Biomechanics, Control, and Energetics of Human Locomotion in the Real World

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Humans contend with a variety of walking surfaces in the real world, including unpaved ground, street curbs, and imperfect sidewalks. However, much of what is known about the biomechanics, control, and energetics of locomotion is confined to flat and level surfaces. But in real-world terrain, it is also difficult to gain quantitative, biomechanical data without the well-controlled conditions of the laboratory. These gaps need to be addressed through a combination of new techniques expressly designed for the real world, to be applied to well-controlled laboratory studies that better model such environments, and to real-world studies that better quantify how humans actually adapt to different ground surfaces. This program bridges the gap from both directions: the laboratory and the real world. In the laboratory, we will conduct studies adapting conventional biomechanics techniques to uneven terrain. This will be enabled by new methods and experiments. For example, we will use an instrumented treadmill (measuring ground reaction forces under each leg), adapted to allow for changeable surfaces such as uneven terrain, varying in amplitude or compliance. In conjunction with motion capture, this will enable full biomechanical characterization of the effect of terrain amplitude on overall steady-state gait, gait variability, and energetic cost. We will also test how biomechanical task demands vary with terrain, and examine the mechanistic basis for increased metabolic cost. We will also examine locomotion in a variety of real-world environments, using improved biomechanical capture methods. Most wearable sensors provide relatively simple measures such as step timing, but cannot obtain the fine-grained information obtained in the laboratory. We have developed new algorithms to use shoe-mounted inertial measurement units to reconstruct each foot's trajectory in space, from which measures such as stride length and variability, or lifting of the foot, can be obtained with accuracy comparable to laboratory. These methods will characterize how humans adapt their gait to challenging surfaces such as loose gravel or sand on an incline. We will also couple these biomechanical measures with oxygen respirometry to assess metabolic cost. The state of the art in energy cost predictions for uneven terrain is to use regression equations factoring in walking speed, load carriage, and a crude and subjective index of terrain difficulty. With better understanding of how mechanical and metabolic variables co-vary, it will be possible to characterize terrain difficulty more objectively and with better predictability for energy cost. We will also explore how gait changes over long distances, to determine how balance and economy are affected by fatigue. These studies will improve both the capabilities for studying locomotion, as well as understanding of the human adaptations on uneven terrain.