Sternocleidomastoid muscle morphology is independently associated with neck strength and endurance in athletes and para-athletes.

Background: Neck functional capacity is clinically relevant in sports medicine for performance support, screening, and injury prevention in sports characterized by acceleration, contact, and sustained precision demands. Ultrasound can quantify local muscle morphology and may improve interpretation of neck performance beyond crude anthropometry (e.g., neck girth) and global strength indicators. Objective: To determine whether sternocleidomastoid (SCM) cross-sectional area (CSA) and neck anthropometry are associated with neck function (strength, endurance, range of motion), and whether SCM morphology explains function beyond age, BMI, general strength (handgrip), and sport-domain context in a male cohort of athletes, para-athletes, and physically active controls. Methods: Cross-sectional comparative study (n=195 men) including athletes across sport domains, para-athletes, and controls. Outcomes were isometric neck strength (flexor and extensor), neck flexor endurance, and active neck ROM. SCM CSA was assessed by standardized musculoskeletal ultrasound; neck girth served as an anthropometric comparator. Analyses included Pearson correlations and adjusted linear regression models controlling for age and BMI. Explanatory value beyond general strength was evaluated by adding handgrip strength, and independence from sport-domain context by adding sport domain as a covariate. Results: SCM CSA correlated positively with flexor strength (r=0.34, p<0.001), extensor strength (r=0.25, p<0.001), and flexor endurance (r=0.28, p<0.001), and showed a weak inverse association with extension ROM (r=-0.17, p=0.017). In adjusted models (age, BMI), SCM CSA was independently associated with flexor strength (β=1.32 kg per cm²; p<0.001) and flexor endurance (β=0.53 min per cm²; p<0.001), but not extensor strength. SCM CSA remained associated with flexor strength and endurance after adjustment for handgrip strength and sport domain. Additional within- versus between-domain analysis supported a within-domain SCM association for flexor strength and endurance rather than a purely between-domain aggregation effect. Neck girth did not provide comparable independent explanatory value for strength or endurance outcomes. Conclusions: SCM morphology is independently associated with neck performance, particularly flexor strength and endurance. Ultrasound-based SCM assessment provides measurable but limited additional information beyond neck girth, handgrip strength, and sport-domain context. These findings support its cautious use as an adjunct measure alongside objective cervical functional testing, rather than as a stand-alone screening tool.