Do fluctuations in oestradiol and progesterone across the menstrual cycle affect mechanical stiffness in female athletes?

Objectives: This exploratory study investigated the effect of fluctuations in sex hormones, oestradiol and progesterone, during the menstrual cycle, on mechanical stiffness, in naturally menstruating females and hormonal contraceptive users. Design: Observational, within subjects. Methods: Participants were 24 female (18–29 years) National Rugby League Indigenous Women's Academy players. At three phases of the menstrual cycle (early-follicular, late-follicular and mid-luteal), venous blood samples (oestradiol and progesterone) were collected. To quantify mechanical stiffness (vertical, leg and hip, knee, and ankle joint) analogue and co-ordinate data were collected, whilst participants completed three drop jumps, landing on two force plates. Trajectory and analogue data were reconstructed, labelled, and filtered in Vicon Nexus software. A linear mixed model was used to determine if there were significant interactions between hormone concentrations and measures of mechanical stiffness. Results: Ankle joint stiffness was significantly positively associated with oestradiol and progesterone concentration (p = 0.006, p = 0.020, respectively). Knee joint stiffness and leg stiffness were positively associated with oestradiol concentration (p = 0.009, p = 0.006, respectively), but not progesterone concentration. Hip joint and vertical stiffness were not significantly associated with either hormone. Conclusions: Female rugby league players have significantly higher ankle joint, knee joint and leg mechanical stiffness when oestradiol was highest, typically during Phase-2 of the menstrual cycle. It is possible that dynamic stiffness of the lower limb joints acts as a protective mechanism. Future research should confirm this relationship and examine interventions to address changes in athletes' mechanical stiffness across the menstrual cycle in order to enhance performance and injury prevention.