Reflex facilitation by remote contraction: topographic aspects.

This study compares the facilitation of various lower limb reflex arcs brought about by a selective contraction of a well-defined upper limb muscle group in able-bodied healthy subjects. Quantitatively (results expressed as a percentage of maximal motor response), the quadriceps tendon reflex is facilitated more than the soleus and biceps femoris tendon reflexes. The time course of facilitation is similar for the 3 reflexes and exhibits 3 distinct phases. Electrically induced reflexes in the quadriceps are facilitated as much as the tendon jerk but only during the 2nd phase. Complementarily, the effects of contraction of different upper limb muscle groups on a single lower limb reflex (quadriceps tendon jerk) were also investigated. This reflex is facilitated more (especially in the 2nd phase) by contraction of thenar muscle than by that of deltoid muscles and more by wrist extensors than wrist flexors. Differences in facilitation are only apparent when the effects are quantified. They depend on the segmental level of the reflex under test (more marked proximally than distally) and on the conditioning muscle group; both facts must be taken into consideration in the interpretation of the functional role of remote contraction.