AbstractReaction time (RT) to auditory and visual imperative stimuli is reduced as the intensity of the stimulus increases; however similar stimulus intensity effects may not be present when a mechanical perturbation is used as the imperative signal (Evarts and Vaughn, 1978). It was recently suggested that when a perturbation elicits a startle reflex, RT is insensitive to the intensity of the perturbation (Heckman et al., 2011), suggesting a similar mechanism to the auditory StartReact effect (Carlsen et al., 2007). The present study sought to further examine the influence of perturbation duration and intensity on RT and incidence of startle reflex activation. Fourteen participants performed a forearm supination task in response to a pronation perturbation. We manipulated the duration of the perturbation (20-80 ms; 15 ms increments; 1.25Nm) and the level of torque (0.25-1.25Nm; 0.25Nm increments; 35 ms duration). EMG data was collected from right biceps (agonist) as well as startle indicator muscles (orbicularis oculi and sternocleidomastoid). Duration of the perturbation did not influence RT (p=.73), with a mean premotor RT across the five conditions of 82 ms. By contrast, analysis of perturbation torque intensity revealed a significant linear trend (p<.001), whereby the smallest perturbation resulted in a mean premotor RT of 95 ms, while the largest perturbation resulted in a reduced RT of 82 ms. Despite the short RTs across all conditions, a startle reflex was not observed on any trials, suggesting the StartReact effect is not responsible for the early release of a preplanned response via a perturbation.