Does Menstrual Cycle Phase Affect Exercise Performance?

Tokyo 2021 is thought to be the most gender-balanced Olympic Games in history, with 49% of athletes being female. Comparing this to 1984, when just 23% of athletes were female, it is obvious to see that female participation in sport has never been better. ¹ Despite this, due to a combination of conflicting evidence and low-quality research, no evidence-based guidelines currently exist regarding the impact of the menstrual cycle on exercise performance.

A recently published meta-analysis by McNulty et al. ² has rightly received a lot of attention in the sports science world. It set out to establish if there were any differences in exercise performance over the different phases of the menstrual cycle, in order to provide evidence-based recommendations to eumenorrheic (regularly menstruating) women. This article is going to take a look at the findings of this meta-analysis and discuss what it means for eumenorrheic female athletes. But first let’s define what we mean by menstrual cycle phase.

The Menstrual Cycle

The menstrual cycle usually lasts somewhere around 28 days. When studying the cycle, it is usually divided into 3 distinct phases that can be identified by the concentration of the ovarian hormones: oestrogen and progesterone. Throughout the 3 phases (early follicular, ovulatory and mid-luteal) levels of oestrogen and progesterone vary in line with their primary role in the reproductive system. Table 1 describes the fluctuations of these hormones over the course of the menstrual cycle. Note that the 3 phases do not cover the entirety of the cycle; the unmarked days correspond to the transitions between each phase.

Phase	Days of Cycle	Oestrogen	Progesterone
Early follicular	1-5	Low	Low
Ovulatory	13-15	High	Low
Mid-luteal	20-23	High	High

It is currently understood that oestrogen has influence over certain factors that could potentially moderate exercise performance. ² Findings have suggested that higher levels of oestrogen lead to increased glycogen storage and fat utilisation, therefore promoting endurance performance. ³ As well as this, oestrogen is considered to have beneficial effects on muscle strength by increasing the quality of the muscle. ⁴

On the other hand, progesterone appears to have anti-oestrogenic effects. This means that when levels of both oestrogen and progesterone are high during the mid-luteal phase, the positive effects of oestrogen may be downplayed due to the presence of progesterone. ^2,3 However, it is still uncertain whether these factors actually lead to improved exercise performance in a specific phase, as there have been many conflicting findings.

The Study

For any readers who are unfamiliar with scientific methodology, a meta-analysis is a type of study design that involves the examination of data from a large number of studies in a particular subject to determine the general trend of their findings. In this case, McNulty et al. ² found 78 studies that fit the inclusion criteria of their analysis, meaning that in total, 1193 participants were included in the review. All women were healthy, eumenorrheic and not taking any hormonal contraceptives. No restrictions were placed on training status or physical activity levels.

Each of the included studies had made comparisons between the phases of the menstrual cycle and measures of exercise performance. These measures were generated from a number of performance tests, such as total work done, time to completion, time to exhaustion, and rate of force production. VO_2max was also included as an outcome because it is solid indicator of exercise performance.

The Findings

The results of the meta-analysis found that exercise performance was very slightly reduced in the early follicular phase, when concentrations of both oestrogen and progesterone are low. There were no differences in performance between the other menstrual cycle phases. Because the effect size was so small, the paper concluded that it is unlikely to be of any meaning to the general population. Nevertheless it could be worth considering with elite athletes where the difference between winning and losing is minute.

However, it needs to be emphasised that overall the quality of evidence in this meta-analysis was classified as low. Only 8% of the included studies were of high quality, 24% moderate, 42% low and 26% very low. There was also large variation between the studies, potentially as a result of varying methodology or participant characteristics.

To conclude, despite the small reduction in exercise performance in the early follicular phase, no general guidelines can be extrapolated from these findings. Instead the authors suggest taking an individual approach that is both informed by, and tailored to the athlete.

References

1. The International Olympic Committee. Promotion of women in sport through time. https://www.olympic.org/women-in-sport/background. Accessed 18 Aug 2020.
2. McNulty KL, Elliott-Sale KJ, Dolan E, et al. The Effects of Menstrual Cycle Phase on Exercise Performance in Eumenorrheic Women: A Systematic Review and Meta-Analysis [published online ahead of print, 2020 Jul 13]. Sports Med. 2020;10.1007/s40279-020-01319-3. doi:10.1007/s40279-020-01319-3
3. Oosthuyse T, Bosch AN. The effect of the menstrual cycle on exercise metabolism: implications for exercise performance in eumenorrhoeic women. Sports Med. 2010;40(3):207-227. doi:10.2165/11317090-000000000-00000
4. Lowe DA, Baltgalvis KA, Greising SM. Mechanisms behind estrogen’s beneficial effect on muscle strength in females. Exerc Sport Sci Rev. 2010;38(2):61-67. doi:10.1097/JES.0b013e3181d496bc

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