Stretch reflex distinguished from pre-programmed muscle activations following landing impacts in man.

1. Electromyographic activity and joint rotation were measured whilst human subjects jumped down to land on a solid surface 0.45 m below them and then to a false collapsible surface at the same level. The collapsible surface did not produce ankle joint rotation. Objective evidence from post-take-off EMG onset latencies showed that the subjects were surprised by the false surface. 2. Following landing on the solid surface, the gastrocnemius and soleus muscles showed peak responses at latencies of 53 and 56 ms, respectively, and the antagonist m. tibialis anterior showed an early peak response with a latency of 26 ms. These responses occurred following landing on the solid surface but not following passage through the false surface. 3. The rectus femoris and biceps femoris muscles showed no clear reflex activity with this fall distance but pre-programmed activity was clearly present following impact. 4. It is concluded that the post-landing activity in m. soleus and m. gastrocnemius is a short-latency spinal reflex triggered by ankle joint rotation.