menstruation

Menstruation: Cycling proficiency - do the various menstrual phases affect athletic performance?

The female body is a complicated system, with hormones playing an important role in its function. Within that system, the menstrual cycle represents a combination of interactive, and sometimes opposing, hormonal actions with the potential to impact upon the health of an exercising female. Therefore, any coach involved in training female athletes needs a clear understanding of the physiology of menstruation and its effects on performance.

Female athletes are often concerned about the number of days in their cycle and the volume of menstrual flow. In fact, the average cycle runs for the oft-quoted 28 days in less than 15% of cases, with normal regular menses encompassing anything from 21-35 days. The duration of menstruation has also been shown to vary widely, with normal flow lasting for an average of 3-7 days.

The cycle, which begins with menstruation, is regulated by a complex interaction of pituitary and ovarian hormones. In the ‘follicular phase’ (the days leading up to ovulation) oestrogen is the predominant hormone; following ovulation, during the ‘luteal phase’ of the cycle, progesterone exceeds oestrogen, preparing the uterus for pregnancy.

Researchers have shown that physiological changes occur in both the follicular and luteal phases and that both oestrogen and progesterone can be altered by intense exercise. Therefore, the influence of menstrual phase is important to our understanding of performance and training.

Although no significant changes to endurance performance have been reported in connection with the various phases of the menstrual cycle, a slight decrease in aerobic capacity has been noted during the luteal phase. This phase is characterised by an increased ovarian response, leading to a net fluid retention, with consequent changes in electrolytes and minor increases in haemoglobin concentration, for reasons explained below(1).

This same research also failed to identify any significant differences in VO2max, heart rate, cardiac output and stroke volume between menstrual phases, suggesting that the cardiovascular response to exercise is unaffected by the menstrual cycle.

One difference the researchers did discover was that blood lactate was lower in the luteal phase following intense exercise, although this has been disputed by subsequent research(2). It has been suggested that any observed decrease in lactate production during the luteal phase happens as a result of an altered oestrogen: progesterone balance.

Ventilatory response to exercise has been shown to increase during the luteal phase of the menstrual cycle, which is associated with a reduced oxygen supply to working muscles(3). The body compensates for this deficiency by boosting the concentration of oxygen-carrying haemoglobin in the blood, and no differences have been reported in either oxygen uptake or time to fatigue between the follicular and luteal phases.

Research into the effects of the menstrual cycle on anaerobic exercise performance is limited and inconclusive. Decreased cycle (Wingate anaerobic power test) and 50m swim test performances have been reported during menstruation, while an increase in high intensity/low duration work has been noted during the luteal phase of the menstrual cycle(4). More recently, investigators demonstrated no significant difference in maximal cycling or leg power during any of the menstrual phases(5), although it has been suggested that premenstrual and menstrual syndrome symptoms, such as joint, muscle and back pain, may have a negative effect on anaerobic performance, possibly by influencing the stretch shortening cycle of the tendons and ligaments.

Common menstrual disorders

Menstrual disorders are very common among athletic women, with amenorrhea (cessation of periods) occurring in up to 40% of some athletic groups by comparison with 2-5% of women of reproductive age in the general population(6). Amenorrheic athletes show no monthly follicular and luteal phase variations, resulting in ovarian suppression.

Bone density generally relates closely to menstrual regularity and the total number of menstrual cycles. Reduced bone density from long-term amenorrhea often occurs at multiple sites, including bone areas subjected to increased force and impact loading during exercise. Persistent amenorrhea that begins at an early age blunts the benefits of exercise on bone mass and is also linked with increased risk of musculoskeletal injury, particularly repeated stress fractures during exercise.

Disturbances in menstrual function can be seen in the form of primary amenorrhea (absence of menstruation by age 18), secondary amenorrhea (absence of menses for three or more months in a woman who was formerly menstruating) or oligomenorrhea (a menstrual cycle lasting more than 36 days).

A high incidence of secondary amenorrhea has been reported for athletes in sports associated with heavy training levels, while girls who begin athletic training before puberty often experience delayed onset of menstruation (menarche).

The menarche can also be delayed on account of inadequate caloric intake which, in many cases, is related to issues of weight control and aesthetic appearance. Attempts to control weight are associated with disordered eating which, in turn, can lead to reproductive dysfunction. It has been suggested that a minimum percentage of body fat is required for the onset of menstruation and the maintenance of a regular cycle, based on the hypothesis that an energy reserve is essential to sustaining a pregnancy.

Even in regularly menstruating competitive athletes, lower-than-expected progesterone levels during the luteal phase have been observed. Over time this can lead to impaired fertility and/or an increased susceptibility to osteoporosis.

A proactive approach

The American College of Sports Medicine (ACSM) has taken a proactive approach to menstrual dysfunction in athletes, recommending intervention within three months of the onset of amenorrhea. Their guidelines recommend a nonpharmacological behavioural approach, together with diet and training interventions, as follows:

  • Reduce training level by 10 to 20% ;
  • Gradually increase total energy intake;
  • Increase body weight by 2-3%;
  • Maintain daily calcium intake at 1,500mg.

Skin blood flow and the sweating response during rest and activity are also influenced by the menstrual cycle. Scientists have found that a significantly higher core temperature is required to initiate sweating during the luteal phase(7). Although this change in thermoregulatory sensitivity does not affect the ability to exercise, it is worth taking account of menstrual cycle phase when evaluating thermoregulatory dynamics during exercise and thermal stress.

In summary, then, the majority of published studies agree that neither menstrual phase (follicular v luteal) nor menstrual status (menstruating v non-menstruating) significantly alters or limits exercise performance.

However, the combination of intensive exercise (particularly during the pre-pubertal years) and under-nutrition can have an adverse impact on reproductive function and sexual maturation, leading to either primary or secondary amenorrhea.

If an athlete does become amenorrheic, medical treatment should be considered in order to maintain long-term health and reduce the risk of fractures.

Andy Harrison

References

  1. J Appl Physiol 1981; 51 (6): 1493-1499
  2. J Sports Med Phys Fitness 1995; 35 (4): 257-262
  3. Med Sci Sports Exerc 1990; 22 (5): 575-580
  4. Can J Appl Sports Sci 1978; 3 (4): 194
  5. Med Sci Sports Exerc 2000; 32 (2): 486-492
  6. Br J Sports Med 2003; 37: 490-494
  7. Physiologist 1985; 28: 368

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