Background Shoulder injuries are common among flatwater kayak athletes and are predominantly characterised by features of overuse.

Objective This study investigated whether body composition and physical characteristics differ between junior flatwater kayak athletes with and without shoulder pain.

Design Cross-sectional study.

Participants The study included 178 Japanese junior kayak athletes (116 boys and 62 girls) (age 13.4 ± 0.9 years, height 162.1 ± 7.1 cm, weight 54.1 ± 8.5 kg) who participated in a national junior training camp (2015–2018). Participants were categorised into 2 groups based on a self-reported history of kayaking-induced shoulder pain.

Main Outcome Measurements Body composition was measured using the Body Impedance Analyser (InBody 720) device, and range of motion was evaluated using a goniometer. Shoulder pain was assessed based on a questionnaire, interview, and physical examination performed during the national training camp. Logistic regression analysis was performed to determine the factors associated with shoulder pain.

Results In this study, 42.1% (n = 75) of the participants experienced shoulder pain. Multivariate analysis revealed that lower trunk muscle mass per body weight was the most significant predictor of shoulder pain. Trunk muscle mass per body weight (odds ratio [OR] 0.81, 95% confidence interval [CI] 0.74–0.90) and age (OR 1.74, 95% CI 1.31–2.32) were associated with shoulder pain in junior kayak athletes.

Conclusions This study suggests that screening for body composition is essential to treat and perhaps prevent shoulder pain. In addition to evaluation of total and regional body composition, evaluation of muscle characteristics may be useful for injury prevention. Lower trunk muscle mass was shown to be associated with shoulder pain. Therefore, appropriate trunk movements and core strengthening should receive attention in junior kayaking athletes.