Mechanisms of Stability and Energy Expenditure in Human Locomotion.

Abstract

Although humans normally walk with both stability and energy economy, either feature may be challenging for persons with disabilities. For example, in patients with lower-limb amputation, falling is pervasive, and may lead to activity avoidance. Similarly, energy expenditure is higher than for healthy subjects and may deter patients from walking, reducing mobility. A better understanding of the fundamental principles of stability and economy could lead to better prostheses that increase quality of life for patients. When designing a mechanism to assist or mimic human gait, such as orthoses or walking robots, the stability and economy of the resulting gait should be considered. To further our understanding of these fundamental principles of gait, I explore a lesser known balance mechanism, foot heading, as well as the role of muscle force production costs in gait. To investigate the stabilizing role of foot heading, I first characterize a method of measuring natural human gait variability outside of lab environments using foot mounted inertial sensors. Accuracy is found comparable to motion capture, while allowing capture of gait in natural environments. Then, using both a simple model of walking, and a variability analysis of human walking, I present evidence that humans stabilize gait laterally by altering foot heading step-to-step. I then consider the metabolic cost of force production in human locomotion. First, an optimization study of a simple model of locomotion shows that force fluctuation costs have a stronger role in determining gait than force amplitude costs. I then illustrate the connection between force fluctuation and a cost for calcium pumping in muscles using a simple muscle model. Finally, a human subject experiment altering force fluctuation in walking demonstrates the higher metabolic cost of fluctuating forces. While human locomotion is a complex activity involving many muscles, sensory systems, and neural circuitry, we can use basic mechanical models to study underlying principles of gait. A better understanding of stability and economy could have applications to many fields involving locomotion, such as the diagnosis of fall-risk in elderly subjects, the development of rehabilitation techniques, the design of prostheses, and the creation of robust and practical walking machines.