Research articlePost-activation depression in the human soleus muscle using peripheral nerve and transcutaneous spinal stimulation

- Post-activation depression of the H-reflex and REP was studied at 25-200 ms intervals.
- Depression of a second response was induced by a conditioning H-reflex or REP.
- Recovery at rest was ∼30% as compared to during contraction.
- Recovery of the REP → H and H → H conditions were similar.
- The REP → REP condition showed the most depression.

Transcutaneous stimulation of the human lumbar spine can be used to elicit root-evoked potentials (REPs). These sensory-motor responses display notable similarities to the monosynaptic H-reflex. The purpose of this study was to compare post-activation depression of the soleus REP to that of the H-reflex, when conditioned by either an H-reflex or an REP. Paired pulses were delivered 25-200 ms apart and the recovery was characterized using three levels of stimulation. In all conditions, post-activation depression was reduced during contraction as compared to rest (P < 0.001). REP doublets, delivered using an inter-pulse interval of 150 ms, recovered to 68 ± 8% of control during plantarflexion and 20 ± 6% of control at rest. During contraction, recovery of a second REP was 65% of the corresponding recovery for a second H-reflex. The recovery of an H-reflex was equivalent, when conditioned by either an H-reflex or an REP, even though the spinal stimulus activated and/or engaged more afferent and efferent fibers. Our results suggest that the additional elements activated by the spinal stimulus did not affect the recovery of the H-reflex. However, the transcutaneous spinal stimulus produced more inhibition when it was assessed using two low-intensity REPs (P < 0.05) suggesting that the pathway mediating the spinally-evoked response was more susceptible to being inhibited.