Synergic Effect of Isometric Resistance Training and Subthreshold Electrical Neuromuscular Stimulation on the Excitability of Spinal Motoneurons in Humans

The rate of force development (RFD) has been widely used as an indicator of muscle strength in the clinical practice. Our previous study showed that the RFD within short time intervals could be increased by low-load isometric resistance training (IRT) with sub-motor threshold electrical neuromuscular stimulation (SMT ENMS) compared with the low-load IRT alone. However, the effect of the combined method on the excitability of spinal motoneurons remained unknown. Our study aimed to clarify a possible synergic effect of low-load IRT with SMT ENMS on the motoneuronal excitability in healthy adults. Ten healthy subjects randomly received four interventions (low-load IRT combined with SMT ENMS, low-load IRT alone, SMT ENMS alone, and the control). The results showed that the RFD and excitability of spinal motoneurons significantly increased under low-load IRT with SMT ENMS conditions than under control conditions. In conclusion, the increase in the RFD by the above combined intervention is related to a significant increase in the excitability of spinal motoneurons.