Metabolism
Estrogen shifts metabolism toward a greater reliance on fat when you run at a submaximal pace, which spares your muscles' limited store of carbohydrate. Because your ability to run for long periods is influenced by the amount of glycogen in your muscles, by sparing the amount used and relying more on fat for energy, fatigue is delayed and your endurance is improved.
Conversely, during short, fast runs, estrogen promotes the use of glucose by your slow-twitch muscle fibers, providing the fuel of choice for intense running.
Muscle Glycogen
Your ability to run for a long time is greatly influenced by the amount of glycogen stored in your skeletal muscles, with fatigue coinciding with glycogen depletion. Research comparing the amount of muscle glycogen in women eating either a normal diet (2.4 grams of carbohydrate per pound of body weight per day) for three days or a high carbohydrate diet (3.8 grams of carbohydrate per pound of body weight per day) for three days has shown that muscle glycogen content is greatest during the mid-luteal phase (when progesterone is high) after both normal and high carbohydrate diets. Muscle glycogen is lowest during the mid-follicular phase (when estrogen is high).
Although carb-loading has become common practice among runners in the days leading up to a long race, women runners need to increase their total caloric intake as well as their percentage of carbohydrate calories to obtain the same carb-loading effect as men.
Breathing
Progesterone stimulates breathing independent of the intensity of your run, which can increase your perception of effort since runners typically link their perception of effort to how much they're breathing. Breathing is greater during the luteal phase, when progesterone concentration is highest, which means you may feel more winded during your luteal phase workouts compared to your follicular phase workouts.
The progesterone-driven increase in breathing may also increase the oxygen demand of breathing itself since the muscles responsible for breathing need oxygen to work just like your leg muscles do. More oxygen being used by your breathing muscles means less oxygen available to your leg muscles to help you run.
Women runners with asthma often experience a worsening of asthma symptoms and increased bronchodilator use during the mid-luteal phase, when progesterone is at its highest. Interestingly, lung function and asthma symptoms seem to vary cyclically. Thirty-three to 52 percent of asthmatic women report a premenstrual worsening of asthma symptoms, and an additional 22 percent report that their asthma is worse during their period.
More: 6 Tips for Women Running With Asthma
Muscle Strength
Estrogen may influence your ability to get stronger. Research has shown that women's responsiveness to strength training is influenced by the menstrual cycle. One study found that weight training consisting of three sets of 12 repetitions every second day during the follicular phase and once per week during the luteal phase of the menstrual cycle increased maximal quadriceps strength by 32.6 percent compared to just 13.1 percent by training once every third day over the whole menstrual cycle. The ratio of maximal strength to muscle cross-sectional area was also greater following the former training program that capitalized on the estrogen-dominant follicular phase compared to the latter, more traditionally-used program (27.6 vs. 10.5 percent).
Endurance Performance
Although there is a lot of variability between women, in general, you can expect to perform better during times of the menstrual cycle when estrogen is the dominant hormone and perform the worst when progesterone is the dominant hormone.
Many women have their best races in the few days immediately following their periods, with their worst races occurring during the few days immediately prior to and including their periods. You may find that, while harder workouts may be more challenging during your period, easy running may actually improve your mood and alleviate physical symptoms associated with your period.
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