Effects of tripolar TENS of vertebral column on slow and fast motor units: A preliminary study using H-reflex recovery curve method

Introduction: Effect of tripolar TENS of vertebral column on slow and fast motoneurons (MNs) activity of soleus muscle was previously investigated. In this study for better differentiation of the behavior of slow and fast MNs, we exploited H-reflex recovery curve recording in two muscles of soleus and lateral gastrocnemius, respectively as the representatives of slow and fast muscles. Methods: 10 healthy non-athlete women aged 22.7±2.21 years participated in two (control and test) sessions. H-reflex recovery curve of soleus (slow) and gastrocnemius (fast) muscles were recorded before and 15 minutes after applying tripolar TENS (TENS frequency= 100 Hz and pulse width= 300 μs) on vertebral column. For recording the curve, rectangular paired stimuli were applied on tibial nerve (ISI= 40-520, frequency= 0.2 Hz, pulse width= 600 μs). Results: Our findings showed that maximum H-reflex recovery in gastrocnemius muscle appeared in shorter ISIs (240 ms), while in soleus muscle it appeared in longer ISIs (400 ms). H-reflex recovery curve amplitude slightly decreased after applying tripolar TENS. The comparison of H-reflex recovery curve of both soleus and gastrocnemius muscles either in control or test session did not show any significant difference (P>0.05). Conclusion: It is suggested that tripolar TENS excites not only the skin but also Ia and Ib afferents in the dorsal column. Synaptic interactions of these afferents in spinal cord cause the inhibition of type I MNs and facilitation of type II MNs.