Menstrual Cycle Effects on Endurance Performance

Men and women with similar lean muscle mass have an identical power ceiling for sustainable, steady-state exercise. This is the central finding from a new University of Oregon study that directly measured the influence of sex hormones on a key endurance performance marker.

### The Maximal Metabolic Steady State: A Critical Endurance Boundary

The maximal metabolic steady state (MMSS) is the highest exercise intensity you can maintain while keeping your body’s physiological responses—like blood lactate and muscle oxygen use—stable. It marks the boundary between the heavy exercise domain (which is sustainable) and the severe domain (which leads to rapid fatigue). For endurance athletes, identifying this intensity is fundamental for setting training zones, pacing long events, and improving metabolic efficiency.

Researchers from the Oregon Performance Research Lab, led by Schoeberlein, Hudgins, DeVelasco, and Wilkins, designed a study to test if this critical boundary shifts with biological sex or with the natural hormonal changes women experience.

### Measuring Hormones and Muscle Oxygenation

The team recruited 30 endurance-trained cyclists and runners, evenly split between men and women. The women’s average VO2 max was 48.1 mL/kg/min, while the men’s was 57.3, a common difference largely attributed to body composition. The novel aspect was the measurement protocol. Instead of relying on menstrual cycle phase, the scientists took direct blood samples at each visit to quantify serum concentrations of estradiol, progesterone, and testosterone.

To find each participant’s MMSS at four distinct hormone profiles over four weeks, they used a non-invasive muscle oxygenation (%SmO2) “zero-slope” protocol. Participants cycled through four 4-minute stages at increasing intensities, with rest in between. By plotting the slope of the muscle oxygen signal during each stage against the work rate, researchers could pinpoint the exact power output where oxygen extraction became stable—the MMSS.

### No Sex Difference in Muscle-Specific Power

The initial analysis confirmed that men achieved a higher absolute power output at the MMSS. However, when the researchers normalized this power to each athlete’s lean body mass (LBM)—the weight of their muscle and organs—the sex difference vanished. The LBM-normalized work rate at the MMSS was statistically similar between men and women.

“Male sex was a significant predictor of power at MMSS, but no sex differences in MMSS were found when normalized to lean body mass,” the authors reported. This means the higher absolute output in men is primarily a function of having more muscle mass, not an inherent physiological advantage at the muscle tissue level for sustaining effort.

### Hormone Fluctuations Do Not Alter Sustainable Intensity

A major question for female athletes has been whether performance capability changes with hormonal cycles. This study provides a clear answer regarding steady-state endurance: it does not. The linear mixed models showed that changes in estradiol and progesterone concentrations were not associated with any change in the MMSS work rate. The boundary between sustainable and unsustainable intensity remained fixed across the four testing visits, despite significant hormonal variation.

“Fluctuations in sex hormone profile in women were not associated with differences in the heavy to severe exercise domain boundary,” the authors concluded. This finding challenges a common assumption and suggests that the body’s fundamental metabolic steady state is resilient to these hormonal shifts.

### Practical Implications for Training and Performance

For coaches and athletes, these findings simplify training program design. The core principle of training at or just below the MMSS to build endurance applies equally to men and women. Women do not need to plan their moderate- to high-intensity steady-state sessions around specific menstrual cycle phases based on the goal of sustaining a metabolic steady state.

The research highlights the importance of focusing on individualized metrics over generalizations. An athlete’s sustainable power is best understood relative to their own physiology, particularly their muscle mass. This supports the use of functional threshold power (FTP) tests or critical power assessments as reliable tools for both sexes.

Furthermore, the study underscores that differences in absolute endurance performance between sexes are largely structural. This aligns with the physiological principle that muscle mass is a primary determinant of force production and sustainable power. For a deeper understanding of how different training intensities affect overall fitness, our guide on HIIT vs. moderate exercise for health outcomes explores the distinct benefits of various training zones.

### A Foundation for Future Research

This work, published in the *Journal of Applied Physiology* (PMID: 41460237), moves the field forward by using direct hormone measurement instead of estimating cycle phase. The authors note this method is important for accurately assessing hormonal impact. While the MMSS appears stable, other performance elements like recovery rate, perceived exertion, or substrate use could still be influenced by hormones, warranting further study.

For endurance athletes, the message is empowering and straightforward. The ceiling for your longest, hardest sustainable efforts is determined by your muscle’s capability, not your sex. By focusing on proven methods to increase lean mass and metabolic efficiency—such as those outlined in our definitive VO2max training guide—you can systematically raise that ceiling. This research adds to a growing body of evidence, like the benefits discussed in our guide to MICT for cardiovascular health, that effective training principles are fundamentally human, not gendered.