Biomechanics, periods and pregnancy

Race for Life 5km, photo by David Knight

Race for Life 5km

The principles of running are basically the same for everyone, but there are some specific differences between men and women which affect running.  This chapter looks at issues from the perspective of women runners, including biomechanical differences, the menstrual cycle, running during pregnancy, and issues of personal safety.

Biomechanical differences

As compared to men, women have wider hips and shoulders. Women are on average about 12cm shorter and 18kg lighter than men and they have about 8-10% more body fat.[i] Women’s legs are turned outwards more than men’s, resulting in a greater misalignment of kneecap tracking, and women have a greater propensity to suffer from knee and hip injuries.

As a result, the prevalence of different types of running injuries varies between men and women, and the appropriate treatments are slightly different.  On average, women suffer more than men from pain in the knee area. There is not much you need to do about this, apart from follow good advice to avoid injury and ensure that any medical advice you get is from a properly qualified and experienced practitioner.

Because of these biomechanical differences, women’s running shoes ought to be designed to be rather different from men’s (for example, they should have greater control to prevent over-pronation).  There are running shoes made especially for women, but studies have found that they don’t make much difference. They are generally more comfortable, however, because they are designed for narrower feet.[ii]

Running & periods

It is not clear whether exercise performance changes much during the menstrual cycle.  Individual women appear to respond in different ways, and many women show no effect at all.

On the whole, unless you are sure that your period affects your running, there is no strong reason to adjust your running around your menstrual cycle. However, if you are an endurance athlete, and you suffer from water retention and weight gain at certain times during your cycle, this may reduce your performance at particular times of the month.

It is worth keeping track of your menstrual cycle in your training log (see Chapter 2), in case you are one of those runners whose performance is affected by your menstrual cycle. (This information will also help you to rule out this possible explanation of a poor performance in the future.) If your performance is related to your periods, then a complete training log will help you to identify the times of the month when you are likely to be affected by your cycle.

The impact of running on your periods

Many women runners find that exercise can improve their mood and reduce discomfort before and during their periods.  Moderate exercise can relieve the physical symptoms of premenstrual tension, including tender breasts.

In some runners, particularly those who are training for long distances, running can lead to amenorrhea – that is, erratic or absent periods. The reasons for exercise-induced amenorrhea are not well understood.  It seems to be caused by some combination of low body fat, inadequate nutrition and stress.  Young runners who train intensely seem to be especially prone to menstrual irregularity.[iii]

Amenorrhea induced by exercise is generally temporary, and periods return when the training load is reduced.  It may, however, have a long‑term negative impact because it increases the chances of osteoporosis as a result of low levels of oestrogen, a hormone which helps to protect calcium in bones.  There is also evidence of lower bone density in women who miss their periods; and this can lead to increased risk of stress fractures and brittle bones later in life.  Amenorrhea is also associated with increased risk of cancer of the uterus and breast.

If you suffer from irregularities in your period, you should see a gynaecologist, both to rule out other, more serious, causes; and to try to identify steps that you can take to restore your normal cycle.  You should look carefully at your diet, to ensure that you are getting enough calcium, protein and fat (see below). You should also consider whether you are suffering from other symptoms of overtraining, such as elevated heart rate, insomnia or fluctuating weight (see Chapter 8).  Persistent amenorrhea can be treated with hormone therapy.

Running & pregnancy

Most doctors now agree that moderate exercise during pregnancy is good for both mother and baby. There is evidence that women who exercise before and during pregnancy have less complicated pregnancies and births.[iv]

“I kept running until six weeks before my daughter was born.  Just run how you feel, and listen to both your bodies.  I started running again a week after Emily was born.  Fortunately, she loves endorphins in her milk!”

Swenja Surminski, London

However, you should not continue to run during pregnancy as if nothing has changed.   Your body does change with pregnancy, and this means that some activities which are safe when you are not pregnant are not safe for you or your baby when you are pregnant.  For example, ligaments and bones soften during pregnancy, to accommodate the baby, which means that you are more susceptible to injury.  Your temperature regulation mechanisms are strained, which means it is easier to overheat, which could damage the foetus, especially in the early stages of pregnancy.  You should avoid intense exercise with high heart rates (e.g. above 140 beats per minute – see Chapter 10) to avoid depriving the foetus of oxygen.

You should check with your doctor before running while pregnant, both to discuss general guidelines, and to check that you don’t present any particular risk factors which indicate that you should not run while pregnant (such as hypertension, premature dilation of the cervix, and persistent bleeding).

If you do decide to continue to run, here are some guidelines:

  • don’t run to exhaustion; and don’t run at high levels of intensity (e.g. sprinting) which may affect the baby’s oxygen supply;
  • be careful not to overheat, especially during the early weeks;
  • be careful not to dehydrate, which means that you may need to drink more than you are used to;
  • maintain your blood sugar levels, which means you should eat more than you are used to;
  • don’t try to stick to a training schedule, or try stick to your previous heart rate zones; instead, run as you feel inclined and listen to your (and your baby’s) body; you might want to use a heart rate monitor to ensure that you don’t elevate your heart rate too much;
  • when running no longer feels comfortable, consider other exercise, such as swimming or aqua-jogging, to keep fit.  An exercise bike will help you to keep fit, and may be useful later on if you are at home alone with the baby.

If you develop any of the following symptoms, you should stop running immediately and consult a doctor:

  • bloody discharge or amniotic fluid leakage from the vagina;
  • sudden swelling of the ankles, hands or face;
  • persistent, severe headaches or visual disturbance;
  • swelling, pain and redness of the calf in one leg;
  • elevation of pulse rate or blood pressure that persists after exercise;
  • excessive fatigue or any palpitations or chest pains;
  • persistent contractions (they may suggest the onset of premature labour);
  • unexplained abdominal pain;
  • insufficient weight gain.

Breast feeding

If you decide to breastfeed your baby, monitor the baby’s weight gain carefully.  If the baby does not appear to be putting on weight as quickly as expected, this could be because running is reducing the amount of milk you produce, and you may need to cut back on exercise.

It is usually more comfortable for your breasts to run after a feed.  You can, however, safely breast-feed as soon as you like after running.  You should be aware that immediately after exercise your breast milk may contain lactic acid. This will not harm your baby, but some babies don’t like it.  You may need to wait for an hour after your run for the lactic acid level to return to normal.


Some runners suffer from incontinence; this can especially affect women runners after having a baby.   Many women have found that pelvic floor exercises and Pilates can prevent this. (I’m told that these exercise are also good for your sex life.)

[i] Atwater, A. E. ‘Gender differences in distance running’, Biomechanics of Distance Running, P. R. Cavanagh, 1990.  Champaign Illinois, Human Kinetics: 321-362.

[ii] E. M. Hennig, Gender Difference for running in athletic footwear, Proceedings of the 5th Symposium on Footwear Biomechanics (ed Hennig & Stacoff), 2001.

[iii] Speroff, L., Redwine, D.B. (1980) Exercise and menstrual function Physician and Sportsmedicine 8 (May), 42-52.  Also Lutter, J.M., Cishman, S. (1982) Menstrual patterns in female runners. Physician and Sportsmedicine 10 (September) 60-72.

[iv] Dagny Scott, The Complete Book of Women’s Running, 2000.  Rodale Press.

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