Healthy aging results in a loss of strength at the shoulder that can be exacerbated by a degenerative rotator cuff tear, a common injury predominantly affecting older adults. Many important tasks necessary to maintaining independence require strength and coordination in the upper limb. The objective of this dissertation was to evaluate the relationship between shoulder strength and function and develop methods that enhance our understanding of how muscle morphology relates to strength. Traditional motion capture combined with a musculoskeletal modeling approach were used to evaluate shoulder strength requirements necessary for successful task completion and determine how age- and rotator cuff tear-related strength losses affect reserve strength at the shoulder. It was discovered that neither age nor age-related rotator cuff tears affect the strength required at the shoulder to successfully complete upper limb functional tasks and older adults with and without a rotator cuff tear maintain an excess of reserve strength for these task demands. Limitations of cross-sectional studies in humans can be overcome by the use of an animal model. Gross morphological analyses of middle aged and older adult vervet monkey upper limb muscles were performed to determine if the vervet could be used for future longitudinal studies of upper limb musculoskeletal aging and determine if upper limb muscle volume affected measures of function:​ walking speed, percent time hanging, and percent time climbing. Vervet monkeys experience upper limb muscle volume loss similar to humans, although this loss was not associated with changes in function, suggesting that they too maintain sufficient upper limb strength necessary for successful task performance. A musculoskeletal modeling approach was implemented to quantify the biomechanical impact of employing an extra cellular matrix graft material in a bridging repair of a torn supraspinatus to restore muscle function. Biomechanical properties of the available graft material severly limit the force-generating capacity of the supraspinatus when they are used as a bridging device in a simulated repair. I developed a novel approach to quantify volumetric distribution and spatial clustering of intramuscular fat to help elucidate the three-dimensional presentation which will ultimately assist in our understanding of how intramuscular fat limits force-generating capacity of muscle tissue. Using this approach, I detected that older adults with a rotator cuff tear have more fat that is more highly clustered in the distal portion of the supraspinatus compared to healthy older adult controls. The dissertation work provides a foundation by which clinicians can better identify those at risk for progression to disability and subsequently allow for earlier and more effective interventions.