Introduction and Aims: Hormonal factors are one plausible explanation for differences in musculotendinous stiffness (MTS) and knee laxity between men and women, and the resulting higher risk for ACL injury in women. This study examined MTS and ACL laxity over the course of the menstrual cycle in women and investigated the interaction of warm-up.

Method: Eight female netball players aged between 16–18 years (mean = 16.3 ± 0.8 years) participated in this study. None of the participants were using oral contraceptives and all demonstrated regular menstrual cycles. Venous blood samples and MTS data were collected each week over the 28-day menstrual cycle. MTS was assessed prior to, and following a standardised warm-up consisting of light cycling and jumping. ACL laxity was determined at the beginning of each test session using the KT2000™ knee arthrometer (MEDmetric Corporation, San Diego, USA).

Results: Repeated measures ANOVA results revealed significant (p < 0.05) main effects of warm-up and test session on MTS. For the effect of warm-up, MTS was found to significantly decrease by 4.2% following the warm-up intervention, indicating that relatively low levels of activity can acutely alter the viscoelastic properties of muscle. Post hoc contrasts for test session revealed that MTS was significantly lower at week three (corresponding to the ovulatory phase), in contrast to weeks one and two (7% and 4.5% decrease, respectively). For ACL laxity, repeated measures ANOVA revealed no significant (p < 0.05) differences at 30 lb anterior force across the menstrual cycle. The results did however demonstrate a trend towards increased ACL laxity during ovulation (week three) when the lower limb musculature was most compliant.

Conclusion: While there were no significant differences in static ACL laxity, reduced MTS is one of the main causes of prolonged electromechanical delay (EMD) and has particular relevance for ACL injury. In conclusion, females are at greater risk of incurring an ACL injury during ovulation when oestrogen levels are highest.