Backgrounds: The function of trunk joint affecting shoulder joint which plays an important role in throwing motion, needs to be assessed from various points of view. There are previous study assessed relationship between trunk rotation function and upper injury but studies on which musculoskeletal characteristics affect the range of motion(ROM) and muscle strength of baseball players are lacking. Therefore it is necessary to analyze the relationship between ultrasound characteristics of muscles and the function of each joint using muscle ultrasound and myotonometer.

Methods: Sixteen male high school baseball players aged 16 to 19 were participated in this study. Along with body composition, trunk rotation(TR), shoulder range of motion(ROM) and maximum voluntary isometric contraction(MVIC) were assessed. The muscle thickness(MT) and echo intensity(EI) within a target muscle, including rectus abdominis(RA), external oblique abdominis(EO), internal oblique abdominis(IO), transverse abdominis(TRA), pectoralis major(PM), upper trapezius(UT) and infraspinatus(INFS) were measured using ultrasound device. The mechanical characteristics of muscle within target muscle, including RA, EO, PM, UT and INFS using myoton PRO

Results: There were no significant differences between right and left in TR ROM and TR MVIC. Left RA showed significant increased creep than right(R = 1.26 ± 0.02, L = 1.38 ± 0.05 p < 0.01). Each MT of left EO (right, 1.15 ± 0.04; left, 1.28 ± 0.05, p < 0.05) and IO (right, 1.34 ± 0.05; left, 1.52 ± 0.06, p < 0.01) showed significant differences, respectively. Right TRA showed significant increased EI than left (R = 23.02 ± 2.63; L = 17.41 ± 2.85, p < 0.05). There were no significant relationship between TR ROM, MVIC and shoulder ROM, MVIC. Left TR ROM had significant correlation with EI of right RA (r = 0.635, p < 0.05). Left TR MVIC strength had significant correlation with MT of right TRA (r = 0.635, p < 0.05) and left TRA (r = -0.633, p < 0.01) respectively. Left TR MVIC strength had significant correlation with EI of left EO (r = -0.633, p < 0.05), left IO (r = -0.549, p < 0.05) and left TRA (r = - 0.662, p < 0.01) respectively. SER MVIC had significant correlation with EI of INFS(r = -0.598, p < 0.05).

Conclusions: Although there were no difference in trunk rotation range of motion and strength between dominant and non-dominant sides, it can be inferred that trunk muscle structure and mechanical characteristics changes in high school baseball players precede trunk rotation range of motion and strength changes. It can be seen that trunk muscles and shoulder muscles have different patterns to muscle strength and range of motion.