Blood flow restriction to improve performance and injury recovery
Blood flow restriction or BFR has become popular in the last few years in the training and physical therapy field.
High performance sport is becoming more equal, and women participate as much as men in most disciplines. Nevertheless, scientifically speaking, most studies still focus on men to carry out research on sports performance, so there is much less evidence regarding women’s exercise physiology. This is of great importance, given that the results observed in men may not directly apply to women as they have different physiological characteristics, the menstrual cycle being one of them.
The menstrual cycle involves important variations in hormones, such as oestrogens or progesterone, which allows us to simply differentiate three different phases: the follicular phase (characterized by low levels of oestrogens and progesterone, which begins with the menstrual bleeding), the ovulatory phase (characterized by high levels of oestrogens and low levels of progesterone), and the luteal phase (characterized by high levels of both oestrogens and progesterone) (see Figure 1). Although variations of these hormones mainly benefit reproduction, they are likely to affect variables related to sports performance as well. In fact, oestrogens have shown to cause anabolic effects on the skeletal muscle, while progesterone seems to cause opposite effects.1
Therefore, it is normal that there’s a concern over the potential effects of the menstrual cycle on sports performance. But what can science tell us about this? To answer this question, a systematic review has recently been published in the prestigious Sports Medicine journal.2 The authors analysed studies involving adult eumenorrheic women (i.e. with a normal menstrual cycle) who did not take contraceptives, and comparing the effects on sports performance between the early follicular phase (taken as reference) and the other phases of the menstrual cycle. In the end, the authors found 76 studies with these characteristics that included more than a thousand participants. When generally meta-analysing those studies, results showed non-significant, trivial effects (ES = -0.06) on performance during the early follicular phase in comparison to the rest of the phases, finding some studies that showed better performance during some phases and others during different ones. Moreover, they found similar results when analysing the markers of strength and endurance performance separately.
As this meta-analysis concludes there is not enough evidence to support a significant effect of the menstrual cycle on performance, it has been proposed that performance in the early follicular phase may be affected due to the low levels of progesterone and oestrogen of this phase. In fact, due to the potential benefits of oestrogens at a muscle level (producing anabolic effects and increasing the antioxidant response and the stabilisation of muscle fibre membranes, which could reduce muscle damage) the benefits on performance could potentially appear during the late follicular and ovulatory phases, when the levels of progesterone are low. However, as we have seen, the evidence does not consistently support that the menstrual cycle has an influence on performance.
Considering the potential negative effect of low oestrogen levels on performance, another factor that could be argued to be detrimental in athletes are oral contraceptives (prescribed not only as a contraceptive measure but also to regulate the menstrual cycle and alleviate symptoms of dysmenorrhea and menorrhagia) as they decrease the endogenous production of oestrogens and progesterone. In this sense, the same research group of the meta-analysis mentioned above published another meta-analysis in the same journal assessing the effects of contraceptives on performance.3 For that, they included studies comparing the performance of women who took contraceptives and women who did not. In general, they did not find any differences, and if they had done, they would have been trivial differences (in favour of those women who did not take contraceptives). Moreover, the authors analysed studies comparing the same participant on and off contraceptives (both because they stopped taking them and they were in a phase they did not have to), and the effects were again trivial and non-significant. Finally, the authors tried to find randomised clinical trials comparing contraceptives and placebo, and they could only find one study. It is worth highlighting that, in that study, taking contraceptives was related to a worse aerobic capacity (maximum oxygen consumption) and an increase of skin folds, but there were no differences in other markers for strength, speed or endurance.4
As we can see, despite the potential effects of variations in hormones such as oestrogen and progesterone during the menstrual cycle or when taking contraceptives, current evidence does not consistently show any significant effects on performance. This does not mean that the menstrual cycle should not be controlled. In fact, this will be important to analyse its regularity and to identify potentially stressing situations affecting it. Also, what most studies have in common is the heterogeneity observed among the different participants. Therefore, although there were no significant effects in general, the individualisation of training and competition should be prioritised in order to adapt the stimuli to each athlete according to their conditions at any given time.
Pedro L. Valenzuela