Female sex hormones estrogen and progesterone fluctuate monthly across the menstrual cycle, affecting moods and energy levels. New research from the University of Oregon finds that those fluctuations don't change a woman's ability to exercise hard, but they do influence how difficult that work feels.
The findings were published in the Journal of Applied Physiology.
The association between menstrual cycle hormonal fluctuations and exercise performance has been poorly understood, said the study's lead author Mira Schoeberlein, a third-year doctoral candidate in the Oregon Performance Research Lab in the UO's College of Arts and Sciences .
"Women have historically been very understudied in physiology, and especially exercise physiology, and even more so across our menstrual cycle," Schoeberlein said.
Hormones play a major role in the body's regulation, particularly in metabolism and energy levels. Many studies comparing exercise and female hormone levels have focused on the early follicular phase of the menstrual cycle, between the onset of bleeding up until ovulation, Schoeberlein said. Estrogen and progesterone are at their lowest during that time and more similar to men's hormone profile. (Men also produce low levels of progesterone and estrogen hormones). But those studies don't capture the effects of estrogen and progesterone rising and falling throughout the menstrual cycle.
Schoeberlein wanted to know how those changes affect an athlete's ability to maintain intense exercise under a specific threshold known as the maximal metabolic steady state. That is the boundary between exercise that can be sustained and exercise that is unsustainable, "the highest intensity that one can sustain while relying fully or primarily on oxidative energy production," commonly known as aerobic exercise, Schoeberlein said.
The boundary between sustainable and unsustainable exercise is particularly important for athletes because it's an ideal spot to train in. Elite marathon runners, for example, run their races just below that threshold.
"That [threshold] can be a very important performance indicator because if you're exercising above it, you cannot sustain exercise for very long," said Brad Wilkins , the director of the Oregon Performance Research Lab and an assistant professor of human physiology.
Schoeberlein and the research team recruited 15 women and 15 men from the Eugene area to participate in the study. For four weeks, participants came in weekly to use a stationary bike, biking at progressively harder levels during their workout. The aim was for the experiment to capture participants' highest level of exercise at a steady state.
Before each session, participants had their hormone levels measured using a blood sample. Throughout the session Schoeberlein measured various biological benchmarks such as heart rate, oxygen uptake and carbon dioxide production. Participants also reported how difficult their workout felt afterward. Ultimately, the maximal steady state exercise intensity didn't change throughout the study.
"That ability to go and work is still at the same level across the whole menstrual cycle," Schoeberlein said. Interestingly, it was the self-reported data that changed. Overall, women said their workouts felt more difficult when their progesterone was high. Progesterone peaks roughly a week after ovulation in the luteal phase, the second half of the menstrual cycle.
The team also found that both men and women were able to do similar intensities of work, after accounting for differences in total muscle mass.
"What you can sustain appears to be the same across the menstrual cycle, but how it feels might be different," Schoeberlein said. "If you can take those together to inform your training or your race, then you can probably maintain or optimize your performance."
This is the first study to look at a range of hormonal fluctuations affecting maximal metabolic steady state, not just one phase of the menstrual cycle, the authors say. It's also one of the few that included participants using multiple types of birth control, including IUDs and oral contraceptives.
The results underscore that there's no one-size-fits-all approach to training, Schoeberlein said. What works for one person at a certain point in their menstrual cycle might not work for someone else. Schoeberlein hopes the results can act as another tool in athletes' toolboxes and empower them to push themselves.
"How you feel still matters, but don't limit yourself," she said.
— By Jude Coleman
This research was supported by the Wu Tsai Human Performance Alliance at the University of Oregon.
