Experiment 2

Eight healthy young women (160.4 ± 4.8 cm, 57.6 ± 8.8 kg, 23.9 ± 3.1 years) who were experiencing normal menstrual cycles (reported 28–33 day cycles for the last 6 months) participated in Experiment 2. All participants were either sedentary or recreationally active women, but not were involved in any type of exercise training program during the period of Experiment 2. They had not taken oral hormonal contraceptives or other forms of hormones for at least 2 years prior to and during the study.

Each participant was tested in three phases of her menstrual cycle: menstrual phase (1–3 days after menstruation; when estradiol and progesterone concentrations are low), ovulatory phase (1–2 days before or on predicted ovulation day; when estradiol is elevated and progesterone is low), and luteal phase (7–10 days after predicted ovulation; when estradiol and progesterone are elevated). Using the menstrual cycle data obtained from the questionnaire, the length of menstrual cycle for each participant was calculated by averaging the cycle length of her the latest 6 cycles, and thereby the beginning of the next cycle could be estimated. Then, the date of ovulation was predicted by counting back 14 days from the presumed first day of the next cycle. The order of testing was randomized across participants and the three tests were conducted at the same time of day to minimize any influences of circadian rhythm.

The experimental setup and procedure of measuring ankle joint angle and SWE were the same as those used in Experiment 1, except for the muscles studied with SWE (only MG in Experiment 2). In Experiment 2, since the smallest dorsiflexion ROM of the total 24 tests (8 participants × 3 phases) was 13°, the passive stiffness at 0° and 13° was evaluated.

Venous blood samples (~5 ml) were taken from an antecubital vein by a medical practitioner using vacutainers to measure the concentrations of estradiol-β−17 and progesterone. Serum samples were obtained by centrifuging the blood samples at 2900 × g for 10 min at 4 °C, and then stored at −80 °C until analysis. The serum estradiol-β−17 and progesterone concentrations were measured at the Clinical Pathology Laboratory in Kagoshima, Japan.

For each variable, one-way repeated measure ANOVA was performed to compare between the three phases. If appropriate, post-hoc Bonferroni analyses were used.