Debunking the Myths of Cycle-Sync Training in Female Endurance Athletes

The idea of cycle-sync training has become increasingly popular, particularly on social media. The concept is simple: that female endurance athletes should adjust training according to menstrual-cycle phases in order to improve performance. It is an attractive idea — but one that is often presented with more certainty than the evidence allows.

In my own research¹, alongside a growing body of literature^2,3, we have shown that key physiological determinants of endurance performance — including performance outcomes^2-4, metabolic responses^5-7, and autonomic regulation¹ — do not show consistent or meaningful variation across the menstrual cycle. This article explores what the evidence actually tells us, why cycle-sync narratives persist, and how female athletes and coaches can take a more robust, practical approach.

In this image, I am collecting a blood sample from an ultrarunner to measure lactate levels during a ramp test, alongside oxygen consumption, perceived effort, and heart rate at different phases of the menstrual cycle.

What Is A MENSTRUAL Cycle-Sync Training?

Cycle-sync training is based on the assumption that cyclical fluctuations in oestrogen and progesterone meaningfully alter physical performance. The menstrual cycle is typically divided into four phases:

• Menstrual phase (≈ days 1–5): low oestrogen and progesterone

• Follicular phase (≈ days 6–14): rising oestrogen

• Ovulation: peak oestrogen

• Luteal phase (≈ days 15–28): elevated progesterone with moderate oestrogen

  *Note that a normal menstrual cycle can range from 21 to 35 days; therefore, the timing and duration of each phase will vary between women.

  
  

Advocates of cycle-sync training often suggest aligning high-intensity or “hard” training with the follicular phase, while prioritising recovery or reduced load during the luteal phase.

The problem is that this framework assumes predictable physiological responses that are not consistently supported by data and women responses vary between each other.

What Does the Evidence Actually Show?

Multiple studies — including my own work in trained female endurance athletes — have examined whether menstrual-cycle phase influences performance or key physiological systems. The findings are remarkably consistent:

• No meaningful changes in endurance or strength performance across cycle phases

• Stable autonomic nervous system regulation, with no clinically relevant changes in heart rate variability or recovery markers

• No consistent differences in metabolic responses to exercise across the cycle

  
  

Importantly, intervention studies testing menstrual-cycle-based periodisation models have shown no performance advantage compared with traditional training approaches⁸. In other words, structuring training around cycle phases does not improve outcomes.

Perception vs Physiology

One important distinction that often gets lost online is the difference between perceived performance and physiological performance.

Across studies^9,10, including ours, many women report feeling worse in the days leading into menstruation or during menses. However, these perceptions are not accompanied by parallel changes in objective physiological markers or performance outcomes.

This mismatch matters. If we assume that feeling worse automatically means performing worse, we risk unnecessarily down-regulating training, reinforcing negative expectations, and attributing normal day-to-day variability to the menstrual cycle alone.

In my study, participants completed questionnaires to understand how they perceived their performance across the menstrual cycle, as well as a wellbeing questionnaire in each phase to assess changes in symptom patterns.

Why the Cycle-Sync Narrative Persists

Cycle-sync training is often supported by anecdotal experiences, selective interpretation of small studies, or simplified hormone explanations. These narratives are compelling — particularly when they appear to validate lived experience — but they do not reflect the full scientific picture.

The menstrual cycle is complex, and responses vary widely between individuals. Add to this factors such as sleep, energy availability, psychological stress, illness, and life load, and it becomes clear why attributing performance changes solely to cycle phase is overly reductionist.

This is a classic case of confirmation bias: once athletes expect a certain phase to feel “bad,” any dip in motivation or performance is more likely to be explained through that lens.

Individual Differences Matter Most

None of this means the menstrual cycle is irrelevant. Some athletes experience significant symptoms — pain, fatigue, heavy bleeding, mood disruption — that do affect training quality.

The key distinction is this: symptom management and individual responsiveness matter far more than calendar-based phase prescriptions.

A more appropriate approach includes:

• Tracking symptoms and responses over time

• Adjusting training based on how the athlete is actually coping, not the predicted phase

• Prioritising nutrition, recovery, and sleep during more challenging periods

• Referring to healthcare professionals when symptoms are severe or disruptive — this is not normal and may indicate conditions such as endometriosis or PCOS

Practical Guidance for Athletes and Coaches

For female endurance athletes and those coaching them:

• Focus on consistent, progressive training aligned with performance goals

• Use subjective feedback and readiness, not cycle phase alone, to guide day-to-day decisions

• Avoid rigid cycle-based training rules

• Educate athletes about the menstrual cycle while reinforcing that it is one variable among many

• Support symptom management through appropriate fuelling, hydration, and recovery strategies

  
  

This flexible, athlete-centred approach builds resilience rather than fragility.

The Role of Symptom Management

Where menstrual symptoms are pronounced, targeted management can meaningfully support training continuity. Strategies may include:

• Heat therapy or gentle mobility work for cramping

• Temporary volume adjustments during heavy bleeding days

• Monitoring iron intake and status where indicated

• Stress-management strategies to reduce overall physiological load

  
  

There is also emerging evidence that chronic omega-3 intake may help reduce menstrual symptoms¹¹, although this requires consistent intake over several months, given the time needed for fatty acids to incorporate into cell membranes.

The Bottom Line

Cycle-sync training is an attractive idea, but it is not supported by strong evidence as a performance-enhancing strategy for female endurance athletes. Physiology remains largely stable across the menstrual cycle, even when perception fluctuates.

Rather than prescribing training based on hormonal calendars, the priority should be individualised coaching, symptom management, and flexible decision-making grounded in real-time feedback. This respects both the complexity of female physiology and the long-term development of resilient, high-performing athletes.

I'm an endurance coach specialised in long-distance races and women physiology, so if you need help preparing for your next race, get in touch.

references:

1. Molina-Gonzalez, I. & Gray, S. R. Physiological and Perceptual Responses Across Menstrual Cycle Phases in Female Ultra-Endurance Athletes: A Pilot Study. medRxiv, 2025.2011.2013.25340155 (2025). https://doi.org/10.1101/2025.11.13.25340155

2. Taylor, M. Y. et al. Menstrual Cycle Phase Has No Influence on Performance-Determining Variables in Endurance-Trained Athletes: The FENDURA Project. Med Sci Sports Exerc 56, 1595-1605 (2024). https://doi.org/10.1249/mss.0000000000003447

3. Docter, H. et al. Running Economy After a Low- and High-Intensity Training Session in Naturally Menstruating Endurance-Trained Female Athletes: The FENDURA Project. Scandinavian Journal of Medicine & Science in Sports 35, e70050 (2025). https://doi.org/https://doi.org/10.1111/sms.70050

4. Goldsmith, E. & Glaister, M. The effect of the menstrual cycle on running economy. J Sports Med Phys Fitness 60, 610-617 (2020). https://doi.org/10.23736/s0022-4707.20.10229-9

5. Matsuda, T. et al. Influence of menstrual cycle on muscle glycogen utilization during high-intensity intermittent exercise until exhaustion in healthy women. Applied Physiology, Nutrition, and Metabolism 47, 671-680 (2022). https://doi.org/10.1139/apnm-2021-0532 %M 35856390

6. Bailey, S. P., Zacher, C. M. & Mittleman, K. D. Effect of menstrual cycle phase on carbohydrate supplementation during prolonged exercise to fatigue. Journal of Applied Physiology 88, 690-697 (2000). https://doi.org/10.1152/jappl.2000.88.2.690

7. Ishikawa, A. et al. Effects of the menstrual cycle on EPOC and fat oxidation after low-volume high-intensity interval training. J Sports Med Phys Fitness 63, 1165-1174 (2023). https://doi.org/10.23736/s0022-4707.23.15209-1

8. Kubica, C., Ketelhut, S. & Nigg, C. R. Polarized running training adapted to versus contrary to the menstrual cycle phases has similar effects on endurance performance and cardiovascular parameters. European Journal of Applied Physiology 124, 3433-3444 (2024). https://doi.org/10.1007/s00421-024-05545-9

9. Jones, B. P. et al. Menstrual cycles and the impact upon performance in elite British track and field athletes: a longitudinal study. Frontiers in Sports and Active Living 6 (2024). https://doi.org/10.3389/fspor.2024.1296189

10. Bruinvels, G. et al. Prevalence and frequency of menstrual cycle symptoms are associated with availability to train and compete: a study of 6812 exercising women recruited using the Strava exercise app. Br J Sports Med 55, 438-443 (2021). https://doi.org/10.1136/bjsports-2020-102792

11. Behboudi-Gandevani, S., Hariri, F.-Z. & Moghaddam-Banaem, L. The effect of omega 3 fatty acid supplementation on premenstrual syndrome and health-related quality of life: a randomized clinical trial. Journal of Psychosomatic Obstetrics & Gynecology 39, 266-272 (2018). https://doi.org/10.1080/0167482X.2017.1348496