Background: The influence of player workload on hamstring and calf injuries in professional baseball players remains unknown. Hypothesis: Increased workload would significantly increase hamstring and calf strain injury risk. Study Design: Case-control. Level of Evidence: Level 3. Methods: All professional baseball players who sustained a hamstring or calf injury between 2011 and 2017 were identified using the Major League Baseball (MLB) Health and Injury Tracking System (HITS). A separate player usage dataset was used to determine workload. We then compared these variables between player games 2, 6, 12, and >12 weeks before a documented calf strain to player games from a noncalf injury control group. In a paired analysis, we compared the acute workload (2, 6, and 12 weeks) before injury with the injured player`s workload >12 weeks before injury. Results: There were 175 unique calf strains (71% occurred in position players) and 1042 unique hamstring strains (97.8% occurred in position players) documented in the MLB HITS database from 2011 to 2017. In all time periods, position players with fewer days rest, more innings fielded, and more plate appearances were associated with a higher risk of subsequent hamstring and calf strains. Having only 1 day rest was associated with a 2.08-fold increase in percentage of players who sustained a subsequent calf strain compared with those with 4 days rest. Position players with >5 days of rest had a 4.03-fold decrease in percentage of players who sustained a hamstring strain compared with players with 1 day rest. Position players who fielded >9 innings per game had a 2.01-fold increase in percentage of players with a subsequent calf strain and a 3.68-fold increase in percentage of players who sustained a subsequent hamstring strain compared with players who only fielded 1 innings per game. Conclusion: Position players with fewer days rest, more innings fielded, and more plate appearance per game had an increased risk of sustaining a hamstring or calf injury. Clinical Relevance: Increased workload had a direct relationship with risk of hamstring and calf strains in professional baseball players. Teams should consider more days rest for position players to avoid calf injuries.
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