Running After the Flu – When to Exercise Safely Again?

Running After the Flu – When to Exercise Safely Again?

After a respiratory infection you can gradually start training again according to your own feelings. If you have symptoms such as fever, muscle aches, sore throat or fatigue, you must not exercise.

I’m sure you know that feeling when you’ve got your workout momentum and you are hitting your fitness goals – and then you catch a cold. Autumn is the most wonderful time for trail running, but unfortunately also the golden time for catching respiratory infections.

According to research, regular exercise reduces infectious diseases, but I've already learned that autumn flu comes as certainly as the leaves fall from the trees. Almost everyone gets the flu, no matter how much you take care of hand hygiene and maintain a healthy lifestyle.

And if you’ve got asthma, respiratory infections tend to be prolonged and the wheezing lasts a long time. When you are sick, sports are neither useful nor fun, and exercising is even dangerous. So how do you know when you can return to training again?

Flu and sports

There is surprisingly little scientific research on flu and exercising. It is known that regular exercise reduces the number of infections. It is also known that a lot of endurance training or a lot of very heavy training and 'overtraining' increase the occurrence of respiratory infections.

What is not yet known is the amount of optimal exercise in relation to staying healthy. The effects of training vary from person to person, partly due to different immune systems.

The connection between sports and respiratory infections has been studied especially in runners. Based on the results, it has been concluded e.g., that:

  • Running more than 780 kilometers (485 miles) caused an increased risk of infection
  • After an ultrarun of 56 km (35 miles), the risk of getting a respiratory infection doubled during the two weeks after the run
  • Fitness runners who ran more than 25 km (15.5 miles) and an average of 42 km (26 miles) had one respiratory infection during the two-month study period, while the group of runners who ran less than 25 km (15.5 miles) and an average of 12 km (7.5 miles) had no flu
  • Those who ran a lot had flu more often and the symptoms lasted longer than non-athletes

The connection between sports (running) and respiratory infections is not linear but a J-curve. According to research, the healthiest are runners who train regularly but moderately.

When not to exercise?

If you google "running after flu" or "exercise after flu", you may find articles that only focus on poor training results and completely ignore the risks of exercising when sick. Although the common cold is not dangerous at all, exercising during the cold or too soon after the cold or fever can even lead to a life-threatening condition.

According to UKK Institute, you should not exercise if you are experiencing symptoms such as "fever, unusual fatigue, accelerated resting heart rate, muscle and joint pain or a general feeling of illness". Such symptoms are typical of a common cold.

After the flu, the cough and runny nose may continue for a long time. If the symptoms last a very long time, check your ferritin levels. Iron deficiency is common among female runners.

Why not exercise when sick

When sick, especially with a fever, your body is in a state of stress. During a respiratory infection, training is useless in the sense that you don't get a training effect from exercise. Your muscles don't develop, no matter how much you lift the weights in gym.

However, not getting the training benefits is not the reason why you should not exercise when you are sick. Exercising may weaken your body's immune system and worsen the infection. Especially your heart muscle is stressed during viral infections, and strenuous exercise may cause dangerous heart symptoms.

What is myocarditis?

Myocarditis is inflammation of the heart muscle. Inflammation is most often caused by viruses. Myocarditis is usually triggered by a common respiratory infection, i.e. the flu.

Myocarditis symptoms (Mayo Clinic):

  • Myocarditis symptoms vary from no symptoms to mild and severe symptoms
  • Chest pain
  • Fatigue
  • Swelling of the legs, ankles and feet
  • Rapid or irregular heartbeat (arrhythmias)
  • Shortness of breath, at rest or during activity
  • Light-headedness or feeling like you might faint
  • Flu-like symptoms such as headache, body aches, joint pain, fever or sore throat

Viral myocarditis can often be symptomless, so you may return to training as soon as the fever has subsided. However, too much training may lead to dangerous arrhythmias.

When to return to training after the flu?

An international consensus group appointed by the International Olympic Committee published a new review and recommendations on the treatment of respiratory infections in athletes and their return to exercise. The consensus statement was published in the British Journal of Sports Medicine in 2022. A Finnish expert group has also made a statement on returning to sports after a respiratory infection based on the review.

After a respiratory infection you can gradually start training again according to your own feelings. If you have symptoms such as fever, muscle aches, sore throat or fatigue, you must not exercise.

When the symptoms of a respiratory infection ease, you can start to increase your activity with light exercise such as walking. Only when your health is good can you start running or other heavier training. Tip: Nordic walking is also a good workout for runners while you are getting your strength back!

You shouldn't immediately rush into interval training or marathon distances. Start with short and light runs. The longer you have been sick, the longer you should exercise lighter than usual. You have to be especially careful in sports that put a lot of strain on the heart muscle.

Listening to your own body while working out

Before starting trail running, I often had the flu. The health benefits of trail running were immediately visible in the first fall. Even though I was running a lot, I was healthier than ever: the flu hardly hit me, minor colds went away quickly, and my asthma symptoms decreased.

Getting sick with corona almost two years ago seems to have reset the health benefits of trail running in terms of flu and asthma, or at least the flu symptoms last a long time again and incite asthma symptoms.

It's really hard to 'waste' the great fitness accumulated by my trips to mountains in the summer and long-distance hikes in the fall just resting. But there's no way around it: if you start working out too quickly after a flu, you easily end up yo-yoing week after week (if not even the whole winter season!) with fluctuating health.

However, listening to your own body is easier said than done. In the fall, when the weather gets colder, your nose may run even when healthy, and asthmatics may get more respiratory symptoms during exercise. Then, when it gets dark, those who get the winter blues begin to feel tired in the body and recovery from running may be delayed without any flu.

When listening to your own body, the most important thing is to try to think about how the training has affected your health. Tip: Take a diary in which you write down:

  • What kind of training did you do (basic run, interval training, easy long run)?
  • How did the training go and what symptoms did you have during the training?
  • How did you feel right after the workout, and what about a few hours later?
  • Take a break of at least two days between training sessions and record any changes in recovery and strength.
  • Only move on to heavier training when you don’t get any symptoms from the training and you feel that you have fully recovered from the workout.
  • If you experience training pressure due to social media, also take a long social media break!

Halotherapy – help from the salt room?

Since I got a flu and associated asthma symptoms almost immediately after my autumn hike this year, I started looking for alternative treatment methods to prevent sinusitis. I ended up reading about salt room therapy that could help if you are prone to prolonged colds or coughs and sinusitis.

So, I started visiting a salt room to breathe the salty air. It is still too early to say how much the salt therapy will help, but after two visits I am hopeful. Right after halotherapy, I haven’t been coughing anymore and the asthma symptoms triggered by trail running have also been reduced.

It is not possible to speed up recovery from the flu, but the symptoms can be treated with rest and possibly also with salt room therapy. On the other hand, the flu can be prolonged and worsened by returning to training too quickly.

Wishing you great autumn trail runs!

Trail running Wellness


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