Yank-related changes in corticospinal recruitment gain in humans

The generation of muscle force is a key component of movement. Force is generated by increasing the number and firing rates of motor units (MU), but limited information is available on the corticospinal (CS) recruitment gain across the spectrum of magnitude and rate of force (yank). To assess the effects of force magnitude and yank on motor outputs, we recorded force (motor evoked twitches, METs) and EMG (motor evoked potentials, MEPs) in response to transcranial magnetic stimulation (TMS) during nearly isometric hand gripping tasks in 20 healthy subjects. Motor cortex TMS was applied during force ramps at different levels of voluntary force magnitude and yank. Yank robustly facilitated METs, maximally at lowest magnitudes of voluntary force, but this facilitation saturated at forces > 50 % of the maximal voluntary contraction (MVC). At each level of force magnitude there was a linear relationship between the yank of voluntary force and MET amplitude. The slope of this relationship, representing the CS recruitment gain, decayed exponentially with increasing magnitude of force. Changes in CS recruitment gain associated with force magnitude and yank were better explained by METs than by MEPs. A positive yank resulted in larger estimated maximal MET at 0 % force compared to static contractions. These results suggest that METs capture a robust yank-related increase in CS recruitment gain, which is blunted by limited MU availability at high force magnitude. We showed that the estimation of maximal motor outputs is optimally achieved in tasks in which force production is achieved with high yank.