The menstrual cycle implies a basic difference in the biology of women and men but the effect of the hormonal variation on training protocols and physical performance is still not fully understood. Despite no existing evidence, the advice to periodize exercise according to the menstrual cycle has been widely spread among elite athletes, coaches, and sports federations. The advice is based on underpowered studies with considerable methodological weaknesses regarding determination of cycle phase, inclusion of athletes and lack of adequate control groups.

The purpose of this randomized, controlled study is to evaluate the effect of exercise periodization on aerobic fitness during different phases of the menstrual cycle. Further, the effect will be related to premenstrual symptoms, body composition and skeletal muscle morphology, sex hormone receptors, metabolic enzymes, and markers of muscle protein synthesis. This study will be well controlled and follow methodology recommendations for menstrual cycle research in sports and exercise.

Female athletes of fertile age will be randomized to different training regimens during three menstrual cycles (12 weeks):

Group A: Training three times a week throughout the menstrual cycle. Group B: Follicular phase-based training five times a week during the follicular phase and thereafter once a week during the luteal phase.

Group C: Luteal phase-based training five times a week during the luteal phase and once a week in the follicular phase.

The exercise will consist of high intensity intermittent spinning classes. Assessment of aerobic fitness and power will be performed at baseline, and again after three completed menstrual cycles. On the same day, body composition will be examined by DXA and blood samples will be collected for analysis of hormones and binding proteins. To confirm menstrual cycle phase, blood samples will be collected for hormone determination, and urinary stick will be used for detection of ovulation. Subjective ratings of menstrual cycle related symptoms will be performed every day. In a subgroup of women, muscle biopsies will be collected from m vastus lateralis at baseline and at the end of the study. This study will contribute to improved knowledge about exercise periodization in relation to the menstrual cycle. Well-grounded data is crucial to give evidence-based recommendations to female athletes when planning their training protocol to optimize training results and performance.

Inclusion Criteria:

• Women exercising at least 3-4 times a week on a high level
• Regular menstrual cycle with 26-32 cycle days.
• Fully healthy and be able to follow the training program.

Exclusion Criteria:

• Hormonal contraceptive users,
• Users of regular medication
• Chronic disease
• Past or present neurological disorder
• Recent musculoskeletal injury

The testleader will be blinded to which group the subject belongs to.