The Influence of Sex Hormones on Neuromuscular Function and Premenstrual Symptoms

The menstrual cycle with fluctuation in female sex hormones has been suggested to influence neuromuscular function. However, variation in ligament laxity, soft tissue stiffness, and skeletal muscle strength across the menstrual cycle has led to conflicting results. Furthermore, premenstrual symptoms (PMS) have been suggested to have a negative impact on neuromuscular performance and contribute to the risk of musculoskeletal injury in physically active women. On the other hand, the use of oral contraceptives (OCs) might protect from injury. The mechanisms of the influence of sex hormones on neuromuscular function and PMS are not fully understood. The overall aim of this thesis was to explore the effect of endogenous sex hormones and OCs on muscle strength, postural control, and PMS in healthy women with moderate physical activity. A further aim was to explore the expression of sex hormone receptors in skeletal muscle during three well-defined phases of the menstrual cycle. Postural control was measured during the active and hormone-free phase of OC treatment in physically active women with or without PMS as evaluated by prospective symptom rating. In the same cohort of women, muscle strength and hop performance were measured during one OC cycle and one normal menstrual cycle at three specific phases, using a cross-over design. Furthermore, changes in PMS in the same women starting to use or discontinuing the use of OCs were evaluated. In another cohort of healthy women, muscle biopsies were collected from the musculus vastus lateralis in the follicular phase, the ovulatory phase, and the luteal phase of the menstrual cycle for determination of mRNA and protein levels of sex steroid hormone receptors. Women with PMS displayed a significant change in postural control (greater displacement area) during the active OC phase compared to the withdrawal phase of OC treatment, whereas women without such symptoms showed no variation in postural control during OC treatment. Muscle strength and hop performance did not vary during the different phases of the normal menstrual cycle, or during OC treatment. In women with PMS, OC treatment decreased ratings of premenstrual somatic symptoms, but not of negative mood symptoms. Gene and protein levels of estrogen receptor alpha and the progesterone receptor varied significantly during the three hormonally confirmed phases of the normal menstrual cycle. The results of this thesis indicate that PMS influences postural control and OC treatment decreases PMS of somatic type. Furthermore, muscle strength and hop performance are not influenced by endogenous and exogenous sex hormones. The variation in expression of sex hormone receptors in skeletal muscle may have an impact on the effects of muscular training and sports injuries in women.