Quadriceps excitability is enhanced by a conditioning tap to the Achilles tendon

In an effort to more fully investigate spinal reflex pathways in humans, we measured the isometric force-time characteristics of the patellar tendon-tap reflex in ten college-age subjects. We also conditioned the tendon jerk with a tap to the contralateral Achilles tendon or the ipsilateral Achilles tendon. The results clearly demonstrated that a conditioning tap to the Achilles tendon produced a marked (greater than 200 percent) excitatory effect onto the quadriceps muscle. Also, this effect occurred sooner when the quadriceps was conditioned by an ipsilateral stimulus rather than a contralateral stimulus. It is concluded that in accord with other conditioning studies to date, the quadriceps muscle is characterized by a predominantly excitatory effect caused by a conditioning stimulus, and that this arousal is independent of the origin of the conditioning stimulus.