Remote facilitation of supraspinal motor excitability depends on the level of effort

Stretch reflexes and motor-evoked potentials (MEPs) of a muscle are facilitated when performing intensive contraction of muscles located in a different segment (remote effect). We investigated to what extent the remote effect on MEPs in the flexor carpi radialis (FCR) in humans is modulated during sustained maximal and submaximal voluntary contractions of the ipsilateral quadriceps (remote muscle). We found that even when the force of maximal voluntary contraction (MVC) of the remote muscle declined during sustained MVC, the magnitude of the remote effect on MEPs remained constant. Maximal electrical stimulation of the remote muscle and transcranial magnetic stimulation of the corresponding motor cortex revealed that the level of voluntary activation gradually decreased during the sustained MVC. The motor response in the FCR following magnetic stimulation at the level of the foramen magnum, which preferentially elicits muscle response as a direct response of the corticospinal tract, was not modified by the remote effect during the sustained MVC. This finding suggested that the excitability of the spinal motoneuron pool remained constant. In contrast to the sustained MVC, during sustained submaximal contraction of the remote muscle, the magnitude of the remote effect on MEPs gradually increased as muscle fatigue developed. These findings suggest that the remote effect on MEPs was dependent on the level of effort driving the remote muscle, but not on the actual level of force output of the remote muscle, and that the origin of the remote effect was supraspinal, putatively upstream of the primary motor cortex.