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Anterior cruciate ligament injuries are more common in females than in males. Research has failed to isolate a cause for this bias. Hormones have been implicated in increased injury rates, increased ligamentous laxity, decreased stiffness, and increased electro-mechanical delay. We compared active and passive muscle stiffness, electromechanical delay, and extensibility of the knee flexor group between menses and ovulation in eumenorrheic women. No significant differences in active and passive muscle stiffness, electromechanical delay, or extensibility of the knee flexor group were measured between menses and ovulation. Menstrual cycle phase may not have an effect on active and passive muscle stiffness, electromechanical delay, or extensibility. Therefore, increased anterior cruciate ligament injury rates in females may not be caused by hormonal changes in muscle stiffness.