The myokinetic stimulation interface: activation of proprioceptive neural responses with remotely actuated magnets implanted in rodent forelimb muscles

Objective: Proprioception is the sense of one's position, orientation, and movement in space, and it is of fundamental importance for motor control. When proprioception is impaired or absent, motor execution becomes error-prone, leading to poorly coordinated movements. The kinaesthetic illusion, which creates perceptions of limb movement in humans through non-invasively applying vibrations to muscles or tendons, provides an avenue for studying and restoring the sense of joint movement (kinaesthesia). This technique, however, leaves ambiguity between proprioceptive percepts that arise from muscles versus those that arise from skin receptors. Here we propose the concept of a stimulation system to activate kinaesthesia through the untethered application of localized vibration through implanted magnets. Approach: In this proof-of-concept study, we use two simplified 1-DoF systems to show the feasibility of eliciting muscle-sensory responses in an animal model across multiple frequencies, including those that activate the kinaesthetic illusion (70 - 115 Hz). Furthermore, we generalized the concept by developing a 5-DoF prototype system capable of generating directional, frequency-selective vibrations with desired displacement profiles. Main results: In-vivo tests with the 1-DoF systems demonstrated the feasibility to elicit muscle sensory neural responses in the median nerve of an animal model. Instead, in-vitro tests with the 5-DoF prototype demonstrated high accuracy in producing directional and frequency selective vibrations along different magnet axes. Significance: These results provide evidence for a new technique that interacts with the native neuro-muscular anatomy to study proprioception and eventually pave the way towards the development of advanced limb prostheses or assistive devices for the sensory impaired.