An energy audit of human gait: experimental and computational assessment of how structure and function shape walking economy

The interaction between musculoskeletal structure and function provides a foundation for understanding habitual movement and changes with aging. Older adults often walk with reduced propulsive force and lower tendon stiffness than younger adults, which likely reduce walking performance and increase metabolic costs among the aging population. The goal of my dissertation is to determine how altering structure (tendon stiffness) and function (propulsive force) shapes walking economy in the context of aging. I first test interdependency between propulsive force and walking speed by combining fixed-speed and self-paced treadmill walking with biofeedback to independently prescribe propulsive force separately from walking speed. I show that propulsive force is a clear determinant of walking speed, solidifying interdependent relations between these gait performance metrics, and building confidence that solutions to augment propulsive force should also increase walking speed. Next, I determine metabolic effects of altering propulsive force and tendon stiffness during walking. I prescribe propulsive force changes via biofeedback, alter tendon stiffness via musculoskeletal models, and estimate metabolic costs using bioenergetic simulations. My simulations suggest propulsive force and tendon stiffness independently affect walking metabolic cost and augmenting either could improve walking economy. My results also imply older adults do not reduce propulsive force to mitigate effects from lower tendon stiffness.Fortunately, engineers have developed ankle exoskeletons with potential to augment both structural and functional changes from aging. I assess exoskeleton ability to augment Achilles tendon stiffness, improve push-off intensity, and reduce walking metabolic cost. I found that passive elastic exoskeletons reduce older adult calf muscle metabolic costs but increase costs for a younger adult. Finally, I demonstrate a proof-of-concept to estimate walking metabolic cost using motion data from standard video recordings. I build upon this technological innovation by benchmarking against other local (muscle activity) & global (heart rate) measures of energetic demand. Altogether, I broadly audit the energy cost of walking by identifying impacts from multiple factors associated with aging: walking speed, propulsive force, and tendon stiffness. I show how ankle exoskeletons may address some of these factors and I showcase a new technology to estimate walking metabolic cost rapidly and at scale.