Influence of Menstrual Cycle Phase on Adaptations to Sprint Interval Training

Sprint interval training improves endurance performance and induces metabolic adaptations in muscle. Most research demonstrating these responses has been conducted in males, with limited studies evaluating changes to endurance performance and skeletal muscle oxidative capacity in females. Moreover, it is currently unknown if training in specific phases of the menstrual cycle influences adaptations to training. Thus, the purpose of the present study is to compare adaptations to 2 weeks of sprint interval training performed in the follicular vs. luteal phase of the menstrual cycle in healthy, eumenorrheic women. We will also determine how these responses differ from males.

Inclusion Criteria: * Participating in 150 min of moderate-vigorous physical activity/week * VO2peak of 'good' or above based on American College of Sport Medicine (ACSM) normative values (greater than 35.0ml/kg/min for females and greater than 39ml/kg/min for males) * Body mass index (BMI) between 18-27 kg/m2 * Performing 2-4 structured exercise sessions/week. * Weight stable (within ± 2kg for at least 6 months) * Non-smoker * Females only: Menstrual cycle length of 27-35 days * Minimal menstrual cycle length variability (less than 3 days) * Ovulating monthly evidenced by a positive urinary luteinizing hormone surge * Blood progesterone concentration greater than 16 nmol/L. Exclusion Criteria: * Hormonal contraceptive use within the last 3 months * Polycystic ovarian syndrome (PCOS) or endometriosis * Failure to meet MC verification criteria. * Diagnosed with cardiovascular or metabolic disease, hyper- or hypogonadism, and/or PCOS * The use of medication for managing blood glucose or lipid metabolism * Current use of oral contraceptives or use within the last 3 months * Irregular menstrual cycles (\35 days) * Pregnant or post-partum in the last 12 months, lactating or breast feeding within 3 months of the start of study, or menopausal * Recreational smoking tobacco * Inability to perform the study exercise protocols or follow the pre-trial dietary or physical activity controls * Taking medications affecting substrate metabolism (corticosteroids or nSAIDs) * Actively engaging in a low-carbohydrate diet (e.g., ketogenic, Atkins)