Simplified Assistance at the Center of Mass During Human Locomotion

Book excerpt: The field of exoskeletons has undergone an evolution from complex full-body exoskeletons that did not (yet) produce the expected results towards simpler single-joint exoskeletons that can improve the mobility of people. While full-body and single-joint exoskeletons certainly have appropriate applications, we need to get a better understanding of the distal and proximal assistive mechanisms and provide insights that are currently lacking on how to assist walking in an even simpler way than single-joint exoskeletons. This dissertation details an iterative approach toward the development of simplified and efficient assistance strategies for improving human locomotion. We first conducted an experiment to observe the human response to proximal and distal perturbations by altering the treadmill grade and footwear inclination. The results indicate that the metabolic rate is predominantly sensitive to changes in the center of mass (COM) mechanics and further motivate the development of devices that can assist walking at the level of the COM. We then developed a robotic tether system that allows applying desired cyclic force profiles as a function of step time to provide whole-body assistance during walking. By leveraging the system capabilities, we performed an experiment and simple pendulum simulation to investigate the effects of timing and magnitude for non-constant force profiles at the COM. Through these experiments and the simulation, we found that assistance at the COM during the double stance phase can efficiently reduce the metabolic rate of walking half. Surprisingly, assisting propulsion did not maximize the reduction in metabolic rate, and our pendulum model revealed that the reduction in metabolic rate can instead be explained by the assistance of COM acceleration at the beginning of the step. Ultimately, our long term goal is to develop similar strategies to populations with gait disabilities, but as a primary step, we investigated the biomechanical mechanisms to assist lower limb joints using timed forward forces at the COM. To that end, we assessed the underlying mechanisms of muscle and joint parameters that explain the effects of timing and magnitude of horizontal forces at the COM on metabolic rate. The results show that the metabolically optimal timing assisted the ankle muscles that are responsible for push-off, and the knee and hip muscles that are responsible for collision. Based on these findings, it seems possible to assist different joints by different amounts by varying the timing of forces at the COM. This could be useful in clinical populations for providing ‘targeted’ joint-specific assistance without having to switch between different exoskeletons. We expect our experimental findings to provide knowledge on optimal force profiles that could be used for treadmill exercise therapy, motorized ‘rollator’-style assistive devices for walking, and even it could even inspire new strategies for combined actions of the ankle, knee, and hip of full-body exoskeletons. Timed forces at the COM could be used to assist patients with impaired gait and facilitate proactive user participation that has been identified as a critical factor in improving locomotor outcomes for rehabilitation robotics.