Date of Award


Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program


First Advisor

Graham E. Caldwell

Second Advisor

Richard E. A. Van Emmerik

Third Advisor

Brian Umberger

Subject Categories

Biomedical Engineering and Bioengineering


The goal of this dissertation was to understand the role of age-related changes in muscle mechanical properties in the control of upright posture in humans. First, a methodology for estimating subject-specific muscle properties in healthy young and older individuals was developed. Magnetic resonance and ultrasound imaging were used in conjunction with dynamometer experiments, musculoskeletal modeling, and numerical optimization to estimate the properties of the dorsiflexor and individual plantarflexor (gastrocnemius and soleus) muscles for 12 young and 12 older adults (balanced for gender). With aging there were declines in maximal isometric strength and increases in series-elastic stiffness in the male subjects, but no differences in the female subjects. Regardless of gender, there were age-related changes in the shape of the force-velocity relation, such that the older subjects produced less relative force during both concentric and eccentric muscle contractions. The second study tested the balance abilities of the same subjects under a variety of static (quiet stance, leaning forward/backward) and dynamic (swaying at preferred/imposed frequencies, maximal reaching, external perturbation) conditions. The older adults performed more poorly on most of the balance tasks. While maximal isometric force made a smaller than expected contribution to predicting balancing ability, the force-length, force-velocity and force-extension properties of the muscles were all predictive of the age-related declines in balance control, explaining ~40% of the variance as independent predictors and ~50% when these factors were combined. Finally, a feedback-driven inverted pendulum model of postural control was developed, which incorporated realistic representations of young and old dorsiflexor and individual plantarflexor muscles using the previously estimated mechanical properties. A sensitivity analysis was performed by manipulating the properties of the plantarflexor muscles. The balancing ability of the model was most influenced by the optimal length of the contractile component and the slack length of the series elastic component of the plantarflexor muscle models. The quiet stance model highlighted the importance of the force-length relation of muscle to the stabilization of upright posture. This dissertation demonstrated that there are age-related changes in the dorsi- and plantarflexor mechanical properties, and these changes are associated with the declines in postural control that accompany aging.