Interaction of the Jendrassik maneuver with segmental presynaptic inhibition

Since its initial description in 1883, the Jendrasik maneuver (JM) has been used in clinical neurological practice as an effective means of potentiating the tendon tap in neurologically impaired patients. The JM also potentiates its electrical analogue, the Hoffman (H-) reflex, but the mechanism of the reflex modulation has not been clearly established. We studied soleus H-reflex modulation in neurologically intact subjects while at rest and during a mild plantarflexion contraction (EMG level equivalent to similar to 10% maximum voluntary contraction). The control H-reflex was elicited by stimulating the tibial nerve in the popliteal fossa with single pulses of 1 ms duration. Conditioning of the reflex was by: (1) increasing segmental presynaptic inhibition via common peroneal nerve (CP) stimulation; (2) pulling the arms and clenching the teeth (JM); or (3) applying both together (JM+CP). CP stimulation significantly (P<0.05) suppressed the H-reflex (50% Hmax), while JM significantly (P<0.05) facilitated it during contraction. From either an analysis of the grouped data or by a within-subject analysis, we found that the combined effect of stimulating JM+CP was significantly lower than JM alone, but did not differ from control values or from CP alone. The simplest mechanism would be that the effects of the two sum algebraically on the interneurones producing segmental presynaptic inhibition of the H-reflex.