AbstractMechanical perturbations to the arms produce short (SLFR; 25-50ms) and long-latency (LLFR; 50-100ms) feedback responses in stretched muscles. The SLFR is generated by spinal circuitry and remains immutable to intention. By contrast, the LLFR engages supra-spinal regions shared with voluntary control and possesses the capacity to modulate in a manner similar to volition (Pruszynski et al., 2008). The present study investigated whether the overt execution of a voluntary response in stretched muscle is required for facilitation of the LLFR. Participants engaged in kinesthetic motor imagery of a compensate task (Experiment 1) or were instructed to execute voluntary responses in a non-stretched contralateral muscle (Experiment 2). The LLFR in stretched right wrist flexors were compared on Imagery and Contralateral trials to standard "Not-Intervene" and "Compensate" conditions. Our findings revealed that on ~40% of Imagery and ~50% of Contralateral trials, a partial voluntary response "leaked-out" into the stretched right wrist flexor muscle. On these "leaked" trials, the early portion of the LLFR (R2: 50-75ms) was increased compared to the Not-Intervene condition and appeared indistinguishable to the facilitation observed on Compensate trials. The latter portion of the LLFR (R3: 75-100ms) showed further modulation that mirrored the patterns of voluntary activity. By contrast, the LLFR on "non-leaked" Imagery and Contralateral trials appeared similar to the Not-Intervene condition. These findings suggest that even though a hastened voluntary response cannot account for all LLFR facilitation, the overt execution of a voluntary response in stretched muscle is required for instruction-dependent modulation of this rapid feedback response.