Development of effective hip-knee-ankle exoskeleton assistance for different walking conditions
Keywords: Mechanical Engineering.

Lower-limb exoskeletons could assist people in a variety of locomotor activities such as walking, running, jumping, and carrying loads. These devices could be beneficial to first responders, military personnel, laborers, and in the future, may be able to assist patient populations and older adults. Recent successful strategies for able-bodied individuals have reduced the metabolic cost of walking by up to 24% when assisting the hips, ankles or both joints. However, there has been limited exploration into simultaneous assistance at the hips, knees, and ankles which may lead to the greatest metabolic reductions of any joint configuration. It is currently unclear how to effectively assist the whole leg as well as how that effective assistance should vary with gait condition. In my doctoral research, I developed a bilateral lower-limb exoskeleton emulator and used it to optimize hip-knee-ankle exoskeleton assistance in a variety of gait conditions. This device is a flexible research testbed that can quickly apply a wide variety of assistance strategies by simply updating the device controller. We used the bilateral lower-limb exoskeleton emulator to optimize hip-knee-ankle exoskeleton assistance through human-in-the-loop optimization, a strategy that adjusts exoskeleton assistance in real time using online user performance measurements. In the first optimization study, we optimized exoskeleton assistance to minimize metabolic cost at slow (1.0 m/s), medium (1.25 m/s) and fast (1.5 m/s) walking speeds. Exoskeleton assistance reduced the metabolic cost of walking relative to walking in the device without assistance by 26% for slow walking, 47% for medium-speed walking, and 50% for fast walking. In the second study, we optimized exoskeleton assistance to minimize the metabolic cost of walking with no load, a light load (15% of user body weight), and a heavy load (30% of user body weight). The weight was applied through a weight vest. Exoskeleton assistance reduced the metabolic cost of walking by 48% with no load, 36% with the light load, and 43% with the heavy load. The results of these studies show that hip-knee-ankle exoskeleton assistance can substantially decrease the metabolic cost of walking at a variety of speeds and with different worn loads. The results from these studies could inform the design of future exoskeleton products and influence future exoskeleton experiments. Exoskeleton products could use the optimized torque profiles found here to dictate needed device capabilities, and the metabolic reductions from these studies provide a benchmark for expected performance. Future exoskeleton experiments could use the optimized torque profiles as a starting point to investigate useful exoskeleton assistance in novel gait conditions, during non-steady state walking, and with patient populations or older adults. While optimizing exoskeleton assistance to reduce metabolic cost was effective for able-bodied adults, it may be beneficial to investigate alternative performance metrics for patient populations like increasing self-selected walking speed or enhancing balance. The results of these studies could inform effective exoskeleton assistance for future products and studies for years to come