The present study aimed to investigate changes in the Hoffmann reflex (H-reflex) at the same time on different days and to compare the findings between genders. Four males and five females in healthy subjects agreed to participate in the study and provided informed consent. All subjects were nonsmokers, and none of the females were consuming any oral contraceptives. The H-reflex was elicited in the right soleus muscle by constant current stimulation of the tibial nerve in the popliteal fossa with the subjects in the prone position. As a reference, the maximal amplitude of the H-wave relative to that of the M-wave (Hmax/Mmax) was measured to determine changes in α-motoneuron excitability. Hmax/Mmax of 6 to 9 trials and was represented by the coefficient of variation (CV). The CV of Hmax/Mmax in females was significantly higher than that in males. The level of Hmax/Mmax for males was highly reproducible, suggesting that fewer factors alter α-motoneuron excitability in males compared with those in females. Several female subjects experienced symptoms of depression and suffered from body and menstrual pain. Variations in Hmax/Mmax values were considerably greater for females than for males, suggesting that the α-motoneuron might be influenced by the menstrual cycle phase as well as by individual differences
