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Enhancing human metabolic economy in stair climbing via an elastic crutch mechanism

Crutching provides a significant increase in mobility for those with limited walking ability. While level ground walking with crutches has been studied in many different forms, stair climbing is a more energetically taxing activity and the upper arm and shoulder strength required is not always available in weaker or severely injured patients. We posit that the introduction of parallel springs spanning the elbow joint will improve the crutching experience by helping patients attain a metabolic reduction compared to unassisted locomotion. Here, we present a foundation for achieving metabolic reduction with joint-spanning elastic elements. Our approach includes three parts. First, we present an augmented crutch design with an elbow spring that can be modified with different stiffnesses. Second, we put forth a clinical testing protocol in which we measure metabolic economy via the pulmonary gas exchange technique (Vo2avg). Simultaneously recording electromyographic (EMG) signals from the primary active muscles provides a neuromuscular interpretation of the crutching activity not captured by the black-box metabolic techniques. We complete the picture by modeling the energetics of the effective elbow muscle by incorporating empirical measurements of maximum angular velocity achieved under a range of isotonic conditions. The metabolic data exhibits trends consistent with our hypothesis of metabolic reduction; although, more subjects are needed to confirm these results. All subjects reported a feeling of augmentation at the optimal stiffness condition. An analysis of the EMG results show a clear transition in muscle usage patterns from a triceps only power stroke to a combined usage of both triceps and biceps. Where the triceps are maximally active during the non-augmented state, as stiffness increases the biceps become more active and the total activation level drops, suggesting the this shift is at least partially responsible for the observed metabolic reduction. While the model correctly predicts the relative shape of the observed curve, the optimal stiffness predictions are higher than their empirical equivalents. This is most likely due to the extra help the triceps muscles received from active stabilization and power muscles not considered in the model. With a more complete muscular picture one could begin to construct an accurate method of prediction and tuning of optimal stiffness. Thesis Supervisor: Hugh Herr Title: Associate Professor of Health Sciences and Technology Associate Professor of Media Arts and Sciences Thesis Reader: Daniel Frey Title: Associate Professor of Mechanical Engineering Associate Professor of Engineering Systems Acknowledgments There are many people who have supported me in the processes of producing this work. I would like to express my sincere thanks to: My thesis advisor, Hugh Herr, for giving me the freedom to find my own way and for his insightful guidance and direction. My fellow members of the Biomechatronics Group who have been my academic peers and companions these past two years. Most especially, I would like to thank Grant Elliot for many deep discussions on life, the universe, and everything and for his willingness to dedicate his brilliance to others so readily; Andy Marecki for his help with all things mechanical; Ernesto Martinez for his motivational talks, advice on metabolic analysis, and fabulous photography skills; Jared Markowitz for many unprecedented consultations on biomechanics and his hilarious jokes that kept me entertained at all hours; Michael Eilenberg for helping me work out the kinks in my experimental design and data processing steps and for his consistently cheerful demenor; Jing Wang for her companionship and laughter which always kept me in lab longer than I expected; Bruce Deffenbaugh for sharing his intuition and experience on approaching all types of problems; Sarah Hunter for her common sense, organization, and ability to make the impossible possible; and all the undergraduate students I have worked with who have done everything I have asked and consistently exceed my expectations in every way. My subjects who exhibited extreme patience for many hours while I collected their data. Bob Emerson and his staff who helped design and manufacture the crutch cuffs and provided much needed insight on how best to attach to the human form. Daniel Frey, for graciously offering to read my thesis. Daniel Montana and Xiao-Yu Fu for many late night conversations, last minute revisions, and words of encouragement. My friends both at MIT and elsewhere who have kept me sane and cheerful though thick and thin. Finally, I'd like to thank my parents and sister for putting up with my ridiculous ideas and somehow still encouraging me to pursue my goals. I am forever indebted to your love, unwavering support, generosity and commitment to my well-being.