A comparative study on soleus H reflex parameters using vertebral column and peripheral nerve stimulation

Although low-threshold transcutaneous electrical nerve stimulation (TENS) is a modality used for control of pain, but its effect in the control of spasticity and change of excitability of a motor neuron is controversial. Therefore, this study was conducted to evaluate the effects of 15-min tripolar TENS of vertebral column and bipolar TENS of common peroneal nerve (300 µs width, 50 Hz) on synaptic activities of spinal cord. For this purpose, soleus H-reflex, Mh wave (M wave on the H-reflex signal) and M wave parameters were studied. Twelve healthy and non-athletic female volunteers were tested in two groups. First group received vertebral column stimulation and second group received peripheral stimulation in 4 separate experimental sessions (two control and two test sessions). In the first group, tripolar TENS (cathode on T 11, one anode 3 cm above and the other 7 cm below the cathode) was applied. In the second group, bipolar TENS (cathode on common peroneal nerve at the level of fibular head and anode 3 cm below the cathode) was used. Then, the acquired data were analyzed statistically. The results showed that peak to peak amplitude (PP), area under the curve and average rectified value (ARV) of H-reflex in the first stimulation group decreases (p<0.05), but in the second group, there is no significant changes. In addition, these parameters increased for Mh wave in the first group, but in the second group, there were no significant changes. It is suggested that tripolar TENS of vertebral column has augmented activity of the fast motor units. In this respect, it is possible that Renshaw cells are more activated and suppressed the activity of the slow motor units. These effects tended to increase the sensitivity of soleus motor end plate and decrease H-reflex amplitude. It can be concluded that tripolar TENS of vertebral column (depending on the site and method of stimulation) appears to be effective in synaptic activities of the spinal cord, especially the fast motor units and Renshaw cells, while bipolar TENS of common peroneal nerve does not produce such effect.