The makers of tampon ads are seemingly obsessed with making women run, jump, swim - even snorkel - their way through their periods. But for many women, the problem with exercise doesn't come with menstruation, but in the run-up to it, when even the most dedicated gym-goer skips the odd session. Who really feels like trampolining with tender breasts and a bloated tummy?

Research shows that, generally speaking, women's physical performance declines when they are premenstrual. Claire Kowalchik, author of the Complete Book of Running for Women, says she can see a dip in her performance: "I ran my fastest and easiest marathon on the first day of my period one year ... the week before menstruation my breathing becomes laboured and if I'm doing speed work, my times will be disappointingly slow."

Hormones released during the three stages of the menstrual cycle may create these differences in performance. Between days one and 13 - the menstrual or follicular stage - levels of oestrogen are low, but as follicles start to grow in the ovaries, oestrogen levels increase. At ovulation, usually on day 14, more hormones are released. If pregnancy doesn't occur, from then on, levels of progesterone increase while oestrogen levels start to drop.

In terms of exercise regimes, some women have reported that, during the first two weeks of their cycle, tempo work, speed runs and interval training feel better, while in the second half, longer, slower runs seem easier.

Different people are affected by hormones to differing degrees, as Dr Karen Birch, a lecturer and researcher into exercise physiology at Leeds University, explains: "Some individuals respond worse to hormonal fluctuations, but the consensus is that elite athletes aren't affected. Women with hormonal irregularities - loss of periods, irregular cycles, or post- menopausal women - have more problems."

Rosalyn Carbon, team doctor for the Great Britain triathlon team, agrees that elite athletes are affected less by hormonal changes. "In my experience, professional athletes suffer less from hormonal influences than women in the general population," she says. "Some women find they need to rest more during menstruation; others don't.

"Most women find that prior to their period, they'd much rather not have to compete in a major competition, and for those, we do suggest that they might go on the contraceptive pill, which irons out hormonal fluctuation significantly. There are also legal drugs such as tranexamic acid that will stop a woman bleeding when she does come on; that might be appropriate for an athlete who doesn't want the hassle of bleeding on a major competitive day in a national championship."

Some women do use the Pill to help their performance, although the trend varies across sports. "In my experience," says Carbon, "most women feel better off the Pill" - so it's not common practice. However, according to Jodi Coffer, science and medicine manager for British Swimming, most elite swimmers do use the Pill. "It just helps regulate cycles and makes competing easier, especially in a pool environment. Care is taken to get the right product, but most are happy to use this form of help with their careers."

For those of us exercising on a more mundane level, working your training around your menstrual cycle can still help manage performance highs and lows. Keep a diary of how you perform and train for a few months - and of premenstrual symptoms such as breast tenderness, bloating and slight weight gain, which affect up to 30% of menstruating women. If the two coincide, tailor your training to fit. Not only will you keep to your gym routine, but the right exercise could even help alleviate your period cramps or PMT.

Are there any sports you shouldn't do while menstruating? "It's generally thought by most yoga teachers that its not a good idea to do handstand or headstands while having periods, as some say it can cause the blood to flow the wrong way and possibly contribute towards endometriosis," says Pierre Bibby of the British Wheel of Yoga. According to scientists, this is nonsense. Dr Gerry Conway, consultant endocrinologist at University College London, says: "There's no real reason physiologically how you could affect your blood flow adversely by standing on your head."

If you suffer from severe cramps while menstruating, though, it's probably best to move away from a high-energy workout. "Low- intensity exercise has been shown to help dysmenorrhea (painful menstruation) as it calms the cramping," says Birch. "Exercise does help PMT symptoms too, although it appears the effect is more psychological than physical. The science is not fully understood, but women do feel better from being active when suffering from mood changes and PMT symptoms."

If you do feel premenstrual, try switching from long cardio sessions at full pelt to slower, longer workouts, doing lower-impact sports - swap running for the cross trainer, recumbent bike or rowing machine. And make sure you are well fuelled before your workout - women are less tolerant of sugar crashes when premenstrual, so have a high-carb snack before and after. Make sure you keep hydrated; even if you are bloated, you need to drink water.

In fact, looking after your nutrition throughout your cycle can help reduce premenstrual symptoms and help you work out better. "Concentrate on eating a healthy diet, with plenty of iron-rich foods such as red meat, poultry (leg meat) and fish (if you are vegetarian, green, leafy vegetables, apricots, prunes), to balance iron loss," says Henrietta Bailey, a nutritional therapist at Pure Sport Medicine.

"Keep up your blood sugar levels throughout the day with healthy snacks, such as fresh fruit, nuts and seeds. And when premenstrual, eat more carbohydrates and less sugar, dairy produce and caffeine, all of which cause more blood sugar fluctuations and contribute to rattiness and premenstrual disharmony."