Functional relationships between myotatic reflex arcs of the lower limb in man: investigation by excitability curves

In 30 normal subjects, the influence of the reflex activation of one myotatic reflex arc on the excitability of other myotatic reflex arcs of the lower limb has been investigated using excitability curves. Soleus, quadriceps, and short biceps tendon reflexes as well as H reflex at two different intensities (liminal and H max/2) were used either as conditioning or as conditioned responses. The reflex activation of the soleus muscle has opposite effects on antagonistic muscle groups of the thigh: facilitation of the quadriceps myotatic arc and inhibition of the short biceps myotatic reflex arc. Conversely, activation of both quadriceps and short biceps muscles leads to a marked and long lasting (+/- 5000 ms) inhibition of the soleus myotatic reflex arc. The differences of functional organization between proximal and distal myotatic reflex arcs are emphasized and the role of the afferent impulses secondary to the conditioning muscular contraction is discussed.