The effects of a startle on the sit-to-stand manoeuvre.

Simple ballistic movements are executed faster in reaction time task paradigms when the imperative signal (IS) is accompanied by a startling auditory stimulus (SAS). We examined whether this effect also occurs in complex movements such as the sit-to-stand manoeuvre (STS). Nine healthy volunteers performed STS to visual IS, either presented alone (control trials) or together with SAS (test trials). Reaction time, measured as the time between IS and take-off (TO), was significantly shortened in test trials when SAS was applied at an interval of 0 ms with respect to IS. The onset latency of EMG bursts recorded from tibialis anterior, lumbar paraspinal, quadriceps and biceps femoris muscles reduced proportionally to the shortening of TO. However, these effects were not observed if SAS was delivered 150 ms after IS, when the manoeuvre had already started. Our results suggest that stimuli acting on subcortical motor structures speed-up but do not otherwise interfere with the execution of the motor programs underlying the STS manoeuvre.