In these winter months, it is common to battle colds, sore throats, and other ailments. If you’re like me, you still want to get some exercise even though you’re under the weather. How can you know if it is safe to run or lift weights when you’re ill? When it comes to exercise with a fever, there are some points to keep in mind.

Your ability to regulate body temperature could be impaired.

Fever, usually defined as an oral temperature over 100°F (or 37.8°C), is the body’s response to a number of conditions, including infections. It affects hormones and body systems. It constricts blood vessels and limits blood flow to the extremities, which can decrease the ability to dissipate heat. It increases body fluid losses. This dehydration and decreased ability to cool the body can significantly impair your ability to regulate body temperature. If you try to exercise with a fever, especially when you’re dehydrated, you risk more serious increases in body temperature.

Strenuous exercise when you have a fever might worsen your illness.

Little scientific data exists in humans, but animal studies show that strenuous exercise during an illness with a fever could make the illness worse, or in rare cases could be fatal. It makes sense that exercise could increase your symptoms or lengthen the duration of those symptoms.

Fever can hamper your strength and endurance.

Fever has many musculoskeletal effects that decrease athletic performance. It has been shown to be associated with decreased muscle strength and endurance, decreased exercise tolerance, and increased perceived fatigue. Decreased speed, precision, and coordination, which often occur with fever, can potentially impair performance and also lead to injury.

If you decide not to exercise with a fever, don’t just lay in bed.

Illnesses with a fever often impair the output of your heart through effects on cardiac muscle and the circulatory system. It has been shown that people who get out of bed and walk every 30 minutes during the day experience lower declines in these functions. In theory then, you might get up and walk around the house for a few minutes every half hour or so to potentially recover from your illness faster.

Now that we’ve discussed some of the issues with exercise and illnesses that cause a fever, the important question becomes, “Should you exercise with a fever?

The Dr. David Geier ShowShould you exercise when you’re sick? In the Zone segment from Episode 69 of The Dr. David Geier Show

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Temperature threshold

While decisions must be made individually, most physicians recommend that you should avoid exercise or playing sports if you have a temperature above 38°C (or 100.4°F). If you know your normal resting heart rate, you could monitor your temperature and heart rate when you’re sick. If you have a higher than normal temperature and a heart rate 10 beats per minute higher than normal, take the day off.

The “neck check” rule

Many people use the location of symptoms as a guide to making the decision to exercise. Essentially the “neck check” rule divides symptoms into those above and below the neck. If you have symptoms like a runny nose, congestion, or sore throat, then mild-intensity exercise is probably safe. If you have no problems in the early minutes of the exercise, you can continue. On the other hand, if you have muscle aches or a deep cough in your chest, many health practitioners advise that you shouldn’t exercise.

Don’t spread your illness to others.

Take measures to limit the spread of infection to others. Wash your hands and disinfect gym equipment. While it’s natural to want to exercise with other people, do your best not to give them your germs!

Should you exercise with a fever or rest?

Exercise has many positive health benefits. Moderate exercise might generally be protective against infection by boosting your immune system. If you have an illness with a fever, though, it might be a good idea to take it easy.

Diehl JJ, Dick NA. Febrile illness in the athlete. Sports Health. Published online October 11, 2013.