Glenohumeral and scapulothoracic muscle architecture in middle-aged and elderly individuals.

BACKGROUND: Age-related changes in shoulder musculature are poorly quantified, and currently no dataset describes the architecture of middle-aged parascapular muscles, limiting musculoskeletal modeling and clinical interpretation. QUESTIONS/PURPOSES: To compare the architectural properties of rotator cuff and deltoid muscles between middle-aged and elderly individuals; and to establish normative architectural data for parascapular muscles in middle-aged specimens. MATERIALS AND METHODS: Thirty-two cadaveric shoulders were examined, including middle-aged (n = 12) and elderly (n = 14-20, a partly previously reported, partly newly analyzed cohort) individuals. Muscle architecture measurements included muscle mass, muscle length, fiber length, sarcomere length, normalized fiber length, and physiological cross-sectional area (PCSA). A two-way linear mixed model (age group × muscle) was used to compare the five muscles common to both age groups, and a linear mixed model was used to compare all thirteen parascapular muscles within the middle-aged cohort, with specimen as a random intercept in both models. RESULTS: Middle-aged specimens demonstrated greater PCSA in the deltoid and subscapularis compared to elderly individuals (p<0.05); the infraspinatus showed a smaller, non-significant difference (p=0.069), and the supraspinatus and teres minor did not differ significantly by age. These differences were primarily attributable to age-related reductions in muscle mass. Fiber length and normalized fiber length did not differ with age. Among middle-aged specimens, the deltoid was the most massive parascapular muscle and had the highest PCSA, whereas the rhomboid minor was the smallest and had the lowest PCSA. The pectoralis major exhibited the longest fibers, while rotator cuff muscles had the shortest. CONCLUSIONS: Aging reduces force-generating capacity in shoulder muscles largely through size-dependent declines in muscle mass, while excursion-related properties remain preserved. These findings highlight the nonuniform impact of aging across muscles. CLINICAL RELEVANCE: These architectural data provide age-specific parameters that can improve musculoskeletal modeling, inform surgical planning, and refine rehabilitation strategies by accounting for age-related changes in shoulder muscle structure.