Closed-Loop Control and Operant Conditioning of Loading Response during Locomotion After Stroke

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Individuals after stroke often struggle to use their stroke-affected leg to propel themselves during walking. Due to this partial disuse of the affected leg and in compensation to that, people walk slowly and with more effort. Despite advances in exoskeleton technologies and robotic gait rehabilitation in improving gait energetics and speed, it remains challenging to yield sustaining improvements in leg function. In people after stroke, the activation of lower leg muscles and generated forces are diminished, and the ankle joint motion that is critically important for walking is impaired. The goal of this project is to develop a novel gait training method to augment the function of the stroke-affected leg by targeting the ankle joint and its muscles to increase walking power. This project will use a wearable robotic device to impose small movement perturbations about the ankle to strategically activate stroke-affected ankle muscles during walking. A unique conditioning training protocol will be designed to increase the person’s ankle muscle activity and force to propel the body forward during walking. This project will advance fundamental knowledge to enhance the muscle power and propulsion, which is a key barrier to improving gait after stroke. Educational and outreach impact: middle and high school students and teachers with disabilities will have opportunities to be exposed to the wearables and gait-assistance technologies through hands-on lessons in a workshop setting. Broader outreach impact includes a public forum for sharing findings from this project and raising disability awareness with people at the local stroke support group. Undergraduate researchers from underrepresented groups and minorities will be encouraged to participate in these training and dissemination activities. This project seeks to achieve a paradigm shift in neuro-behavioral gait training by integrating a novel operant conditioning protocol and a robotic ankle device to enhance paretic leg function post-stroke. The innovation in this project is the development of a hybrid approach to target the soleus muscle activity within the stance phase of walking and condition its loading response as a strategy to improve hemiparetic walking. The research plan includes two objectives with testing in people post-stroke and able-bodied individuals. Objective 1 will characterize the input-output property of the ankle plantarflexors’ loading response in stance phase. An adaptive closed-loop control algorithm will be designed and evaluated to apply ankle joint rotations using the wearable robot. The joint perturbations will be shifted from the natural ankle kinematics to target the soleus loading response, i.e., activate load sensing Ib afferents, in mid-late stance phase. The closed-loop algorithm will be systematically investigated to customize the perturbation magnitude, speed, and timing for each participant. Changes in soleus electromyography (EMG), joint kinematics, and propulsion will be quantified between perturbed and unperturbed step cycles during one treadmill walking session in 15 participants post-stroke. Findings from this objective will provide insights into the methods to modulate the soleus loading response by manipulating ankle kinematics through applied, controlled joint perturbations using the wearable device (i.e., emulate imposed dorsiflexions during normal gait). Objective 2 will develop a dynamic protocol to enhance (operantly up-condition) the soleus loading response during treadmill walking. With operant conditioning of the EMG evoked response, participants will be rewarded for enhancing the soleus loading response through visual feedback. The conditioning protocol will be implemented in 8 able-bodied individuals to quantify soleus EMG, joint kinematics, and propulsion throughout 6 baseline and 24 conditioning sessions. Findings from this objective will help to advance the knowledge to target and enhance the excitability of Ib pathways to increasing soleus activity and propulsive force generation.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.