How to Tell When a Fever After Excessive Exercise Is Too Serious to Ignore

Working your muscles during exercise generates heat. The more intense and prolonged the exercise, the more heat your muscles generate. Your body dissipates this heat primarily by sweating. As the sweat evaporates, body heat is lost. A slight increase in body temperature is not unusual during or immediately after exercise, especially if the environment is hot and humid.

However, a significant elevation in body temperature could signal heat exhaustion, heat stroke or another serious condition. A high or persistent fever after exercise should not be ignored.

An intense workout or sporting event that involves more physical exertion than you're used to can lead to an excessive elevation in body temperature. Known by the medical term ‌exertional heat illness‌ (EHI), this condition is most likely to occur in hot, humid conditions. A hot, humid environment limits the effectiveness of cooling your body through sweating.

Intense sweating in a hot environment also causes you to lose more body water, which further compromises your body's ability to maintain a normal body temperature. Although less common, EHI can also occur with prolonged or intense exercise in cool environmental conditions, especially if your clothing or protective equipment limits your body's ability to cool itself.

EHI encompasses a group of heat-related conditions that can occur with intense exercise. These conditions range from minor to potentially life-threatening. The level of increase in body temperature along with other factors — including hydration status and electrolyte balance — are key in determining the severity of EHI.

The National Athletic Trainers' Association Position Statement on exertional heat illnesses describes three EHI conditions: exercise-associated muscle cramping, exertional heat exhaustion and exertional heatstroke.

Body temperature is an important factor in differentiating exertional heat exhaustion from heatstroke. A temperature of 104 F or higher typically indicates exertional heatstroke. An elevated body temperature less than 104 F suggests exertional heat exhaustion. However, the capacity to tolerate elevated body temperature varies, so accompanying signs and symptoms are also important.

Muscle cramps can develop with intense exercise at any temperature, but occur more frequently in a hot environment. The development of muscle cramps may be the first sign of overheating. Resting and hydrating are best in this situation.

Continued exercise accompanied by an elevated body temperature could lead to heat exhaustion, which requires quick treatment to cool and rehydrate the body. Common symptoms include:

• Dizziness
• Lightheadedness or fainting
• Nausea, diarrhea or abdominal cramps
• Weakness and reduced performance
• Profuse sweating or cool, clammy skin
• Hyperventilation
• Headache

Exertional heatstroke is life-threatening and requires emergency medical attention. In addition to a body temperature of 104 F or higher, symptoms may include

• Disorientation
• Confusion
• Irritability
• Aggressiveness or irrational behavior
• Staggering or collapse
• Hot skin that may be wet or dry
• Vomiting or diarrhea
• Fainting
• Seizures or coma

A fever of less than 104 F after working out is usually related to overexertion and typically returns to normal within an hour or two with rest and hydration. Keep in mind, too, that a fever sometimes occurs coincidentally after intense exercise but is unrelated. Watch for other signs and symptoms of common illnesses, such as a cold, the flu or a stomach virus.

‌Read more:‌ Should You Exercise When You Have the Flu?

A persistent or delayed fever after intense or prolonged exercise might indicate another problem. For example, extreme exercise when you're unaccustomed to it might lead to muscle tissue breakdown. This condition, known as exertional rhabdomyolysis, is characterized by widespread muscle soreness, tenderness and weakness that develops and progresses following excessive exercise.

Fever also usually develops along with tea- or cola-colored urine, due to leakage of muscle proteins into the bloodstream. Immediate medical attention is needed to prevent serious complications, including kidney failure.