Influenza, commonly known as the flu, is a systemic viral infection that affects the nose, throat, and sometimes the lungs. It is distinctly more severe than a common cold, which tends to remain localized to the upper respiratory tract. Flu symptoms often include sudden onset of fever, significant body aches, and profound fatigue. For individuals dealing with these systemic symptoms, the answer to whether they should exercise is generally a firm no.
Immediate Guidance: The Neck Check Rule
A simple, widely-used guideline to determine if exercise is safe during an illness is called the “Neck Check” rule. This rule suggests assessing the location of the most bothersome symptoms to decide on an appropriate course of action. Symptoms located entirely above the neck, such as a runny nose, mild nasal congestion, or a slight sore throat, suggest that light physical activity may be permissible.
If symptoms are limited to the “above the neck” region, activities like a gentle walk or light stretching might be acceptable and may temporarily relieve nasal congestion. However, the intensity and duration of the workout must be significantly reduced, often by 50% or more. If symptoms worsen during the light exercise, the activity should be stopped immediately.
Conversely, any symptoms that occur below the neck are a sign to stop exercising and prioritize rest. These symptoms include a fever (100°F or higher), generalized body aches (myalgia), and chills. Other red flags requiring complete rest are chest congestion, a deep hacking cough, shortness of breath, vomiting, or diarrhea. The presence of below-the-neck symptoms indicates a systemic infection requiring the body’s full energy reserves to recover.
Physiological Risks of Exercising With the Flu
Pushing the body with physical exertion while fighting a systemic viral infection introduces several physiological risks. Strenuous activity diverts blood flow and metabolic energy away from the immune system and towards the working muscles. This diversion can hamper the body’s ability to mount an effective defense against the influenza virus, potentially prolonging the illness or increasing its severity.
A primary risk is the potential for viral myocarditis, which is inflammation of the heart muscle. Influenza viruses can infect the heart tissue, and physical exertion significantly increases the strain on an already compromised heart. This strain can raise the risk of developing heart rhythm abnormalities, heart failure, or sudden cardiac arrest. Research suggests the risk of a heart attack can be six times higher in the week following a flu diagnosis.
Exercising with a fever creates a dangerous environment related to thermoregulation. A fever is the body’s natural response to fight infection by raising the core temperature. Adding exercise increases the body’s heat production and places a burden on its cooling mechanisms. This combination can lead to severe dehydration and dangerously high core body temperatures, increasing the risk of heat exhaustion.
Gradual Return to Physical Activity
The decision to resume exercise must be approached cautiously and systematically to prevent a relapse or complication. A person should be completely symptom-free for a minimum of 24 to 48 hours before attempting any physical activity. This includes being fever-free without the use of fever-reducing medications like acetaminophen or ibuprofen.
When restarting, the initial activity should be extremely light and low-intensity, such as a short, leisurely walk. A good protocol is to begin at no more than 50% of the usual duration and intensity of pre-illness workouts. This initial period of light activity should be maintained for a few days to ensure the body tolerates the effort without any recurrence of symptoms.
The return to a normal routine should be gradual, typically taking a full week or two to ramp back up to previous levels. It is important to monitor the body closely for signs of overexertion, such as unusual fatigue, dizziness, chest pain, or a return of muscle aches. If any symptoms reappear, the activity should be immediately stopped, and further rest is required.