Rotator cuff tears are a highly prevalent musculoskeletal injury affecting 20-50% of older adults, and prevalence increases with more advanced age. Symptoms of a rotator cuff tear include muscle atrophy, fatty infiltration, decreased strength, and a loss of functional ability. However, a healthy aging process is also associated with atrophy, reduced strength, and declines in physical function. The goals of this dissertation were to understand how muscle volume and strength are affected with healthy aging, and how the symptoms of a rotator cuff tear may further compound age-associated changes to muscle morphology, strength, and function.

We measured upper limb muscle volume and strength in a cohort of healthy older adults and compared these findings to previously reported measures for healthy young adults. We also studied a group of older individuals with a rotator cuff tear and healthy age- and gender-matched controls. We found that healthy older adults have reduced muscle volume and strength compared to young adults, and these measures are further reduced in subjects with a rotator cuff tear. In addition, quantitative assessments of muscle atrophy and fatty infiltration are not significantly related to semi-quantitative clinical assessments of these qualities. Older patients with a rotator cuff tear report worse physical function on self-report measures of shoulder function and demonstrate altered joint kinematics during functional upper limb task performance.

We developed individualized computational musculoskeletal models of older adult subjects with and without rotator cuff injury, scaling the model’s peak muscle forces using subject-derived measurements. We performed dynamic simulations of movement for recorded kinematics. Results demonstrated that patients with a rotator cuff tear used a more abducted and internally rotated posture during task performance and had reduced peak resultant joint reaction forces with reduced compressive force components at the glenohumeral joint.

Results of this dissertation suggest that the age-associated changes to upper limb musculature and strength are further compounded in subjects with a rotator cuff tear. This work lays the foundation to better inform clinical assessment techniques for older patients with a rotator cuff tear and rehabilitation strategies to prolong functional independence in the older adult population.