Shoulder pain is one of the most common musculoskeletal complaints, and its prevalence and consequences increase with age [1]. A significant proportion of the world's population has experienced shoulder pain daily, yearly, and throughout their lifetime [2]. In addition, shoulder functional disabilities are common and can interfere with work, hobbies, social, and sporting activities. They may also be associated with psychological distress and reduced quality of life [3,4]. Hence, shoulder pain and functional assessment is a crucial component in clinical practice, especially in an aging country like Japan.

Among various instruments used to evaluate shoulder function, the Constant Score (CS) [5] has been one of the most widely used tools for assessing shoulder pain and function irrespective of diagnosis. The CS has a maximum of 100 points, of which 35 are allocated to subjective components answered by the patient and 65 to objective components measured by the medical profession. The subjective component is composed of 5 items that assess pain and activities of daily living, and the objective component includes the measurement of the active range of shoulder motion and strength. The pain-free active range of shoulder motion is measured in forward and lateral elevation with a goniometer, internal and external rotation is measured by performing composite functional movements, and isometric strength is measured in abduction.

The modified CS was developed in 2008 [6] and has been translated and culturally adapted into numerous languages, such as Greek [7], Turkish [8], Chinese [9], Spanish [10], Arabic [11], Persian [12], and Brazilian Portuguese [13]. However, no validated Japanese version of the CS exists to date. The lack of a culturally and linguistically adapted Japanese CS represents a significant gap, impeding standardized clinical assessment and limiting the comparability of research data across international studies involving Japanese-speaking populations. The establishment of a Japanese version of the CS would not only standardize shoulder function assessment within Japan but also facilitate participation in multicenter and multinational research endeavors.

Therefore, this study aimed to translate and culturally adapt the original CS into Japanese and examine its reliability and validity.