Validation of Vibrotactile Feedback to Improve Selective Motor Units Recruitment

Recently, muscle interfaces have been used to control external devices through the activation of single motor units. In the present study, we proposed and validated two different vibro-tactile feedback strategies designed to convey information about the firing rate of two motor units active at the same time. In a two-alternative forced-choice task, participants had to discriminate the higher vibration frequency among the ones of two vibrotactile stimulators placed on their arms. Spike strategy and continuous strategy were tested in two different experiments and motor units’ activation was simulated. The spike strategy directly translates the discharge activity of motor units into a vibratory burst with a 1-to-1 conversion, i.e. each spike of one motor unit triggers a single burst of the corresponding vibrator. This was evaluated in two body configurations, i.e. same forearm versus different arms. The continuous strategy mapped the discharge activity of motor units into a continuous vibration exploiting the entire operative range of vibrotactile stimulators. A single-body configuration was tested (i.e. two different arms). Participants’ responses were fitted with a psychometric sigmoid curve (i.e. a psychometric model commonly applied to detection and discrimination tasks), and the discrimination accuracy index was used to evaluate the feedback strategies. Results from Experiment 1 showed that the continuous strategy worked better when the stimulators were placed on two different arms, but overall discrimination performance was poor. Experiment 2 showed that both the continuous strategy conveyed vastly superior overall performance compared to the continuous strategy.