Elasticity of the Scalene Muscles in Collegiate Baseball Pitchers Using Shear Wave Elastography

Background: Thoracic outlet syndrome is more common in overhead athletes. The anterior and middle scalene muscles have been associated with thoracic outlet syndrome; however, the relationship between the elastic moduli of the scalene muscles and the overhead-throwing position has not been evaluated. Purpose: To investigate the elastic moduli of the anterior and middle scalene muscles in simulated throwing positions using shear wave elastography. Study Design: Descriptive laboratory study. Methods: We enrolled collegiate baseball players who underwent preseason medical checkups in January and February 2020. The anterior and middle scalene muscles were visualized in the short-axis view using ultrasound. In this view, the elastic moduli of the anterior and middle scalene muscles were measured using shear wave elastography in the following arm and neck positions: (1) adduction and neutral rotation of the shoulder (neutral), (2) 90 degrees of abduction and external rotation of the shoulder (ABER), and (3) ABER with neck rotation toward the nonthrowing side (ABER+NR). Repeated-measures analysis of variance and a paired t test were used to compare the elastic modulus values among the 3 positions and between the 2 muscles, respectively. Results: Overall, 30 baseball players were included. In both scalene muscles, the elastic moduli were significantly greater in the ABER versus neutral position (anterior scalene: 14.4 +/- 3.8 vs 9.4 +/- 2.0 kPa, respectively [P < .001]; middle scalene: 15.8 +/- 5.0 vs 10.9 +/- 3.4 kPa, respectively [P < .001]). In the neutral position, the elastic modulus was significantly greater in the middle scalene than the anterior scalene (10.9 +/- 3.4 vs 9.4 +/- 2.0 kPa, respectively; P = .03). In the ABER+NR position, the elastic modulus was significantly greater in the anterior scalene than the middle scalene (18.1 +/- 4.6 vs 15.8 +/- 2.6 kPa, respectively; P = .03). Conclusion: Because the ABER position was a simulation of the throwing position, these results indicate that this position could be associated with high elastic moduli of the scalene muscles, and both the anterior and middle scalene muscles may be involved.