Preparedness for landing after a self-initiated fall.

A startling auditory stimulus (SAS) causes a faster execution of voluntary actions when applied together with the imperative signal in reaction time tasks (the StartReact effect). However, speeding up reaction time may not be the best strategy in all tasks. After a self-initiated fall, the program for landing has to be time-locked to foot contact to avoid damage, and therefore advanced execution of the program would not be convenient. We examined the effects of SAS on the landing motor program in 8 healthy subjects that were requested to let themselves fall from platforms either 50 or 80 cm high at the perception of a visual imperative signal and land on specific targets. In trials at random, SAS was applied either together with the imperative signal (SAS(IS)) or at an appropriate prelanding time (SAS(PL)). As expected, the latency of takeoff was significantly shortened in SAS(IS) trials. On the contrary, the timing of foot contact was not significantly different for SAS(PL) compared with control trials. No changes were observed in the size of the electromyograph bursts in the two experimental conditions with respect to the control condition. Our results indicate that the landing program after a self-initiated fall may in part be organized at the time of takeoff and involve precise information on timing of muscle activation. Once launched, the program is protected against interferences by external inputs.