AbstractMechanical perturbations delivered to the upper limbs can elicit stretch reflexes of short (M1:25-50ms) and long-latency (M2:50-100ms). When acting against the perturbation, a pre-planned voluntary response can also be elicited at latencies within the M2 epoch (< 100 ms). Such short latencies have served as a basis for the view that the hastened voluntary response may be characterized as a perturbation-evoked "triggered response." The purpose of this study was to investigate whether these response latencies are affected by the expectancy of the perturbation (simple vs choice), type of response (compensate vs assist responses), and perturbation limb (ipsilateral vs contralateral). Participants performed pre-planned, rapid right wrist-supination responses under simple and choice conditions. In Simple RT, an expected pronation (or supination) perturbation was delivered to the right limb. In Choice Compensate/Limb, a pronation perturbation was delivered to either left or right limb. In Choice Assist/Limb, a supination perturbation was delivered to the left or right limb. In Choice Comp/Assist, a pronation or supination perturbation was delivered to the right limb. In the final Choice condition, all four perturbation types were presented randomly. Ensemble EMG from right biceps showed consistent, short-latency proprioceptive premotor RTs (~ 90-110 ms) to all four perturbation types. Average response latencies were not affected by perturbation expectancy. Compensate responses against the perturbation also did not differ from Assist responses going with the perturbation. Latencies were delayed (~10 ms) when the perturbation was delivered to the contralateral limb. Results suggest that pre-planned responses can be triggered early via proprioceptive perturbations.