The aim of the study is to investigate the effect of the different menstrual cycle phases on the recovery from exercise-induced muscle injury in eumenorrheic women. For this purpose, in a cross-over, randomized study, at least 10 healthy eumenorrheic women aged 18-35 years will participate. In a random order, the participants will perform 45 min downhill (-15% slope) running on a treadmill at 70% HRmax followed by a maximal time-trial (95% HRmax) to exhaustion: i) during the follicular phase and ii) during the luteal phase. Before the exercise protocol, as well as at 24 h, 48 h and 72 h following exercise, complete blood count, delayed onset of muscle soreness (DOMS), creatine kinase activity, countermovement jump, isometric, concentric and eccentric strength of knee extensors and knee flexors, will be assessed. In addition, lactic acid concentration will be assessed before and immediately following exercise, and DOMS will be assessed immediately after the end of exercise. Following a washout period of ≥28 days (depending on the length of the menstrual cycle), participants will repeat the exact same procedure for the remaining phase of the menstrual cycle.

Research in the field of athletic training and rehabilitation, has traditionally focused on men, ignoring the physiological differences between sexes that may significantly affect athletic performance and recovery following exercise-induced muscle damage (EIMD). According to recent reviews of research in sports and exercise medicine, female representation is particularly limited, with only 4% - 13% of studies including women [1]. However, the number of females' participation in regular exercise and sports constantly increases, therefore it is imperative to define EIMD in women.

Except for being limited, most of the existing studies in women have been conducted during the early follicular phase of the cycle, when estrogen and progesterone levels are low, so that women's hormonal profile resembles that of men [2], in order to avoid any fluctuations [3]. However, this approach limits our understanding of the effects of the other phases of the cycle, such as the luteal phase, on performance and exercise-induced inflammation, and thus on women's ability to train and perform.

Although limited, some data suggest the influence of different phases of the menstrual cycle on both performance and exercise-induced muscle injury. For example, women's thermoregulatory levels are higher during the luteal phase of the cycle, which may affect their cardiovascular performance and endurance [4]. Similar data suggest that hormonal fluctuations may also affect recovery from EIMD [5]. Indicatively, the concentration of CK and IL-6 24 h and 72 h following 90 min of continuous running at 70% of VO2max was higher during the follicular phase [6].

Considering the above, research regarding the effect of the different phases of the menstrual cycle on performance and EIMD, it is crucial for the effective design of individualized training programs depending on the phase of the menstrual cycle, to improve performance and avoid injuries in women.

The aim of the study is to investigate the effect of the different menstrual cycle phases on the recovery from EIMD in eumenorrheic women.

According to an initial power analysis performed (probability error: 0.05, power: 0.80), a number of 8-10 individuals is required in order to identify statistically significant differences. Therefore, at least 10 participants will be included in the present study.

The design of the present study is cross-over, randomized and will be conducted in two cycles. The participants, after being informed about the study, as well as the benefits and potential risks, will sign an informed consent for participation in the study. Before the initiation of the data collection, familiarization of the participants with the assessment tests and the exercise protocol at low intensity, will precede. Also, the participants will provide a 7-days diet recall before their participation in the first experimental condition. Subsequently, baseline measurements will be performed at the Biochemistry, Physiology and Exercise Nutrition Laboratory (SmArT Lab), Department of Physical Education and Sports, University of Thessaly: anthropometric characteristics (body height, body mass, body mass index), body composition (body fat percentage, lean body mass, fat mass, bone density), aerobic capacity (VO2max). Subsequently, in a random order, the participants will perform 45 min of submaximal (at 70% HRmax) downhill (-15%) running on a treadmill followed by a maximal (95% HRmax) time-trial to exhaustion: i) during the follicular phase and ii) during the luteal phase. The randomization of the menstrual cycle phases will be done by a random integer sets generator, available online (Random.org). Before the exercise protocol, as well as 24 h, 48 h and 72 h following exercise, complete blood count (CBC), exercise-induced muscle injury [delayed onset muscle soreness (DOMS), creatine kinase (CK) activity] and muscle performance [(countermovement jump (CMJ), isometric, concentric and eccentric strength of knee extensors and knee flexors)] will be assessed. In addition, metabolism (lactic acid) will be assessed before and immediately following exercise, and DOMS will be assessed immediately after the end of exercise. Following a washout period of ≥28 days (depending on the length of the menstrual cycle), participants will repeat the exact same procedure for the remaining phase of the menstrual cycle.

Inclusion Criteria:

• Physically active subjects (VO2max ≥35ml/kg/min)
• Absence of musculoskeletal injury (≥6 months)
• Abstinence from the use of ergogenic supplements (≥1 month)
• Abstinence from anti-inflammatory drugs (≥1 month)
• Abstinence from participating in exercise with eccentric content for at least 7 days before exercise
• Abstinence from alcohol and energy drinks before exercise

Exclusion Criteria:

• Recent history of musculoskeletal injury (<6 months)
• Use of ergogenic performance supplements (<1 month)
• Taking anti-inflammatory drugs (<1 month)
• Participation in exercise with eccentric content in the previous 7 days before exercise
• Consumption of alcohol and energy drinks before exercise