One week you're flying.
The weights move well. Your cardio feels strong. You leave the gym thinking you've finally hit your stride.
Then another week arrives.
The same dumbbells suddenly feel heavier. Your normal workout feels exhausting. You're bloated, uncomfortable and wondering whether you've somehow lost fitness in the space of a few days.
If you menstruate, there's an obvious question:
Could your menstrual cycle be affecting your workouts?
Possibly.
But the answer is considerably more interesting than the simplified version often presented on social media.
Hormonal changes occur throughout the menstrual cycle and some women notice very clear changes in symptoms, energy and how exercise feels.
What scientists haven't established is a universal calendar telling every woman exactly when she'll be strongest, weakest or should perform a particular type of exercise.
In fact, one of the largest systematic reviews examining menstrual-cycle phase and exercise performance found only a trivial average reduction in performance during the early follicular phase, which generally includes the beginning of menstruation. Importantly, the researchers concluded that the evidence did not support universal recommendations and that an individualised approach was more appropriate.
That's a very different message from:
“Never lift heavy during this week of your cycle.”
So let's unpack what is actually happening.
First, What Happens During the Menstrual Cycle?
A menstrual cycle is counted from the first day of one period to the first day of the next.
The often-quoted 28-day cycle is only an example. Real cycles vary between people and can also vary within the same person.
For simplicity, researchers often discuss several broad phases.
Menstruation and the early follicular phase
Day one is the first day of menstrual bleeding.
At this point, oestrogen and progesterone concentrations are relatively low.
Some women experience cramps, fatigue, headaches, gastrointestinal symptoms, back pain or simply feel generally uncomfortable.
Others feel almost completely normal.
That difference between individuals is going to become very important.
The follicular phase
After menstruation begins, follicles in the ovaries develop and oestrogen generally rises as ovulation approaches.
Ovulation
Ovulation occurs when an egg is released from an ovary.
It doesn't necessarily happen on day 14. The timing depends on the length and characteristics of an individual's cycle.
The luteal phase
Following ovulation, progesterone rises alongside changes in oestrogen.
Later in this phase, if pregnancy hasn't occurred, hormone concentrations decline and menstruation follows.
Some people experience premenstrual symptoms during this period, including bloating, headaches, breast tenderness, irritability, tiredness, sleep disturbance and changes in appetite.
The NHS recommends keeping a symptom diary for at least two cycles if PMS symptoms are problematic, because identifying an individual's pattern can be useful.
So Do Hormones Actually Change Exercise Performance?
This is where things become messy.
There's a perfectly reasonable biological hypothesis that fluctuations in oestrogen and progesterone could influence exercise physiology.
Researchers have therefore studied strength, endurance, power, fatigue, metabolism and perceived effort across different menstrual-cycle phases.
The results haven't produced a neat monthly performance chart.
A major systematic review and meta-analysis involving 78 studies found that exercise performance might be trivially reduced during the early follicular phase compared with other phases.
But the researchers emphasised enormous variation between studies and rated much of the evidence as low quality.
Their practical conclusion was that a personalised approach should be taken based on an individual's own response to the cycle rather than universal rules.
More recent research hasn't completely resolved the debate.
A 2025 systematic review restricted to studies using higher methodological standards found that some studies detected phase-related differences, but the direction and size of those differences varied. Maximum and explosive strength were largely unaffected, and the researchers again highlighted methodological problems and uncertainty.
In other words:
The menstrual cycle can matter without affecting every woman in exactly the same way.
But I've Definitely Felt Weaker During My Period
And that's perfectly compatible with the research.
Population averages and individual experiences aren't the same thing.
Imagine ten women.
Some perform almost identically throughout their cycles.
Several feel noticeably worse around menstruation.
Another feels worse premenstrually.
Someone else notices little difference in strength but finds endurance training harder.
Average all of those people together and the overall effect can appear small.
That doesn't mean the individual experiences aren't real.
There's another distinction too:
Your muscles being measurably weaker and your workout feeling considerably harder aren't necessarily the same thing.
Cramps, headache, poor sleep, bloating and fatigue could make a training session unpleasant even if your maximum strength hasn't dramatically changed.
That's why your own symptoms matter.
Does Strength Change Throughout the Cycle?
This remains debated.
A 2024 meta-analysis specifically examining maximal strength found some phase-related differences, including advantages for certain strength measures during late follicular or ovulatory phases.
But another critical review concluded that the overall evidence was too inconsistent and methodologically weak to establish meaningful menstrual-cycle effects on acute strength performance or long-term adaptations to resistance training.
That's not science failing.
That's science telling us the evidence isn't strong enough yet to make the confident claims you sometimes see online.
So if somebody tells you:
“Women should always lift heavy between days 8 and 14 and deload during their period.”
They're going beyond what the evidence currently supports.
Why Can the Same Weight Feel Heavier?
There are many possibilities.
Your menstrual cycle is one.
But so are:
Poor sleep
Stress
Accumulated training fatigue
Eating less than usual
Dehydration
Alcohol
Illness
Heat
A harder previous workout
Changing exercise order
Not allowing enough recovery
Psychological stress
Simply having a bad day
Your body isn't a machine that produces exactly the same performance every Monday at 6pm.
A disappointing workout doesn't automatically mean your programme has stopped working.
One Bad Workout Doesn't Mean You've Lost Strength
This is worth remembering.
Strength adaptations don't disappear because one session felt terrible.
If you normally squat a particular load and one day it feels unusually heavy, that is information about today's performance, not necessarily evidence that your long-term fitness is declining.
This is one reason experienced lifters use concepts such as perceived exertion and repetitions in reserve.
Training can respond to the body you brought into the gym that day.
Should You Lower the Weight During Your Period?
Not automatically.
If you feel good, there's no compelling reason to reduce your weights simply because an app says you're on a particular cycle day.
If you feel terrible, there's equally no prize for pretending you don't.
Suppose your planned weight normally feels like a comfortable 7 out of 10 effort.
Today it feels like 9 out of 10.
You could:
Reduce the load slightly.
Perform fewer sets.
Keep the weight but stop the set earlier.
Choose a less demanding variation.
Perform an easier session.
Or take a rest day.
That's called autoregulation.
And it's useful regardless of sex or menstrual cycle.
Should You Exercise During Your Period?
If you feel well enough, yes.
Having a period isn't itself a reason someone must stop exercising.
The NHS actually lists gentle activities such as walking, swimming, cycling and yoga among things that may help ease period pain.
Research supports the possibility that regular exercise can help with primary dysmenorrhoea, the medical term for period pain without another underlying pelvic condition.
A 2026 systematic review and meta-analysis involving 29 studies and 1,704 participants found therapeutic exercise was associated with reductions in pain and symptom severity, although the researchers rated the certainty of evidence as low to very low and warned that results varied considerably between studies.
Another large 2024 analysis of 49 studies and more than 3,000 participants also found exercise interventions were associated with reductions in menstrual pain.
So movement may help.
But that does not mean:
“Your cramps hurt because you haven't exercised enough.”
Nor does it mean you need to force yourself through a hard workout while in severe pain.
What About Bloating?
This is another area where people can unnecessarily panic about their progress.
You look leaner one week.
Then suddenly your stomach feels swollen and the scales are up.
Have you gained fat?
Not necessarily.
Fluid balance can fluctuate during the menstrual cycle.
One study repeatedly measured 42 women throughout their cycles and found body weight was approximately 0.45 kg higher during menstruation, with the difference largely attributable to increased extracellular water rather than meaningful changes in body composition.
That's roughly a pound.
And importantly, that's an average from one relatively small study, not a rule saying every woman gains exactly 450 grams during her period.
A newer 2026 study also found regional changes in body water across cycle phases even though overall body weight and total body water did not significantly change.
So bloating can be real while the amount and timing vary.
Bloating Isn't Body Fat
This distinction can save people a lot of unnecessary anxiety.
Body weight can change because of:
Water
Food sitting in your digestive system
Stored carbohydrate and its associated water
Bowel contents
Hormonal changes
Salt intake
And many other short-term factors.
The scales measure your total mass.
They don't provide a live readout of body fat.
If your weight rises around your period and then falls again, you haven't necessarily gained and lost fat within several days.
Should You Weigh Yourself During Your Period?
You can.
But interpret the number intelligently.
If menstrual-cycle fluctuations reliably affect your weight, comparing today's measurement with a completely different cycle phase may create noise.
A better approach for someone tracking weight closely can be to look at longer-term trends.
Some women may also find comparing similar points across successive cycles more informative.
For example, comparing this month's premenstrual week with last month's premenstrual week can sometimes tell you more than comparing Tuesday with Friday.
Does Your Period Affect Cardio?
Possibly for some women, but again there isn't a universal response.
Menstrual-cycle research has investigated aerobic capacity, endurance, ventilation, body temperature and perceived exertion.
The overall literature remains inconsistent.
That means you shouldn't assume every run during your period will be worse.
But if you repeatedly notice that a particular part of your cycle makes cardio feel harder, that pattern is worth recognising.
The key is your repeated pattern, not somebody else's Instagram infographic.
What About Body Temperature?
Progesterone is associated with a small rise in resting body temperature after ovulation during the luteal phase.
That has led researchers to investigate whether exercise in hot conditions may feel or behave differently across the cycle.
For the average recreational exerciser, the practical message is straightforward:
Pay attention to heat, hydration and how you feel.
If you consistently feel hotter or less comfortable during certain sessions, adapt accordingly.
You don't need to redesign your entire programme because your temperature changed by a fraction of a degree.
PMS Can Change More Than Your Body
Premenstrual syndrome can include psychological as well as physical symptoms.
The NHS lists symptoms including:
Mood swings
Feeling depressed or irritable
Feeling anxious or emotional
Tiredness or trouble sleeping
Bloating or cramping
Breast tenderness
Headaches
Changes in appetite
These can obviously affect someone's desire to exercise and their perception of a workout.
A session performed after poor sleep while experiencing cramps and a headache may feel completely different even if your underlying fitness hasn't changed.
Can Exercise Help PMS?
There is some evidence suggesting it can.
A systematic review examining exercise and premenstrual symptoms reported improvements in a range of physical and psychological symptoms, although the research base was heterogeneous and didn't establish one perfect exercise prescription.
That means regular exercise may be helpful.
It doesn't mean you need to punish yourself with HIIT because you're experiencing PMS.
Walking counts.
Strength training counts.
Cycling counts.
Swimming counts.
A gentler session counts.
The goal is movement appropriate to how you're feeling.
What Is Cycle Syncing?
“Cycle syncing” has become a major fitness and wellness trend.
The idea is that you deliberately change your workouts according to menstrual-cycle phases.
A typical social-media plan might say:
Menstruation = yoga and gentle exercise
Follicular phase = harder strength training
Ovulation = maximum intensity
Luteal phase = moderate exercise
It sounds wonderfully scientific.
The problem is that the evidence isn't strong enough to prescribe that schedule universally.
The major meta-analysis on menstrual-cycle performance specifically concluded that general recommendations couldn't be formed from the existing evidence and advocated an individualised approach.
So the idea of paying attention to your cycle is sensible.
The idea that every woman's training calendar should be dictated by the same hormonal timetable isn't currently supported.
A Better Version of Cycle Syncing
Instead of:
“I'm on day 23, therefore I must do Pilates.”
Try:
“I've tracked my training and symptoms for several months and I consistently struggle with high-intensity sessions during the two days before my period. I'll make those sessions easier.”
That's useful personal data.
The difference is enormous.
One approach starts with an internet rule and forces your body to obey it.
The other starts with your body and adapts the training.
Track Symptoms, Not Just Dates
If this topic resonates with you, try keeping a simple record for two or three cycles.
You don't need an elaborate spreadsheet.
Record:
Cycle day
Type of workout
How difficult it felt
Energy level
Sleep
Period pain
Bloating
Mood
Any unusual symptoms
Perhaps your main lift or cardio performance
Over time, you may discover a pattern.
Or you might discover there isn't one.
Both results are useful.
The NHS also recommends keeping a symptom diary for at least two menstrual cycles when investigating problematic PMS.
Don't Forget Hormonal Contraception
Not everyone who menstruates has the same hormonal pattern.
Combined oral contraceptives, progestogen-only methods, hormonal IUDs and other hormonal contraceptives alter reproductive hormones in different ways.
So advice based entirely on a naturally occurring menstrual cycle may not apply.
A systematic review and meta-analysis of oral contraceptive use found any average difference in exercise performance between pill users and naturally menstruating women was likely to be trivial, and performance remained broadly consistent across the oral-contraceptive cycle. The researchers again recommended an individual approach rather than universal performance advice.
A 2024 study comparing naturally menstruating women with oral-contraceptive and hormonal-IUD users similarly investigated strength and power across hormonal phases, illustrating just how much more complicated the subject is than dividing everyone into the same four-phase chart.
What About Perimenopause?
This changes the conversation again.
As someone approaches menopause, cycles may become less predictable and symptoms can change.
That makes rigid calendar-based training even less useful.
Rather than trying to force an irregular cycle into a perfect 28-day fitness plan, training according to symptoms, recovery and actual performance becomes particularly sensible.
This is where we'd internally link to the CardioFire Menopause and Exercise guide.
When “I'm Tired During My Period” Deserves More Attention
Some fatigue around menstruation doesn't automatically indicate a medical problem.
But persistent or significant fatigue shouldn't always be dismissed as hormonal.
Heavy menstrual bleeding can contribute to iron-deficiency anaemia.
The NHS lists symptoms including tiredness, lack of energy, shortness of breath, noticeable heartbeats and headaches. Blood loss is one of the common causes.
If your periods are very heavy and you're repeatedly exhausted, breathless or struggling unusually badly with exercise, speak to your GP rather than simply assuming you're bad at cardio.
How Heavy Is Too Heavy?
The NHS advises seeking medical advice for period problems that affect your life.
Heavy periods can include needing to change period products very frequently, bleeding through clothes or bedding, needing two types of sanitary product together, passing large clots or periods lasting longer than seven days.
Severe period pain that interferes with normal daily activities also deserves assessment.
There can be underlying causes of painful or heavy periods, including conditions such as endometriosis, adenomyosis and fibroids.
Exercise isn't a substitute for investigating symptoms that are significantly affecting your life.
Severe Pain Isn't Something You Have to “Push Through”
Fitness culture sometimes rewards ignoring discomfort.
That's particularly unhelpful here.
If you're experiencing severe or worsening pelvic pain, fainting, dizziness, heavy abnormal bleeding or other concerning symptoms, don't treat completing your workout as some test of character.
The NHS advises urgent medical assessment for certain combinations of severe pelvic pain, heavy bleeding, faintness, breathing difficulty and other concerning symptoms.
Training can wait.
A Practical Way to Train Across Your Cycle
Forget the complicated colour-coded hormone calendar for a moment.
Start with your normal programme.
On days you feel good
Train normally.
If the weights feel strong, use them.
If your planned cardio feels comfortable, do it.
There is no reason to deliberately hold yourself back merely because an app predicts you should be weaker.
On days you feel slightly below normal
Train, but use feedback.
Perhaps leave another repetition in reserve.
Reduce the load slightly.
Take longer rests.
Shorten the cardio session.
On genuinely rough days
A lighter workout, walk, mobility session or rest day may be more appropriate.
That's not losing progress.
That's managing training.
When a pattern keeps repeating
Record it.
If you consistently perform differently during a particular part of your cycle, you can start planning around your own pattern.
That's considerably more defensible than adopting someone else's.
Your Workout Programme Should Serve You
A programme is a plan.
It isn't a contract.
The purpose of training isn't to prove that you can execute exactly the same numbers regardless of what is happening in your body.
The purpose is to create enough training stimulus over weeks, months and years to become fitter, stronger and healthier.
One lighter workout won't ruin that.
Neither will one missed session.
What matters far more is consistency over time.
Stop Judging Progress From One Week
This applies particularly well to the original problem:
“Last week I was strong. This week I'm bloated and my weights have gone down.”
Those things don't automatically mean you're going backwards.
Bloating can reflect fluid changes.
Workout performance naturally fluctuates.
Menstrual symptoms can alter how you feel.
Sleep can change.
Stress can change.
Recovery can change.
Look at the trend.
Are you stronger than three months ago?
Can you perform more repetitions?
Has your walking or running improved?
Are you training more consistently?
Does the same workout generally feel easier?
That's progress.
The Bottom Line
Yes, the menstrual cycle can influence how exercise feels.
For some women, the difference is obvious.
For others, it's barely noticeable.
And that's exactly why the most useful advice isn't a universal menstrual workout calendar.
Current research does not show a large, predictable performance effect that occurs identically in every naturally menstruating woman. Some studies identify phase-related differences; others don't, and the overall evidence remains inconsistent.
What we can say with much more confidence is that symptoms matter.
Pain matters.
Sleep matters.
Bloating matters.
Fatigue matters.
Your training history matters.
And your individual experience matters.
So don't panic because the dumbbells that flew up last week suddenly feel like they've been filled with concrete.
Adjust the session if you need to.
Keep training normally when you feel good.
Track recurring patterns if you're curious.
And don't confuse a temporary bad workout, a bloated stomach or a higher number on the scales with losing all the progress you've worked for.
Your fitness isn't rebuilt from scratch every menstrual cycle.
