LONG-LASTING CONDITIONING OF THE HUMAN SOLEUS-H REFLEX FOLLOWING QUADRICEPS TENDON TAP

Percussion of the quadriceps tendon was used to test the hypothesis th at 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 or 100 ms duration, with the knee at 60 degrees or 90 degrees of flexion. The tap elicited quad riceps stretch reflexes in four subjects, with a mean latency of 42 ms . The major component of the conditioning of the soleus H reflex was s ignificant 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 atte nuation of reflex magnitude was evident, whichever combination of dura tion and force of tap was used. Preceding and/or following this inhibi tion, there was mild facilitation. Static stretch of quadriceps also s ignificantly reduced soleus H reflex magnitude. These results support the spindle receptor origin for the gain attenuation seen during movem ent. The time course of the gain attenuation suggests a spinal route, by which the spindle discharge of the heteronymous extensor muscles in itiates presynaptic inhibition of transmission through the reflex path way.