Form and Function Relationships Governing Complex Muscle-Soft Tissue Interactions Revealed with 3D Modeling: Applications to Aging

The universal experience of aging presents many clinical problems rooted in muscle dysfunction. Muscles actuate movement required for daily life and understanding muscle function prior to the onset of age-related changes is critical in addressing these problems. Muscle function depends on both its fiber architecture (the arrangement of its cells) and its interaction with elastic connective tissues. When muscles have complex architectures and connective tissue interfaces it is difficult to intuit the full relationship between structure and function. As a result, questions about how age-related structural changes alter movement remain unanswered. Physics-based computational modeling allows us to represent complex three-dimensional (3D) muscle and connective tissue in order to relate tissue morphology and material properties to biomechanical function. In my dissertation, I have developed finite element models to investigate unique examples of architecturally complex muscle-driven systems that experience significant mechanical dysfunction with age.
I first studied the eye to examine muscle-related vision impairment. The human eye is capable of accommodation, where optical range is adapted for distant and near vision, however, this ability progressively declines with age. Accommodation relies on deformation of the lens surfaces which occurs as tension on the lens is modified during contraction of the ciliary muscle. With tri-sectioned fiber architecture, the ciliary muscle pulls against outer layers of the eye while interfacing with lens through a network of fibers. This complex mechanism is difficult to probe experimentally, so the biomechanics of this process are still unclear. I have developed a model of the eye’s accommodative mechanism to reveal how action of the multi-section ciliary muscle deforms the lens. I then used this model to predict how age-related changes in mechanical properties of different tissues reduce accommodative capacity.
Next, I focused on mobility by investigating the complex triceps surae group which generates plantarflexion power at the ankle during walking. Reduction of this power is the most universal hallmark of elderly gait impairment. I compared imaged-based measurements of triceps surae muscles and Achilles tendons with walking kinetics of young and older adults to understand structure-function changes with age. To elucidate how anatomic variability impacts the mechanics of the Achilles tendon, I created models with different twisted morphologies to predict effects on loading during walking. Finally, I created a model of the soleus, the largest of the triceps surae, to reveal how the morphology and material properties of the interdigitating connective tissue structures within this muscle influence architecture changes in its multiple compartments as it lengthens.
This dissertation advances our knowledge of muscle-driven production of movement by illuminating relationships between the form and function of these complex tissues, thus progressing understanding of vision and mobility impairments that occur with aging.