I’ve been sick. When can I start exercising again?

You’ve had a cold or flu and your symptoms have started to improve. Your nose is no longer running, your cough has calmed down, and your head and muscles no longer hurt.

You’re ready to get off the couch. But is it too early to go for a run? Here’s what to consider when returning to exercise after an illness.

Exercise can boost your immune system – but not always

Exercise reduces your risk of getting respiratory infections by increasing your immune function and ability to fight viruses.

However, intense physical activity can temporarily increase your susceptibility to upper respiratory infections, such as colds and flu, due to short-term suppression of your immune system. This is known as the “open window” theory.

A 2010 study looked at changes in the immune systems of cyclists who trained for up to eight hours after two hours of high-intensity cycling. It found that important immune functions were suppressed, leading to increased rates of upper respiratory infections after intense endurance exercise.

So we need to be more careful after performing exercises that are more difficult than usual.

Can you exercise when you are sick?

It depends on the severity of your symptoms and the intensity of the exercise.

Light to moderate exercise (reducing the intensity and duration of the workout) may be acceptable if your symptoms are a runny nose, nasal congestion, sneezing, and mild sore throat, without fever.

Exercise can help you feel better by opening your nasal passages and temporarily relieving nasal congestion.

However, if you try to exercise at a normal intensity while you are sick, you risk injury or becoming more seriously ill, so it is important to listen to your body.

If your symptoms include chest congestion, cough, upset stomach, fever, fatigue, or general muscle aches, avoid exercising. Exercising while you have these symptoms may make them worse and prolong recovery time.

If you have had the flu or another respiratory illness that caused a high fever, make sure your temperature has returned to normal before resuming exercise. Exercise raises your body temperature, so if you already have a fever, your temperature will rise more quickly, making you sicker.

If you have COVID or other contagious illnesses, stay home, rest, and isolate yourself from others.

When you are sick and feeling weak, don’t force yourself to exercise. Instead, focus on getting enough rest. This can actually reduce the time it takes to recover and get back to your normal workout routine.

I have been sick for a few weeks. What happened to my strength and fitness?

You may think that taking two weeks off from training is disastrous and you fear losing the gains made in your previous sessions. But it could be exactly what your body needs.

It is true that almost all of the benefits of training are reversible to some extent. This means that the fitness you have gained over time can be lost without regular exercise.

To study the effects of detraining on our bodily functions, researchers conducted “bed rest” studies, in which healthy volunteers spent up to 70 days in bed. They found that V̇O₂max (the maximum amount of oxygen a person can use during maximal exercise, which is a measure of aerobic fitness) decreased by 0.3 to 0.4 percent per day. And the higher the V̇O₂max levels before bed rest, the greater the decline.

In terms of skeletal muscle, upper thigh muscles become smaller by 2% after five days of bed rest, 5% at 14 days, and 12% at 35 days of bed rest.

Muscle strength declines more than muscle mass: knee extensor muscle strength declines by 8% after five days, 12% after 14 days, and more than 20% after about 35 days of bed rest.

This is why it seems more difficult to do the same exercises after resting even for five days.

But in bed rest studies, physical activities are strictly limited, and even getting out of bed is prohibited for the duration of the study. When we are sick in bed, we do some physical activities, such as sitting up in bed, getting up, and walking to the toilet. These activities may reduce the rate of decline in our physical functions compared to study participants.

How to get back into exercise gently

Start with a low-intensity workout, such as walking instead of running. Your first workout back should be light so you don’t get out of breath. Take it easy and slow.

Gradually increase the volume and intensity to the previous level. It may take you the same number of days or weeks of rest to return to the previous level. If you have been absent from an exercise routine for two weeks, for example, it may take two weeks for your fitness to return to the same level.

If you feel exhausted after exercise, take an extra day of rest before resuming training. One or two days of rest should not significantly affect your performance.