Haptic interaction with a human partner for ankle training in chronic stroke: a pilot study

Background: Sensorimotor impairments following stroke frequently result in diminished voluntary control of the ankle, contributing to deficits in balance and gait. Robotic training paradigms targeting ankle motor control often use an assist-as-needed strategy, where compliant guidance is provided to assist movements towards a target trajectory. However, interaction with "perfect" reference trajectories may overly constrain movements during training and has been shown to limit learning in many upper-limb contexts; alternatives to robotic assistance have rarely been explored for post-stroke ankle training. Inspired by human-robot-human interaction studies, we investigated whether physical interaction with a therapist-termed human interaction-offers advantages over traditional trajectory guidance regarding short-term learning. Methods: In a within-subject design, nine individuals with chronic stroke (61.6 +/- 14.3 years) performed a 1-DoF visuomotor tracking task while wearing ankle robots designed to train dorsiflexion and plantarflexion movements. Two robotic training methods were evaluated in separate visits: (1) compliant connection to a sinusoidal target trajectory (i.e., trajectory guidance) and (2) compliant connection to a physical therapist who tracked the same target trajectory (i.e., human interaction). In each visit, tracking performance (i.e., errors, movement smoothness) and muscle activation were evaluated during and immediately after training. Results: Both strategies improved tracking accuracy and movement smoothness during training, however there were trends of better tracking performance with trajectory guidance compared to human interaction. In particular, random error was more significantly suppressed during trajectory guidance training. Importantly, we observed that human interaction led to a trend of improvements in unassisted tracking performance and significantly higher dorsiflexor activation immediately after training. Conclusion: Our results suggest that human interaction has the potential to facilitate improved learning during ankle training in chronic stroke participants, likely by providing assistance without over-constraining an individual's movement smoothness or variability. Training while physically interacting with a partner may serve as an effective alternative to conventional robot-guided therapy for post-stroke ankle rehabilitation, though further studies are needed to assess the generalization of this approach regarding long-term retention and functional improvements. Registry: clinicaltrials.gov, TRN: NCT04578665, Registration date: 8 October 2020.