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.