A session of transcutaneous electrical nerve stimulation changes the input-output function of motoneurons and alters the sense of force

Transcutaneous electrical nerve stimulation (TENS) is commonly used in research and clinical settings for pain management and augmenting somatosensory inputs for motor recovery. Besides its functional effect, TENS acutely alters kinesthesia and force steadiness. However, the short-term impact following a session of TENS on proprioception and motor unit behavior is unknown. We evaluated the effect of a session of TENS on the senses of force, joint position, touch, and discharge activity of motor units. Fifteen healthy participants underwent two experiments, each with two visits randomly administering TENS or sham-TENS. The sense of force (Exp. 1) and position (Exp. 2) were evaluated through matching trials by pinching a dial and rotating their wrist (ulnar deviation). Isometric pinch contractions were performed before and after the session of TENS or sham-TENS, in which electromyographic signals were recorded from the first dorsal interosseus (FDI) and abductor pollicis brevis (APB). Results showed that TENS acutely altered the senses of force, position, and touch, but only the sense of force remained altered following TENS. Motor unit discharge rates increased in both FDI and APB muscles for the same force output following TENS. A positive correlation was also observed between motor unit discharge rates and errors in force perception. These findings suggest that a session of TENS may have short-term effects on the input/output function of motoneurons (5 - 10 min in this study), which in turn may alter the sense of force. However, the precise timeline for these short-term aftereffects is unknown.

Methods
The data was collected using a custom built experimental setup that integrated EMG acquisition systems and custom softwares to interact and record relevant data from the participants. Following data collection, all data from different participants and visits were preprocessed and compiled to a uniform dataset for post-processing. Post-processing details have been detailed in the associated manuscript