This study aimed to (1) identify acute changes in supraspinatus and biceps longus tendon thickness after pitching and (2) determine the relationship between changes in tendon thickness and changes in glenohumeral range of motion after pitching. Fourteen healthy men with at least 5 years of baseball experience as baseball players performed 100 full-effort fast ball pitches. Tendon thicknesses and the glenohumeral internal rotation range of motion were measured before, immediately after, and 24 hours after pitching. One-way repeated-measures analysis of variance or the Friedman test with Bonferroni post hoc comparisons was performed to compare changes in tendon thicknesses and glenohumeral range of motion due to repetitive pitching and to identify the relationships between changes in tendon thicknesses and in glenohumeral internal rotation range of motion. The supraspinatus and biceps longus tendon thicknesses increased immediately (p<0.001) and 24 hours after pitching (supraspinatus: p=0.024 and biceps longus: p<0.001). The glenohumeral internal rotation range of motion decreased immediately after pitching (p<0.001) but not at 24 hours (p=0.086). The decreased glenohumeral internal rotation range of motion was associated with increased supraspinatus tendon thickness immediately after pitching (r=- 0.77 and p=0.001). Our results suggest that it takes time for tendon swelling to improve in pitchers after high-velocity repetitive pitches.
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