Long-lasting conditioning of the human soleus H reflex following quadriceps tendon tap

Percussion of the quadriceps tendon was used to test the hypothesis that knee extensor muscle spindle discharge initiates down-regulation of the gain of the soleus H reflex. Seven subjects participated. Soleus H reflex magnitude was observed for up to 15 s, following conditioning tendon taps of 60 N or 80 N force and 10 ms duration, with the knee at 60 degrees or 90 degrees of flexion. The tap elicited quadriceps stretch reflexes in four subjects, with a mean latency of 42 ms. The major component of the conditioning of the soleus H reflex was significant attenuation of magnitude by 30-90% of controls, starting as early as 36 ms post-percussion and lasting as long as 3-8 s. The attenuation of reflex magnitude was evident, whichever combination of duration and force of tap was used. Preceding and/or following this inhibition, there was mild facilitation. Static stretch of quadriceps also significantly reduced soleus H reflex magnitude. These results support the spindle receptor origin for the gain attenuation seen during movement. The time course of the gain attenuation suggests a spinal route, by which the spindle discharge of the heteronymous extensor muscles initiates presynaptic inhibition of transmission through the reflex pathway.