Image source, Sarah MackayImage caption, Sarah Mackay, known as @fitwsarah online, says her energy levels change through her cycle
A woman points to a colour-coded grid dividing the menstrual cycle into four phases, each paired with a different list of exercises.
"When you're bleeding, you want to be doing steady-pace jogging, Pilates, barre, walking, resting," she tells viewers.
This is one of thousands of videos on TikTok and Instagram suggesting that people change their workouts depending on where they are in their menstrual cycles - something known as 'cycle-syncing'.
Most of the advice is shared by fitness and wellness creators rather than medical or research experts.
They divide the menstrual cycle into phases, recommending:
gradually increasing intensity in the follicular phase leading up to ovulation
harder strength or high-intensity training around ovulation
easing off again during the luteal phase (between ovulation and the start of your period)
Kelly McNulty, a researcher at Leeds Beckett University specialising in menstrual cycles and exercise, says there is evidence that some women experience changes in symptoms, energy or perceived strength across their cycle.
But that is different from evidence that all women should train in a particular way at a particular point, she tells the BBC, adding that these videos are "a trend based on marketing rather than any kind of research or evidence".
"There isn't any research to support that kind of phase-based prescription," she says.
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Should you sync your exercise routine with your period?
A key issue is that menstrual cycles are highly individual, McNulty adds - they can differ between women, and one woman's experience can also change from one cycle to the next.
That makes the blanket advice often found on social media difficult to apply, she explains.
Many cycle-tracking apps and programmes are also designed around relatively regular menstrual cycles that are 21 to 35 days, which can make their recommendations less applicable to people with irregular periods, or those using hormonal contraception.
"It's not so much about doing certain exercises in certain phases, it's about working with your own individual body and what your needs might be."
Image caption, There are now thousands of videos on Instagram and TikTok recommending women sync their workouts to their menstrual cycles
Cesci Burgoyne, a nurse based in Sydney, Australia who is also a qualified personal trainer and nutritionist, adjusts her training depending on how she feels in her cycle. She doesn't follow a prescriptive plan, she says, but instead makes changes intuitively.
"In my luteal phase I'll try and ease off in the training," she tells the BBC. "In my follicular phase, I'm so energised, I feel like I can lift so much heavier. I like to try and get my long runs in."
Burgoyne also welcomes the growing attention on women's health, and says that exercise and nutrition research has historically focused heavily on male bodies, with findings then often applied to women.
Image source, Sarah MackayImage caption, Sarah says she has sometimes hit personal records while on her period
Fitness creator Sarah Mackay, known as @liftwsarah on Instagram and TikTok, has also noticed her energy, motivation, cramps and mood change through the month.
But she deliberately avoids strict plans because, she says, "there are too many variables at play".
Some online plans advise women to avoid intense training during their period, she says, yet she has sometimes hit personal records while menstruating.
"The problem is not that the menstrual cycle is irrelevant," she says, but that "not everything is predictable".
Dr Faye Bate, a trained medical doctor and women's health expert, is also wary of advice suggesting all women should train differently according to their menstrual phase.
"A one-size-fits-all model of 'you should be exercising this way in this phase and a different way in a different phase' shouldn't be advice that applies to all women," she says.
She is concerned that repeatedly telling women they are likely to be weaker or less capable at particular points in their cycle could create a "nocebo" effect - where negative expectations themselves influence how someone feels or performs.
McNulty tells the BBC that routinely easing off resistance training could also limit progress. Strength training is particularly important for women, who face a higher risk of osteoporosis later in life.
Building strength requires consistent, progressively challenging training, she says, and "we don't want to create another barrier by suggesting women train less for almost half the month."
Dr Bate also warns against normalising severe symptoms: if pain, fatigue or mood changes interfere with exercise or daily life, women should seek medical advice rather than assuming it is an inevitable part of their cycle, she says.
While cycle-syncing itself may not be backed by strong evidence, the experts we spoke to did suggest being "cycle-aware".
McNulty says tracking your cycle can be a useful way to understand your own patterns, while Dr Bate also encourages women to become more familiar with how they feel at different points in the month.
Both welcome the fact that menstrual health is being discussed more openly, but say the focus should be on individual experience, rather than rigid, phase-based rules.
In practice, that might mean having painkillers ready before your period, taking a rest day when symptoms are worse, or easing off a workout if you feel unusually fatigued.
For Dr Bate, "the most important part is tuning in to how you feel".
In other words, use your cycle as information rather than a set of rules.
