A new study has debunked a popular TikTok wellness trend called cycle syncing, which claims that tailoring a workout routine to match the hormonal changes that occur during the different phases of the menstrual cycle provides a fitness benefit.

Around 2021, the hashtag #CycleSyncing started gaining popularity on TikTok. By October 2022, it had amassed approximately 125 million views that, by May 2024, had jumped to 294 million.

Despite its widespread popularity – or because of it – a new study led by researchers from McMaster University, Canada, has questioned the science behind the popular ‘wellness concept’ that claims that aligning exercise routines with the hormonal changes that occur during the menstrual cycle optimizes fitness outcomes.

“Our findings conflict with the popular notion that there is some kind of hormonal advantage to performing different exercises in each phase [of the menstrual cycle],” said the study’s lead author Lauren Colenso-Semple, PhD. At the time of the study, Colenso-Semple was part of the Exercise Metabolism Research Group in the Department of Kinesiology at McMaster. “We saw no differences, regardless of cycle timing.”

The ‘textbook’ 28-day menstrual cycle is divided into four phases, which are set out below along with the recommended exercise for each phase according to the cycle syncing concept:

• Menstrual phase (days 1 to 5): Hormone levels drop, leading to shedding of the uterine lining. Energy levels may be lower during this time. Gentle exercise such as yoga or walking is recommended.
• Follicular phase (days 6 to 14): Estrogen levels rise as the body prepares to release an egg, often resulting in increased energy and improved mood. Moderate-intensity workouts like strength training or cycling are best during this phase.
• Ovulation (around day 14): There’s a peak in estrogen and luteinizing hormone, leading to the release of an egg. Many experience heightened energy during this phase. As a result, high-intensity interval training (HIIT) or vigorous cardio sessions are suggested.
• Luteal phase (days 15 to 28): Levels of the hormone progesterone rise to prepare the body for a potential pregnancy. If pregnancy doesn’t occur, levels fall, which can lead to premenstrual symptoms such as mood swings or fatigue. Best to engage in lower-impact activities such as Pilates, yoga, or gentle stretching.

The researchers recruited 12 healthy females with an average age of 19. All participants reported having regular menstrual periods, as determined by a tracking app, and hadn’t used any form of hormonal contraception for at least six months before the study. Each participant completed two six-day study phases in a randomized order: one in the late-follicular phase, when estrogen is highest, and one in the mid-luteal phase, when progesterone is highest. Participants performed unilateral resistance exercise, working out one leg in one phase and the other in the next phase. Over six days, the researchers measured muscle protein synthesis and myofibrillar protein breakdown after exercise or during rest.

Muscle protein synthesis (MPS) directly measures muscle growth using stable isotope tracers. These are ‘tagged’ amino acids that are swallowed, enter the bloodstream and are incorporated into new muscle proteins when the body repairs and rebuilds muscle after exercise. Muscle biopsies were taken from the participants at different times post-exercise and analyzed to see how much of the tagged amino acid got used to build new muscle proteins. If more of the tagged amino acids appear in the muscle over time, it means the body is building more muscle (MPS).

Myofibrillar protein breakdown (MPB) is the process of breaking down muscle proteins into their smaller components, amino acids. The body naturally does this to remove damaged proteins, recycle amino acids for energy, and build new proteins. Researchers use metabolomics to track the process, which, in simple terms, is a way of checking the leftovers of body processes. Every process in the body creates byproducts called metabolites that get released into the blood, urine, or tissues. MPB is measured using blood and urine samples, stable isotope tracers, and muscle biopsies. By measuring MPB alongside MPS, researchers can see if muscles are gaining or losing protein over time, which helps to determine if certain factors (such as menstrual cycles) affect muscle growth or breakdown. If MPB is high, muscle loss might be happening. If MPS is higher than MPB, muscle growth is occurring.

The researchers found that the resistance exercise stimulated MPS, but participants’ menstrual cycle phase – neither follicular nor luteal – affected resting or exercise-stimulated MPS or MPB.

“Our work shows that women who want to lift weights and recondition their muscles should feel free to do so in any phase of their cycle,” said Professor Stuart Phillips, the Canada Research Chair in Skeletal Muscle Health at McMaster and the study’s corresponding author. “There is no physiological difference in response to the exercise. It is important to tailor your training to how you feel.”

By the way, the notion that a menstrual cycle lasts 28 days is more myth than reality. Only about 13% of women have a 28-day cycle; the average is 29.3 days long.

The study was published in the Journal of Physiology.

Source: McMaster University