The present study aimed to investigate the changes in the mechanical properties of human muscle and tendon during the menstrual cycle in vivo. The subjects were young healthy women (n=8, age 22.5+/-0.9 years) with a normal menstrual cycle. Cycle phases were divided into the menstrual (when estradiol and progesterone concentrations were low), ovulatory (when estradiol was elevated and progesterone was low), and luteal (when progesterone was elevated). Measurements included maximal isometric voluntary contraction (MVC), muscle activation level (using interpolated twitch method), and tendon properties (using ultrasonography) in knee extensors and plantar flexors. No significant changes in MVC and muscle activation level were found during the menstrual cycle. Similarly, there were no significant differences in the maximal elongation and stiffness of tendons among the three phases. These results suggested that the changes in female steroid hormones during the menstrual cycle did not affect the mechanical properties of human muscle and tendon.