Assessing the Elasticity of the Flexor Pronator Muscles After Throwing Using Ultrasound Shear Wave Elastography.

Abstract

Background: The medial ulnar collateral ligament (UCL) of the elbow joint is the primary restraint to valgus stress during the throwing motion. The flexor pronator muscles (FPMs) also stabilize the elbow joint against valgus forces; however, assessment of FPM stiffness in baseball players has been limited.

Purpose: To use ultrasound shear wave elastography (USWE) to evaluate the change in tissue elasticity of the FPMs due to pitching.

Study designs: Descriptive laboratory study.

Methods: Included were 14 healthy male amateur baseball players (mean age, 28.0 ± 2.6 years) who had played intermediate-level high school or college baseball. The tissue elasticity of the flexor digitorum superficialis (FDS) in the superficial layer of the FPMs and the flexor digitorum profundus (FDP) in the deep layer of the FPMs were measured using USWE before, immediately after, and 24 hours after a throwing session of 100 pitches. The intra- and interrater reliability of the measurements were evaluated using the Pearson correlation coefficient and intraclass correlation coefficient (ICC). The elasticity of each muscle was analyzed using repeated-measures analysis of variance followed by post hoc analysis.

Results: Reliability of the USWE measurements was good to excellent (intrarater ICCs, 0.78-0.94; interrater ICCs, 0.85-0.96). The mean tissue elasticity values before, immediately after, and 24 hours after pitching were 22.3 ± 4.4, 41.0 ± 13.8, and 38.3 ± 11.2 kPa, respectively, for the FDS and 27.1 ± 5.8, 48.0 ± 22.3, and 29.6 ± 11.5 kPa, respectively, for the FDP. For both the FDS and FDP, elasticity was significantly higher immediately after pitching than before pitching (P < .001 and P = .0027, respectively). While the elasticity of the FDS at 24 hours after pitching remained significantly higher compared with before pitching (P = .0011), the elasticity of the FDP at 24 hours decreased to the same level as before pitching (P = .91).

Conclusion: Results using USWE indicated that the elasticity of the FDS did not decrease 24 hours after pitching and remained significantly higher than before pitching, suggesting that it may not be fully functioning as a dynamic stabilizer during this period.

Clinical relevance: Pitching in such a condition may result in greater stress on the UCL and increase the risk of UCL injury.