Zoom Link: https://gatech.zoom.us/j/5374686038

Advisor: 

Greg Sawicki, Ph.D. (Georgia Institute of Technology) 

Thesis Committee: 

Young-Hui Chang, Ph.D. (Georgia Institute of Technology) 

Young Jang, Ph.D. (Georgia Institute of Technology) 

Sabrina Lee, Ph.D. (Simon Fraser University) 

Rich Mahoney, Ph.D. (Intuitive) 

Interaction of ankle exoskeleton assistance with age-related changes in physiology to reduce metabolic cost of walking

Difficulties with mobility were the most commonly reported disability for those age 65 and over. It is well known that older adults are slower and less economical during walking compared to young. This is thought to be brought on by reduced ankle push off power and a redistribution of positive power generation to more proximal joints (e.g., hip). Ankle exoskeletons have been shown to increase ankle push off, increase self-selected speed, and reduce metabolic cost in young adults for a near immediate improvement in walking performance. There is a critical gap in understanding whether beneficial exoskeleton assistance strategies for younger adults will also benefit older adults and if so, what the underlying mechanism is that enables exoskeletons to reduce metabolic cost across age.

Older adults have more compliant tendons than young, or a less stiff spring, operate with shorter less optimal muscle lengths, and exhibit reduced push-off power leading to a loss of the ‘spring in their step’. This necessitates higher muscle activations and reliance on muscles at less efficient joints like the hips, increasing metabolic cost during walking. Passive ankle exoskeletons have been shown in younger adults to lower the demand at the ankle, optimize complicated muscle-tendon dynamics during stance, and reduce metabolic cost. Muscle level changes in young adults in response to ankle exoskeletons to reduce metabolic cost led to wondering how ankle exoskeletons interact with age-related changes in physiology to reduce metabolic cost. The near-term objective of my work was to evaluate the calf muscles and tendon’s role in modifying metabolic cost during walking with passive, and active ankle exoskeletons across age. My central hypothesis was ankle exoskeletons can offset age-related changes in physiology to reduce metabolic cost to that of young walking economy.

I used electromyography to measure muscle activity, B-mode ultrasound to track muscle level changes, and a portable indirect calorimetry system to measure metabolic cost in young and older adults with passive and active exoskeleton conditions. These aims yielded a greater understanding of how people interact with ankle exoskeletons to modify metabolic cost. These outcomes can improve the design and control of ankle exoskeletons to improve the cost of walking across age, leading to greater mobility and increased quality of life. The aims also clarified whether passive or active control is best for young or older adults. Passive devices are lighter weight, require less maintenance, and are easier to conceal but they are less tunable and have shown lower reductions in metabolic cost. Active devices can be optimized for each person and provide more assistance at any timepoint in the gait cycle. However, motors and batteries make a lightweight device difficult to create and complicates usage with maintenance, battery life, bulkiness, and noise. Completion of the aims, pave the way for studies in more functional measures such as increasing self-selected walking speed, improving balance, and reducing fatigue that may translate more directly to improved quality of life.