Relation between forearm flexor–pronator muscle elasticity and ulnar collateral ligament laxity of the elbow joint in high school pitchers with and without medial elbow tenderness: evaluation using shear wave elastography

Background The forearm flexor–pronator muscle contributes to the dynamic stabilizer against valgus stress at the elbow during pitching motion. This study used shear wave ultrasonography to assess the relation between the elasticity of each forearm flexor–pronator muscle and the laxity of the ulnar collateral ligament (UCL) of the elbow joint in high school pitchers with and without medial elbow tenderness. Methods One hundred and fifteen high school baseball pitchers (average age 16.5 years) underwent a medical checkup during the off-season of 2022. We investigated instances of medial elbow tenderness on the pitching side at that medical check. The 115 participants were divided into two groups: 25 pitchers with medial elbow tenderness (medial elbow tenderness group) and 90 without medial elbow tenderness (non-medial elbow tenderness group). The shear wave velocities (SWVs) of the pronator teres (PT), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), flexor digitorum profundus (FDP), and the common flexor–pronator muscles (CFPM) were measured as the index of muscle elasticity using shear wave ultrasonography. Ultrasound examinations were conducted with and without gravity valgus stress of the medial aspect of the bilateral elbows. We compared the elasticity of the forearm flexor–pronator muscle and the degree of laxity of the ulnar collateral ligament of the elbow between the two groups. Also, we analyzed the relation between the elasticity of each forearm flexor–pronator muscle and the laxity of the ulnar collateral ligament of the elbow between the two groups. Results Between the two groups, no significant difference was found for any forearm flexor–pronator muscle of the throwing side. In the group without medial elbow tenderness, a weak but significant negative correlation was observed between the SWV of the FDS and the medial ulnohumeral joint width with and without gravity stress on the throwing side (r = -0.24, P < 0.05). In the group with medial elbow tenderness, no statistically significant association was detected; however, these findings remain inconclusive due to the limited sample size and low statistical power (0.26). Conclusion Our findings suggest that higher elasticity of the FDS may be associated with a modest reduction in UCL laxity, suggesting it may play a limited role in dynamic stabilization.