Should I train when I'm feeling sick?

General Guidelines for Exercising When Sick

A common recommendation is the “neck rule.” This [1]suggests:

Above the neck symptoms (e.g., mild sore throat, runny nose, or sneezing) = exercise may be safe in moderation.
Below the neck symptoms (e.g., chest congestion, body aches, fever, or digestive issues) = it’s best to rest.

Resistance Training When Sick

Mild illness (e.g., common cold):
- What the science says: If symptoms are mild and only above the neck, low-intensity resistance training might be okay. Some studies suggest that light exercise can boost immune function temporarily. However, intense resistance training can suppress the immune system for hours after the workout, potentially worsening the illness.
- Recommendation: Stick to lighter weights or bodyweight exercises and limit workout duration. Avoid heavy lifting or pushing yourself to failure, as it can stress the body.
Severe illness (e.g., flu or fever):
- What the science says: When sick with more severe symptoms, such as fever or body aches, resistance training can increase your risk of complications (like heart issues) due to the stress it places on the cardiovascular and immune systems.
- Recommendation: Skip resistance training entirely until recovery.

HIIT When Sick

Mild illness:
- What the science says: HIIT is an intense form of exercise that places significant strain on the cardiovascular and immune systems. While light exercise may have mild immune-boosting effects, intense training like HIIT can depress immune function temporarily, making it harder for your body to fight off an illness.
- Recommendation: Even if you feel mild symptoms, it’s best to avoid HIIT when sick. HIIT requires maximum effort and taxes the body, which could prolong illness or make symptoms worse.
Severe illness:
- What the science says: High-intensity exercise while dealing with a more severe illness (e.g., flu or fever) can lead to further complications, such as myocarditis (inflammation of the heart) or dehydration.
- Recommendation: Avoid HIIT completely during severe illness and until you are fully recovered.

Cardio When Sick

Mild illness:
- What the science says: Low- to moderate-intensity cardio, like walking, light cycling, or jogging, can be safe for some people with mild symptoms above the neck (e.g., common cold). A light cardio session might even help relieve nasal congestion by opening nasal passages.
- Recommendation: Stick to low-intensity cardio if you feel up to it. Avoid pushing your limits—exercise should feel easy, and you should be able to talk comfortably.
Severe illness:
- What the science says: Engaging in cardio when experiencing severe symptoms, particularly fever or body aches, is strongly discouraged. The combination of illness and exercise-induced stress can lead to dehydration, heart strain, and a longer recovery period.
- Recommendation: Take a break from cardio workouts until severe symptoms subside and your energy levels return.

Specific Concerns to Consider:

Fever: Any form of exercise with a fever is discouraged, as the body is already under stress. Exercising while feverish can lead to dehydration, heart issues, and increased fatigue.
Inflammatory Responses: High-intensity workouts (e.g., HIIT and heavy resistance training) can cause inflammation in the body as part of the recovery process. When combined with illness, this can overwhelm the immune system, delaying recovery.
Immune System and Recovery: Research shows that moderate exercise can strengthen the immune system over time, while intense or prolonged exercise can suppress immune function for up to 24 hours. This is particularly important when considering HIIT or heavy weightlifting while ill.

Summary Recommendations:

Mild Illness (Common Cold): Light cardio and moderate resistance training are okay if you feel up to it. Avoid HIIT.
Moderate-to-Severe Illness (Flu, Fever, Chest Congestion): Rest is crucial. Avoid all forms of intense exercise, including resistance training, HIIT, and cardio.
Always Listen to Your Body: If you feel more fatigued than usual or symptoms worsen, it's best to rest.

References and additional sources: