments for MSK injuries.
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