The increase in women’s involvement in sports has increased interest in the influences of women’s unique physiology on athletic performance. The effect of the menstrual cycle on athletic performance, is of particular interest. Socially, the menstrual cycle is construed as a time of weakness or inability and often symptoms are dismissed or ignored. Mainstream media encourages women to stop training or change training styles. These psychosocial factors imposed by our society relay strong frailty narratives around a woman’s body and decrease a woman’s likelihood to train. The following data explores where these ideas come from and how drastically these changes are affecting training and performance.
To start, in science we support the null hypothesis (the assumption that there is no difference between groups or conditions) until there is overwhelming evidence to the contrary. Going forward, the null hypothesis we need to reject will be:
There is no reliable difference between the different phases of the menstrual cycle on performance in strength sports.
In order to make broad recommendations to the public about training we need a large clear, concise, repeatable body of evidence indicating a benefit to one phase of the menstrual cycle for performance and training. The following articles will break down the physiology behind why people hypothesize there is a difference between phases and evaluate strength and lifting performance across the menstrual cycle.
A Basic overview of hormones
A hormone is a chemical messenger released from an organ or tissue into the bloodstream to influence organs, tissues, etc. at distant sites. Steroid hormones are derived from cholesterol and are converted through a series of enzymatic reactions to their final form. Once released into the bloodstream, a hormone circulates throughout the body until it binds to a receptor. Receptors can be located inside a cell, on the surface of a cell, and sometimes both. After binding the hormone and receptor complex creates a cascade of molecular, cellular, and genetic effects all resulting in changing physiology. Hormones are measured as a concentration, e.g. how many hormone molecules will likely be found in a certain volume of blood.
The hypothalamic-pituitary-gonadal axis (HPG axis) is the main regulatory pathway for reproductive hormones, which are also known as sex steroids. The sex steroids , estrogen and testosterone, are hormones related to the differentiation of sexual characteristics. The hypothalamus is a specific group of neurons in the brain and one of its many physiological roles is releasing the Gonadotropin-Releasing hormone (GnRH). GnRH binds to receptors in the Anterior Pituitary Gland that result in the release of Luteinizing hormone (LH) and Follicle Stimulating Hormone (FSH). LH and FSH bind to receptors on the ovaries and stimulate the production and release of Estrogen and Progesterone. LH is also important for male sex hormones (testosterone) but that is beyond the scope of this article
These hormones will also act on the tissues that secrete their stimulating hormones in a system called negative feedback. Negative feedback reduces the amount of stimulating hormone that is released, thus the amount of hormone in the bloodstream is regulated by its own concentration. For example, estrogen can bind to receptors on the hypothalamus and the anterior pituitary to reduce the production of GnRH and FSH/LH respectively thus down-regulating their production and in turn down-regulating estrogen itself. This tightly regulated system has the ability to increase and decrease hormone levels depending on external factors and internal cyclical rhythms such as the menstrual cycle.
- Estrogen – Estrogen has 3 subtypes that exert similar physiological effects (ie: estradiol, estrone and estriol) but estradiol is the primary hormone we will focus on in humans. Estrogen binds to estrogen receptors (ER) that are located all over the body. Each of these receptors is located within or on a target cell. When activated this receptor will enact a certain mechanism depending on the cell it’s located in. Estrogen receptors are located all over the body such as in the brain, in the muscular tissue, and on organs. Estrogen is primarily responsible for the stimulation of breast development, female fat deposition and other secondary sex characteristics. Estrogen also has influences on the musculoskeletal system (ligaments, tendons, bone and muscle) thus resulting in the hypothesis that it may influence training performance and outcomes. Further, estrogen is implicated in multiple diseases (cardiovascular disease, osteoporosis, lupus, metabolic disorder, obesity, cancer, endometriosis, uterine fibroids etc.) thus indicating not only the importance of estrogen in the body but the complexity of its action on physiology. Hamilton et al., 2017
- Of note, estrogen doesn’t float around the body freely. Instead, it is bound to proteins like albumin and sex hormone binding globulin (SHBG). When estrogen is bound to, it can be used by the body, but when it is bound to SHBG it is not available.
- Progesterone – Progesterone is released by the ovary. This hormone is responsible for preparing the body for pregnancy. This hormone is actually a precursor for testosterone. Progesterone also has effects on MSK physiology.
- LH – is the main hormone responsible for ovulation. The rapid increase in LH concentration is the body’s signal to release the egg from the ovaries.
- FSH – is also important in sexual development. FSH promotes the growth of the egg in the ovaries before the release of the egg.
- Relaxin – A hormone related to tendon laxity that increases during the luteal phase of the menstrual cycle. This hormone is discussed more often in terms of injury reduction. Powerlifting and other gym sports have very low injury risk and thus this hormone is beyond the scope of this article.
There are other hormones that are involved in the development and maturation of the egg during the menstrual cycle, but for brevity, the most relevant hormones are progesterone and estrogen.
The Menstrual Cycle:
Now that we’ve introduced all the main characters, we can dive into the menstrual cycle. The menstrual cycle is a set of monthly changes a woman’s body goes through to release an egg and thicken the endometrium (lining) of the uterus to prepare for the possibility of implantation. The menstrual cycle is broken down into two main phases that are delineated by menses and ovulation. The follicular phase begins when menses (the period) begins and the uterine lining is shed. This is considered day 0 of the menstrual cycle. On average this phase lasts 12-14 days until ovulation occurs. Bull 2019 Ovulation is the release of the egg from the ovary and signals the start of the luteal phase where the body is spending energy preparing for the egg to implant into the lining of the uterus. This phase can last from 10-22 days. These two phases can further be broken down into early mid and late phases to more accurately describe the hormone fluctuations occurring.
The main hormones involved with the menstrual cycle are LH, FSH, estrogen, and progesterone, which fluctuate around 28 days on average, but can range from 18 to 40 days Fehring et al., 2006. Both menses and ovulation are triggered by changes in hormonal concentrations. LH and FSH spike right around the time of ovulation. Estrogen is highest during the end of the follicular phase and the middle of the Luteal phase. Progesterone is elevated through a large portion of the luteal phase.
The menstrual cycle does exhibit a decrease in estrogen levels during the early follicular phase. The reduction in progesterone is also diminished during this time. These are transient changes, and are quite different from the chronic reduction in estrogen and progesterone seen during menopause (more on menopause below).
These hormones have not only local effects on the reproductive system, but their effects also reach systemically via the blood-stream. Between women, there are fluctuations in the concentrations and timing of their hormones and even within one woman there will be changes from cycle to cycle. These individual differences between women are further compounded by the differences in responses women have to these hormone changes. For example, even two women who have the same length of cycle and have similar levels of fluctuation will have different symptoms (muscle pain, headache, bloating, breast tenderness, appetite changes, poor concentration) across the duration of the cycle.
Basal body temperature fluctuates across the menstrual cycle and is often used to track ovulation. Basal body temperature (BBT) is measured when a person gets up every morning. There is a small increase in body temperature around the time of ovulation. This change can indicate ovulation and suggest what phase of the menstrual cycle a woman might be in. Without actual hormone levels being measured, specifically estrogen and progesterone, we cannot determine with certainty which phase of a cycle a woman is in Janse de Jonge et al 2019.
Lastly, menstrual cycles can deviate from the above cited pattern Janse de Jonge et al 2019. Oligomenorrhea (irregular and inconsistent menstrual blood flow) and amenorrhea (absent menstruation during the reproductive years of a woman’s life) are indicators of health issues for women. Riaz et al 2020 Nawaz et al 2020 These health issues are a concern for performance and training and should be managed by a physician (MD or DO).
Due to their systemic effects and varying effects, there is a lot of research investigating effects of these hormones and their subsequent influence on women’s physiology. What is most important to recognize is how these hormone fluctuations actually affect training and performance. This is where the differentiation between statistically significant and clinically significant becomes meaningful. Statistical significance indicates the correlation between two changing variables is likely not due to random chance. Clinical significance is the likelihood that a change in variables results in change in treatment. For resistance training, we would need strong evidence that women reliably see better performance or results with different programming based on specific phases of the menstrual cycle.
This is a complicated question we’ll attempt to answer over the next four parts of this series. Importantly, the background knowledge described above lays the foundation for these upcoming articles and it may be beneficial to comb through this article a few times to become familiar with the physiology.