The flu is a viral illness that causes fever, gastrointestinal distress, muscle aches and pains that can last for several days. When the symptoms of the flu are severe, you have no desire to get out of bed, let alone go out and train. As you begin feeling better you may be ready to get out of the house after being trapped inside with the virus. Before going for your first run post-flu, make sure your body is ready to handle the additional physical stress from a run.
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Take your temperature. Do not go for a run if you are still running a fever. An elevated temperature is a sign your body is still fighting off the flu virus. You need to let your immune system clear the flu out before physically straining your body by running.
Make sure your stomach issues are gone. Vomiting and diarrhea, both common with the flu, cause dehydration. Your sweat will add to the dehydration as you run, increasing the chance that you could become dangerously dehydrated, leading to problems such as nausea, diarrhea, weakness and fainting.
Follow a 24 hour rule. Don't go running until your symptoms have improved for 24 hours or more. This includes any fever, nausea, vomiting, diarrhea, muscle aches or cramps. Since flu symptoms can sometimes disappear for a short time then reappear, you need to make sure the flu is really gone before you lace up your sneakers to run.
Reduce your mileage. You won't be able to go out and run your usual training runs after you recover from the flu. Your body has been through a lot fighting off the virus, and doesn't have the same stamina and energy level you're used to. Back off and listen to your body.
Adjust your schedule. When recovering from the flu, it can take weeks for you to get your strength back. Don't expect to be able to cram in a bunch of races or go on several tough runs for the first few weeks. Your body needs that strength-building time to recuperate. If you overdo it while you're weak, you risk lowering your immune system and becoming sick again.