Collaborative Research: Use of Wearable Sensors to Track Muscle-Tendon Loading during Exosuit Assisted Locomotion

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Advances in exosuit technologies are enabling the use of powered assistance to enhance walking performance in healthy individuals and assist individuals who exhibit gait pathologies, e.g. individuals with stroke. Unlike rigid exoskeletons, exosuits are lightweight and use soft materials that provide a comfortable and unobtrusive fit with the body. A pack worn at the waist uses battery-powered motors to generate forces that are transmitted to the ankle and hip. In spite of demonstrated success in decreasing the energy needed to walk, it remains challenging to tune exosuit assistance patterns for individual users. Thus, the long-term goal of this work is to enable individualized assistance that can adapt in real time to a user’s unique gait patterns and to the environment. To do this, novel sensors, termed shear wave tensiometers, will be used to track adaptations in knee and ankle muscle loading that arise when assistance is provided by a powered ankle exosuit. Studies will be performed to determine how different exosuit assistance control patterns modulate internal muscle loading under varied walking conditions, including with and without exosuit assistance, with and without carrying a load (backpack) and walking in an outdoor/real-world environment (declines, inclines and variable walking speeds.) Theses studies will enhance the fundamental understanding of neuromuscular responses to exosuit assistance and thus enable human-in-the-loop implementations that adapt assistance based on the needs of an individual. Educational and outreach impact will be achieved by using fundamentals underlying the robotic, biomechanics and sensor technologies developed in this project as a platform for engaging K-12 students in STEM. Simplified versions of the exosuits and sensors will be incorporated into the annual engineering outreach event at the University of Wisconsin-Madison which reaches thousands of K-12 students and their teachers every year. Also, the Soft Robotics Toolkit hosted by Harvard will be used to create engaging content that describes human-machine interaction, biomechanics, physiology and gait.The goal of this project is to use novel tissue load sensors, termed shear wave tensiometers, to investigate biomechanical adaptations to exosuit assistance within and beyond the laboratory environment. Though current exosuit technologies have been shown to lower the metabolic cost of walking in healthy subjects and improve propulsion, ground clearance, and symmetry in stroke survivors, the extent of these benefits varies widely across subjects. The project builds on a new collaboration between the lab that invented the tensiometer method to directly gauge tendon loading by measuring the propagation speed of shear waves along the tendon’s axis (University of Wisconsin-Madison) and a lab that is recognized for leadership in developing the next generation of soft exosuits (Harvard.) The Research Plan is organized under three aims, with each aim being evaluated in 10 human subjects. The FIRST Aim is to develop and incorporate a wearable shear wave tensiometer into an ankle exosuit to continuously monitor Achilles tendon loading during prolonged treadmill walking trials. Ankle joint torque determined from tensiometer measurements will be compared to measurements based on motion capture. The result of this aim will be a validated wearable sensor for quantifying changes in tendon tissue loading induced by exosuit assistance. The SECOND Aim is to evaluate relationship between ankle exosuit assistance magnitude and change in muscle-tendon loading during walking with and without added mass. Results of this aim will provide novel insights into the relationship between exosuit assistance and biological soft tissue loads, with expectations that the tested variables (exosuit force, exosuit timing, and added mass) will have various effects on the user’s muscle-tendon load. The THIRD Aim is to evaluate the effect of ankle exosuit assistance on muscle-tendon loading while walking in an outdoor circuit that includes inclines, declines, comfortable speed and fast walking. Measurements include tendon loading with the mobile tensiometer, muscle kinematics with ultrasound, and biomechanics and suit data with suit IMUs and load cells. Results of this aim are expected to demonstrate that, similar to how a motion capture and force plate setup in a lab environment allows for estimating joint moments, the suit sensors and tensiometer will allow for evaluating joint kinematics and tendon kinetics in outdoor environments. Success of the project is expected to lead to a new design of exosuits with integrated tensiometer sensors, produce new understanding of biomechanics with exosuits, and potentially inform optimization of personalized wearable exoskeletons for clinical and/or aged populations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.