Age-related Differences in Shoulder Strength and Muscle Coordination


Everyday tasks, such as putting on a jacket or reaching up to a shelf, often become more challenging for older adults. These tasks rely on the ability to generate three-dimensional torques about the shoulder and to adapt these torques across activities. Shoulder strength and muscle coordination impact shoulder torque production, yet how these factors change with age is not well understood. Shoulder strength and muscle activity are typically measured during planar motions that mimic clinical assessments but describe only a small portion of the directions used in daily activities. In this thesis, I have addressed these limitations by quantifying shoulder strength and muscle coordination during three-dimensional torque production. Three major contributions have come from this work. First, I developed new tools for quantifying three-dimensional shoulder strength that are sensitive to the changes that occur with aging. I then used these tools to quantify age-specific differences in shoulder strength and found that from 20 to 80 years old strength decreased by an average of 39% across participants. In addition to decreased overall magnitude, I identified the directions in which older adults were significantly weaker, suggesting potential targets for training and rehabilitation. Finally, I evaluated how changes in neural control might contribute to these differences in strength by quantifying muscle coordination. Interestingly, I found that while younger adults typically activate shoulder muscles in a preferred direction, presumably where the torque production is greatest, older adults activate muscles in a broader range of directions. This breadth of activation is consistent with co-contraction that may serve to stabilize the shoulder but would also limit the maximum torques that can be produced. Together, these results highlight the importance of monitoring changes in shoulder strength and coordination in older adults to identify, address, and ultimately reduce the impact of these differences on daily activities.

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