Throwing can Increase the Stiffness of the Scalene Muscle
Authors: Takenaga Tetsuya , Takeuchi Satoshi , Murakami Hideki , Sugimoto Katsumasa , Yoshida Masahito 
Published: July 2020

Objectives: Thoracic outlet syndrome (TOS) has been reported as a set of symptoms due to the compression of the brachial plexus and subclavian vessels in the region of the thoracic outlet1. As a type of TOS, scalenus anticus syndrome involves the compression of the brachial nerves as they pass through the interval surrounded by the anterior and middle scalene muscles, and the first rib bone or cervical rib2. Recently, exercise-induced TOS is becoming more common in athletes, especially for those who perform repetitive overhead and hyperabduction maneuvers with upper limbs, such as baseball players. However, the effect of throwing on the stiffness of the scalene muscles is unknown. Thus, the purpose of this study was to quantitatively measure the stiffness of the scalene muscles using real-time shear wave elastography (SWE). The stiffness of scalene muscles was hypothesized to increase for the throwing side of baseball players. Methods: Thirty college baseball players (age range 19 to 21 years) were included for this study. Ultrasonic SWE with a 2-10 MHz linear array probe transducer (Aixplorer; SuperSonic Imagine, Aix-en-Provence, France) was used to assess the stiffness of the anterior and middle scalene muscles. Each participant was sited. The measurements were performed in two arm positions; 1) adducted and neutral rotation of the shoulder 2)90 degree of abduction and external rotation of shoulder with elbow flexed to simulate a clinical examination known as Roos test3. In both of the arm positions, the transducer was positioned just superior to the clavicular bone, parallel to its axis. Transducer was moved superiorly and tilted to visualize the superior surfaces of the anterior and middle scalene muscles parallel to the surface of the fifth cervical nerve simultaneously (Figure 1A). In this position, shear wave elastography was performed to measure the elasticity of each scalene muscle as its stiffness. Each muscle was divided into superior and deep areas. In both areas of each muscle, three 3mm-diameter circles were set to measure the elasticities of the scalene muscles and its averaged data in each area was defined as each stiffness (Figure 1B). A repeated-measures analysis of variance (ANOVA) was used to compare the elasticity of superior and deep areas in anterior and middle scalene muscles in throwing and non-throwing side. Values of p<0.05 were considered statistically significant. Results: For the throwing side, higher stiffness was found in the deep part of the middle scalene muscle compared to the superior and deep parts of the anterior scalene muscle with an adducted and neutrally rotated shoulder (p=0.0433). Moreover, the muscle stiffness was significantly higher in the superior and deep part of the middle scalene muscle than in the superior and deep parts of anterior scalene muscle in an abducted and externally rotated position of shoulder (p =0.00187). Meanwhile, no significant difference was found in the anterior and middle scalene muscles for the non-throwing side in both arm positions. Conclusion: In professional athletes with TOS who experienced surgical treatment, moderate to severe hypertrophy of the anterior scalenus muscles has been reported to be found4. Meanwhile, although the stiffness of the scalene muscles can be also related to the compression on the brachial plexus and on subclavian vessels in the region of the thoracic outlet, its quantitative measurements in the scalene muscles has not been reported. In this study, at throwing side, the muscle stiffness significantly increased in the superior area of middle scalene muscle in throwing side. While no contribution was identified in the scalene muscles at non-throwing side. As a result, repeat throwing motion can increase the stiffness of the middle scalene muscle. As a result, the brachial plexus and/or the subclavian artery could be compressed at the interscalene triangle. Throwing athletes with TOS should be treated, considering the stiffness of the middle scalene muscle, even conservative or surgical treatment. Our study was the first study to evaluate the effects of throwing on the stiffness of the scalene muscles in throwing athletes. Repetitive throwing motion can affect the stiffness of middle scalene muscle. Reduction of the middle scalene muscle should be considered to treat throwing athlete who has TOS. [Figure: see text]