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 clown to land on a solid surface 0.45 m below them and the n to a false collapsible surface at the same level. The collapsible surface did not produce ankle joint rotation. Objective: evidence from post-take-o ff EMG onset latencies showed that the subjects were surprised by the false surface. 2. Following landing on the solid surface, the gastrocnemius and soleus mus cles showed peak responses at latencies of 53 and 56 ms, respectively, and the antagonist m. tibialis. anterior showed an early peak response with a l atency of 26 ms. These responses occurred following landing on the solid su rface but not following passage through the false surface. 3. The rectus femoris and biceps femoris muscles showed no clear reflex act ivity with this fall distance but pre-programmed activity was clearly prese nt following impact. 4. It is: concluded that the post-landing activity in m. soleus and m. gast rocnemius is a short-latency spinal reflex triggered by ankle joint rotatio n.