Direct corticospinal control of force derivative.

During simultaneous generation of static and dynamic forces, motor cortical signals only predict the dynamic components, suggesting a key role in the coding of force changes. However, such a role is obscured by uncertainties regarding the representation of dynamic force signals in corticospinal outputs. We used transcranial magnetic stimulation (TMS) of the motor cortex in humans during a task that dissociated the direction of instantaneous net force and that of force derivative. The direction of TMS-evoked force outputs was closely associated with that of the force derivative, and had no relationship with that of the net force generated simultaneously, even though the magnitude of the instantaneous net force largely exceeded that of the force derivative. This observation supports the hypothesis that during dynamic force generation, the motor cortex and the corticospinal system assume a pivotal role in coding the direction of force changes, through selective recruitment of spinal motoneurons.