Recent studies have shown that when normal human volunteers are subjected to a sudden free fall, EMGs of truncal and limb muscles are activated due to the startle reflex originating in the otolith organs. In this study, muscle activities during sudden free fall and voluntary fall were recorded in 18 healthy human subjects. Monophasic or biphasic EMG responses could be recorded from all subjects during sudden free fall; the average latency of initial activity was 29.2msec (SD 3.5msec) in the orbicular oculi muscle, 67.7msec (SD 9.3msec) in the anterior tibial, 73.5msec (SD 9.2msec) in the gastrocnemius and 71.8msec (SD 9.4msec) in the soleus muscles. On the other hand, such initial activities did not appear in voluntary falls, and only activity which coincided with landing could be recorded. It is suggested that free fall stimulation can be used as a diagnostic tool in disorders of otolithic function.
