Instrumented crutches to measure the internal forces acting on upper limbs in powered exoskeleton users

Powered exoskeletons have proven to be a reliable solution to provide mobility to persons with spinal cord injury (SCI), but how users can learn to manage these devices is still an open issue. The training required to learn how to use an exoskeleton for assisted walking is currently based on a subjective evaluation of gait by the patient and his therapist, with a trial and error approach that increases the time and effort required to reach an efficient walking rate. To provide therapists information on the training progress, as well as on upper limb involvement in assisted gait, a measurement system was developed, by instrumenting Lofstrand crutches and integrating them with the sensing framework commonly found in gait analysis laboratories. Each crutch is composed of three strain-gauge bridges for measuring axial and shear forces, a conditioning circuit with transmission modules, a tri-axial accelerometer, and a power management circuit with two batteries. Data are transmitted wirelessly via Bluetooth to a personal computer, avoiding any interference with the user's gait. To estimate upper limbs' internal forces, an inverse dynamics analysis of the measurement results was performed on a biomechanical model developed by the authors. Internal forces acting on shoulders, elbow and neck were computed, to assess loads on the joints, as well as torques acting on the system degrees of freedom, due to the muscular activity of the subject. The presented approach was applied to Rewalk
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users at different stages of their training, and with different walking performances. The paper presents the description of the measurement system, as well as an example of the results of these case studies, showing how this solution could be used to quantify important parameters to guide the user training during assisted gait.