Objectives: To evaluate whether 100 consecutive pitches induce clinically meaningful increases in medial elbow joint space gap (MJS) in high school baseball players and to examine associations with player characteristics, including early multi-sport participation.
Methods: In a randomized controlled trial, 34 healthy male high school baseball players were allocated to a pitching group (n = 17) or a control group (n = 17). The pitching group threw 100 fastballs at maximal effort, whereas controls rested for an equivalent duration. MJS was measured by ultrasonography before and after pitching or rest. The minimal clinically important difference (MCID) for MJS change (mm and %) was calculated using a distribution-based method (0.5 × SD of the pitching group's MJS increase). Group × time effects were tested using a two-way repeated-measures ANOVA, and univariate analyses explored relationships between MJS change and participant characteristics.
Results: No significant group × time interaction was observed (p = 0.427). In the pitching group, MJS increased by 0.2 ± 1.1 mm (8.3 ± 24.6%), remaining below the MCID thresholds (0.55 mm; 12.3%). Early multi-sport experience during elementary school was associated with smaller MJS expansion (p = 0.009), and the number of non-baseball sports correlated negatively with MJS change (r = -0.628, p = 0.007). No associations were found with pitch velocity or body composition.
Conclusion: A single bout of 100 pitches did not produce clinically meaningful MJS widening in high school baseball players. However, early multi-sport participation was related to reduced MJS expansion, suggesting that diversified sport experience may be an important background factor when refining pitch-count guidance.
Keywords: Adolescent; baseball; elbow joint; minimal clinically important difference; rehabilitation.