As reported by some researchers, human responses to Functional Electrical Stimulation (FES) are likely to be affected by several factors, such as spasticity, muscle fatigue, nerve habituation and so forth. In our previous study, it has been shown that, an sub-threshold Auxiliary Stimulation (AS) to the Gastrocnemius, with current frequency ranged from 2000 to 6000Hz, could alleviate the symptom of spasticity and muscle fatigue caused by the stimulation to the Tibialis Anterior, enable comparatively stable and durable function restoration assistance. To understand the underlying neuromuscular processes elicited by the auxiliary electrical stimulation and its qualitative natures, we have measured the Hoffmann-reflex (H-reflex) in human soleus muscle before and after the AS in this study,, and proposed a mathematical model which takes into consideration not only the efferent but also afferent neural pathways, to interpret the effectiveness of the AS. Based on the results from the experiment it can be noted that: H(max)/M(max) became lower after AS, AS could reduce the excessive excitability of Alpha MNs. The mathematical model of pre-synaptic inhibition could help us to understand the underlying neuromuscular processes elicited by electrical stimulation and its qualitative nature, and used it to predict the states of spinal cord more easily, quickly, and reasonably in clinic experiment as a medical evaluation method for the paralyzed humans.