Dissertation Defense: Transients, Variability, Stability and Energy in Human Locomotion

Committee Members

• Dr. Manoj Srinivasan, Chair (MAE)
• Dr. Martin Golubitsky (Mathematics)
• Dr. Giorgio Rizzoni (MAE)
• Dr. Rama Krishna Yedavalli (MAE)
• Dr. Alison Sheets-Singer

Abstract

Most research in human locomotion is limited to steady-state, constant speed and symmetric locomotion behaviors. However, walking and running in everyday life requires us to adapt our locomotion strategies in response to intrinsic noise-like transients, external environmental irregularities and more general practical demands on our moving from place-to-place. Here, we investigate such adaptive locomotion behaviors in three separate but, broadly related studies: I. The dynamics of walking while changing speeds, II. control strategies for running stably in the presence of noise-like deviations and III. the dynamics of different types of asymmetric walking. In part I, we find that the metabolic cost of accelerating and decelerating when walking is significant can constitute 8 to 20% of our daily walking energy budget. Moreover, this increased metabolic cost predicts adaptations in the preferred walking speeds of people walking short distances. In part II, we reveal the adaptive control strategies hidden within the step-to-step variability in human running data, adopted by people in order to run stably in the presence of unavoidable and intrinsic sensorimotor errors. The control is implemented when the leg is on the ground and is well-predicted by deviations in the center-of-mass states during the previous flight phase. We show that people use almost-deadbeat control of horizontal velocity deviations; killing about 70-100% of horizontal velocity deviations within one step by appropriately modulating the placement of the foot and the force applied on the ground by the leg. Further, deviations in the center-of-mass motion predicts the swing foot placement before the swing foot itself. On implementing these strategies on a simple computational biped model, we find that they can withstand deviations up to ten times larger than the step-to-step variability from which they were inferred. This suggests that the control strategies that people use for small intrinsic errors can be extended to deal with larger external perturbations. In part III, we aim to predict the steady-state adaptation behaviors in people constrained to walk asymmetrically; the constraints are in the form of asymmetric masses, asymmetric belt speeds or asymmetric leg lengths. We find that optimization of metabolic energy of simple biped models predicts trends in the stance time asymmetry observed in experiments. For split-belt walking, the qualitative predictions made match with observed behavior over many days of adaptation to the new paradigm. Finally, we study overground asymmetric walking behavior in unilateral amputees wearing passive low-cost prosthetic legs. We find that, similar to able-bodied individuals, the trends in their overground preferred walking speed choices are predicted by the metabolic cost of the movement.