Mental set can modulate response onset in the lower limb muscles to falls in humans

The objective of this study was to investigate the modulation of the ankle muscle electromyographic (EMG) response as a function of mental set. Thirteen young healthy subjects underwent 40 unexpected and self-initiated drops from 30 cm above two separate force-plates. Following unexpected drops, reflex activities were observed in the medial gastrocnemius (MG) and tibialis anterior (TA) at mean latencies (+/- SD) of 83.59 +/- 10.1 and 99.43 +/- 21.82 ms, respectively. Following self-initiated drops, the response latency of the MG was significantly shortened (to 71.98 +/- 10 ms, P<0.05), and the TA was significantly lengthened (to 183.33 +/- 45 ms, P<0.05) when compared with unexpected drops. Such a modulation was associated with a significant reduction of the impact force on landing as compared with unexpected drops (by 17%, P<0.05). Interestingly, a negative correlation was found between the onset of the TA EMG response and the magnitude of the impact force on landing during expected (r= -0.66, P<0.05) but not unexpected drops.