Fig. 3.

A sample of a force trajectory in the finger-pressing task using the index and middle fingers. Virtual finger forces represent the sum of the index and middle finger forces. The target force is determined using 90% and 50% of a maximum voluntary contraction in the virtual finger. The reduction threshold is a reference value calculated by subtracting three standard deviations from the mean force during Phase 1. Phase 1 is the baseline period wherein the participants matched the virtual finger force to the target force before transcutaneous medium-frequency alternating current (tMFAC) stimulation was delivered. Phase 2 is the period from onset of tMFAC stimulation to the point that the virtual finger force recovers to reach the reduction threshold. Phase 3 is the period in which virtual finger force has completely recovered. In Phase 2, t1 and t2 were defined as onset and offset times for blockade of motor neuron signals, respectively. For instance, t1 represents the time from tMFAC stimulation onset to the time when virtual finger force falls below the reduction threshold. Also, t2 represents the time from tMFAC stimulation cessation to the time when virtual finger force begins to increase