Quadriceps mediated changes in soleus motoneuron excitability

In an effort to investigate the interactions of the quadriceps and soleus muscles, the isometric force-time curve of the Achilles tendon-tap reflex (ATR) was measured in 10 college age subjects. The tendon-jerk was conditioned with a tap to the ipsilateral or contralateral patellar tendon, or by an electrocutaneous (3x perception) stimulus applied to the belly of the quadriceps. The conditioning stimulus preceded the test ATR by 25, 40, 55, 70, 85, 100, 115, 130 or 145 ms. The results indicated that the ipsilateral conditioning tendon-tap produced a wave of short-latency facilitation (40 ms) and long-latency inhibition (85-130 ms) to the triceps surae muscles. The ipsilateral cutaneous conditioning, however, did not alter reflex excitability. A contralateral conditioning to the patellar tendon again produced short-latency (50-70 ms) facilitation, whereas the electrocutaneous stimulus did not alter motoneuron excitability. Moreover, a barrage of electrocutaneous impulses to either the ipsilateral or contralateral quadriceps (1 minute/60 pps) did not significantly alter motoneuron excitability of the soleus muscle. These results indicate that motoneuron excitability changes are observed in the soleus muscle with mechanical conditioning but not with electrocutaneous conditioning. Several neurophysiological mechanisms are proposed to mediate these changes.