Return to exercise with caution after a bout of illness

If your fitness regime has been thrown off course by a bout of flu, a cold or the dreaded norovirus, you are certainly not alone. But how, in that sluggish post-viral period, do you gauge whether it is safe to start exercising again?

Your resting heart rate can be a good indicator of whether exercise is appropriate - providing you know what it is when you are 100 per cent well. "Elite athletes check their resting heart rate daily," says Dr Mark Wotherspoon, a sports physician with the English Institute of Sport. "If the resting level is 10 beats per minute above normal, this would be an indicator not to train."

For the rest of us, the nature - and location - of your symptoms is an important determinant of whether you should don your slippers or your running shoes. "We differentiate between 'above the neck' symptoms such as a runny or stuffy nose, watery eyes or a mild sore throat," says Wotherspoon, "and 'below the neck' ones, such as a cough, a congested or tight chest, an upset stomach, muscle aches or fever."

If your symptoms are above the neck and you feel OK, it is fine to do a light work-out. Research from Ball State University in Indiana found that infecting subjects with a mild cold virus did not affect their ability to exercise moderately. Lung capacity of the infected subjects was the same as that of the healthy ones, and running on a treadmill for 15 minutes felt no harder.

Is there any truth in the old "sweating out a cold" adage? "Bringing up your body temperature is a way of fighting a virus," says Dr Alex Nieper, sports physician for Chelsea Football Club. "But keep the activity light to moderate - and brief."

Research from the Common Cold Centre at Cardiff University shows that symptoms normally last around a week - though in about 25 per cent cases, they can linger for up to 14 days.

Studies have found that a long, hard work-out can lower immunity for up to nine hours.

And if your symptoms are below the neck, give your workout a miss regardless of how you feel, or how much you think you need to do it. Exercising with major cold symptoms, particularly a fever, will prolong your illness and can be dangerous.