The menstrual cycle, characterized by fluctuating levels of estrogen and progesterone, significantly influences various physiological processes in women. These hormonal changes not only prepare the uterus for pregnancy but also impact other aspects of body homeostasis, including exercise performance and fatigability. A scientific review by Hugo M. Pereira, Rebecca D. Larson, and Debra A. Bemben explores how different phases of the menstrual cycle affect exercise-induced fatigability and the implications of these findings for women's health and fitness.

Hormonal Fluctuations and Fatigability

The menstrual cycle consists of the follicular phase, ovulation, and the luteal phase, each characterized by distinct hormonal profiles:

• Follicular Phase: Low levels of estrogen and progesterone mark the early follicular phase, while estrogen levels peak in the late follicular phase.
• Luteal Phase: After ovulation, both estrogen and progesterone levels rise, reaching their peak during the mid-luteal phase before declining towards the end of the cycle.

These hormonal variations influence motor output during fatiguing exercises. Estrogen is known to enhance vascular flow and glycogen utilization, while progesterone increases ventilation and body temperature. These effects suggest that exercise performance and perceived exertion could vary across the menstrual cycle.

Research Findings on Fatigability

The review analyzed 46 experimental studies on eumenorrheic women, examining metrics of motor output during both the follicular and luteal phases. Key findings include:

Statistical Differences in Fatigability:

• Out of the 46 studies, 15 reported statistical differences in fatigability between the follicular and luteal phases.
• Of these, seven studies found less fatigability during the luteal phase, while eight reported less fatigability during the follicular phase.
• The variability in results highlights the complex interplay between hormonal fluctuations and exercise performance.

Effect Sizes:

• The effect sizes, which measure the magnitude of differences, varied significantly across studies.
• The range of effect sizes (-6.77 to 1.61) indicates considerable inconsistency, reflecting the diverse methodologies and conditions of the studies.

Influence of Task Type:

• The type of task performed (isometric vs. dynamic) significantly influenced the results.
• For example, some studies found greater endurance during isometric intermittent contractions in the mid-luteal phase, while others showed no significant differences for sustained isometric contractions.

Upper vs. Lower Extremity:

• Differences were also noted based on whether the upper or lower extremity muscles were used.
• Studies on upper extremity muscles (e.g., handgrip exercises) showed mixed results, with some indicating greater endurance during the follicular phase and others during the luteal phase.
• For lower extremity muscles (e.g., knee extensors), some studies reported greater time to task failure during the luteal phase, while others found no significant differences.

Maximal Strength and Endurance:

• Changes in maximal strength across the menstrual cycle were examined, with mixed findings.
• Four studies out of the retrieved pool showed statistical differences in maximal strength between the phases, with variations depending on the limb involved and the task performed.

Cycling and Running Performance:

• For cycling, endurance time during a time trial or constant-intensity test showed no significant difference between the phases in most studies.
• However, one study indicated a 50% longer time to task failure at 90% VO2 peak during the luteal phase.
• Running performance also varied, with some studies showing longer endurance times during the luteal phase at supramaximal intensities, while others found no significant differences.

Ratings of Perceived Exertion (RPE):

• The effect of the menstrual cycle on perceived exertion during exercise was inconsistent.
• Some studies found higher RPE during the luteal phase, while others reported no significant differences between phases.
• Psychological factors and individual perceptions likely contribute to these mixed results.

The inconsistencies in results may stem from variations in study designs, methods for determining menstrual cycle phases, and the specific muscles or tasks assessed.

Potential Mechanisms

Several mechanisms could explain the observed variations in exercise-induced fatigability across the menstrual cycle:

• Metabolism: Estrogen may enhance glycogen sparing, potentially improving endurance during the luteal phase. However, the metabolic response differs between upper and lower extremity muscles.
• Temperature Regulation: Progesterone increases resting body temperature, which could affect performance in heat-sensitive tasks. This temperature increase might lead to higher perceived exertion and fatigability during the luteal phase.
• Psychological Factors: Perceptions of exertion, influenced by hormonal fluctuations, could also impact performance. Progesterone, in particular, has been associated with changes in mood and perception.

Application to Cycle Syncing: Exercise and Workouts

The concept of cycle syncing aligns lifestyle, nutrition, and exercise routines with the phases of the menstrual cycle to optimize health, performance, and well-being. Understanding the effects of hormonal fluctuations on exercise-induced fatigability allows women to tailor their fitness routines effectively. This section expands on how the findings from the review on exercise-induced fatigability can be applied to cycle syncing.

Early Follicular Phase/Period (Days 1-7):

• Hormonal Profile: Low levels of estrogen and progesterone.
• Exercise Recommendations: Engage in gentle activities such as yoga, stretching, and light cardio. These activities are beneficial during menstruation when energy levels might be lower, and the body needs to recover.
• Benefits: These exercises help alleviate menstrual cramps, reduce overall fatigue, and maintain flexibility. They also support mental well-being by releasing endorphins, which can improve mood and reduce stress.

Mid to Late Follicular Phase (Days 8-14):

• Hormonal Profile: Rising estrogen levels, peaking just before ovulation.
• Exercise Recommendations: This period is ideal for high-intensity workouts, including strength training, HIIT (high-intensity interval training), and endurance exercises. The body is more resilient to stress and recovers more efficiently.
• Benefits: Increased estrogen levels enhance muscle strength, endurance, and recovery. Women may experience higher energy levels and improved performance, making this phase suitable for pushing fitness goals and achieving personal bests.

Ovulation: Around Day 14

• Hormonal Profile: Peak estrogen levels with a surge in luteinizing hormone (LH).
• Exercise Recommendations: Continue with high-intensity workouts but be mindful of how your body feels. It’s a good time to challenge yourself with personal bests in strength and endurance exercises.
• Benefits: Peak hormone levels contribute to optimal performance, agility, and strength. The high energy levels and motivation can be harnessed for competitive sports or achieving new fitness milestones.

Early Luteal Phase (Days 15-21):

• Hormonal Profile: High levels of both estrogen and progesterone.
• Exercise Recommendations: Focus on moderate-intensity exercises such as steady-state cardio, moderate weight lifting, and Pilates. Include flexibility and balance exercises to complement your routine.
• Benefits: Moderate exercises help maintain fitness levels and support hormonal balance. The body can still handle significant physical activity, but it’s essential to pay attention to signs of overtraining.

Mid to Late Luteal Phase (Days 22-28):

• Hormonal Profile: Progesterone levels remain high, while estrogen starts to decline.
• Exercise Recommendations: Shift to lower-intensity activities like walking, light yoga, and swimming. Ensure adequate hydration and focus on nutrition to support energy levels and mood.
• Benefits: Low-intensity exercises help manage PMS symptoms, including bloating, fatigue, and mood swings. They also promote relaxation and reduce stress, essential as the body prepares for menstruation.

Practical Tips for Cycle Syncing

• Nutrition: Adjust your diet to support hormonal changes. Increase iron intake during menstruation, and focus on complex carbohydrates and proteins during the luteal phase to manage energy levels and mood. Incorporate foods rich in magnesium and B vitamins to help reduce PMS symptoms.
• Hydration: Maintain optimal hydration, especially during the luteal phase, to manage increased body temperature and prevent fatigue. Drinking enough water can also help reduce bloating and improve overall energy levels.
• Rest and Recovery: Prioritize rest and recovery, particularly in the luteal phase. Ensure adequate sleep and consider incorporating mindfulness practices such as meditation or deep breathing exercises to manage stress and improve overall well-being.
• Tracking: Use apps or journals to track your menstrual cycle and note how your body responds to different types of exercises throughout the phases. This data can help you tailor your fitness routine more effectively and understand your body’s unique patterns.
• Flexibility: Be flexible with your exercise routine. Listen to your body and adjust the intensity and type of workouts based on how you feel each day. If you’re feeling fatigued or experiencing discomfort, opt for lighter activities or rest.
• Personalization: Every woman’s cycle is different, and personalizing your approach to cycle syncing is crucial. Experiment with different types of exercises and schedules to find what works best for you.

By aligning exercise routines with the menstrual cycle, women can optimize their physical performance, enhance recovery, and maintain overall health. This approach acknowledges the natural fluctuations in energy and strength, allowing for a more sustainable and effective fitness regimen. Future research should continue to explore these dynamics, providing more personalized and evidence-based strategies for women’s health and fitness.

Conclusion

This scientific review by Hugo M. Pereira, Rebecca D. Larson, and Debra A. Bemben highlights the complex relationship between menstrual cycle phases and exercise-induced fatigability. While hormonal fluctuations undeniably influence performance, the exact impact varies depending on numerous factors, including the type of exercise and individual differences. By leveraging the concept of cycle syncing, women can better align their fitness routines with their menstrual cycles, promoting overall well-being and optimizing performance. Future research should continue to explore these dynamics, providing more personalized and effective strategies for women's health and fitness.