Longitudinal changes of elasticity in the muscle tissues around the shoulder joint during the growth period have not been assessed using shear wave elastography. This study enrolled male students aged 13-18 years who played baseball or rubber baseball as an extra-curricular activity during junior high or high school or on a baseball team outside of school. The exclusion criterion was a history of surgery for athletic injury. One hundred and twenty-one boys were included in the study. The elasticity of the superior part of the trapezius, the supraspinatus, and the infraspinatus were measured by ultrasound. The shear elastic modulus (SEM), which is the ratio of the strain ratio (SR) in the acoustic coupler to the SR of each muscle, was calculated as a representative value. Six months after the baseline assessment, subjects were evaluated regarding any newly developed pain in the joint of the throwing shoulder, and categorized into either the non-pain group or the pain group. Although all muscle SEMs tended to increase in both the throwing and non-throwing shoulders, no significant difference was observed in the prevalence of shoulder joint pain between ages (p = 0.541). The results of a binominal logistic regression analysis, adjusted for age, body mass index, playing position in baseball, frequency of baseball practice, shoulder range of motion, and muscle strength showed that a decrease in SEM values of the supraspinatus was a risk factor for the development of new pain (odds ratio: 0.056; 95% confidence interval 0.011-0.299; p = 0.001). The elasticity of muscle tissues around the throwing shoulder increased with age, and low tissue elasticity of the supraspinatus of the throwing shoulder was a factor that triggered pain during throwing motions. Copyright © 2019 The Japanese Orthopaedic Association. Published by Elsevier B.V. All rights reserved.