Author(s): DAUVERGNE, M., TESTA, J., MARIANI, A., LA RIVIÈRE, O., BESOMBES, C., MARCOS, M., CADERBY, T., PEYROT, N., Institution: UNIVERSITY OF LA RÉUNION, Country: FRANCE, Abstract-ID: 2614

INTRODUCTION:
Muscle-tendon unit (MTU) stiffness is a key determinant of force transmission, rapid force development and elastic energy exchange, with implications for locomotion, performance and injury risk. Because stiffness is method dependent, passive and active measures reflect distinct mechanisms, whereas whole-leg spring-like behavior integrates MTU properties and neural activation and may therefore diverge from tissue level measures. Sex hormone fluctuations across the menstrual cycle may influence musculotendinous mechanical properties and neuromuscular function (1), as estradiol and progesterone vary markedly across phases. It is unclear whether resistance training-induced adaptations in muscle stiffness are consistent across cycle phases. Therefore, this study examined the effects of resistance training on passive and active vastus lateralis stiffness and leg stiffness, assessed at early follicular (EF), ovulatory (OV) and mid-luteal (ML) phases in naturally menstruating women.
METHODS:
Seven physically active naturally menstruating women were tested PRE and POST a 12 week supervised resistance training program (2 × 75 min per week). Measurements were performed during the EF (days 2-5), OV (24-48h after a positive ovulation test), and ML (7-10 days after OV) phases. Leg stiffness (Kleg) was derived from a bilateral hopping test. Vastus lateralis stiffness was assessed by shear wave elastography at rest (SWE0) and during a 50% maximal voluntary contraction (SWE50) (2). A two-way repeated measures ANOVA was conducted to test the effects of menstrual cycle phase (EF, OV, ML) and training status (PRE vs. POST).
RESULTS:
For SWE0 and SWE50, no significant menstrual cycle phase × training interaction (p = 0.528 and p = 0.097, respectively) or main effects of cycle phase (p = 0.070 and p = 0.640, respectively) or training status (p = 0.882 and p = 0.218, respectively) were observed.
For leg stiffness (Kleg), neither a menstrual cycle phase × training interaction (p = 0.384) nor a main effect of menstrual cycle phase (p = 0.733) was observed. In contrast, a significant main effect of training was found (p = 0.046, ηp² = 0.581), with Kleg increasing by 14.7% from PRE to POST.
CONCLUSION:
These preliminary findings indicate that leg stiffness increased after 12 weeks (+14.7%), independent of menstrual cycle phase. In contrast, vastus lateralis stiffness (SWE0, SWE50) showed no significant effects of either menstrual phase or training. Together, these results suggest that training-related increases in leg stiffness may be driven by functional and/or neuromuscular adaptations rather than detectable local MTU mechanical changes in women. The absence of phase-related effects on muscle stiffness, which contrasts with some previous findings (3), may reflect the small sample size, and warrants confirmation in larger cohorts.

References
[1] Bernstein et al., 2023
[2] Nordez and Hug, 2010
[3] Bouvier et al., 2025