Feeling an unexpected chill or shiver during or shortly after a strenuous run is common. This sensation is a normal physiological reaction as your body transitions from the high-heat state of exercise back to its resting temperature. The sudden coldness signals a rapid shift in your internal thermal balance as the body attempts to regain a stable core temperature. While usually a benign sign of an efficient cooling system, it can also indicate depleted energy and fluid reserves.
The Body’s Thermoregulation System
Running is a highly inefficient process from a thermodynamic perspective, with only about 20 to 25 percent of the energy produced being converted into movement; the remainder is released as heat. This intense metabolic activity dramatically increases your core body temperature, which the body’s control center, the hypothalamus, works to counteract. To shed this excess heat, the body initiates a process called vasodilation, where the small blood vessels near the skin’s surface widen. This increased blood flow to the periphery allows heat to radiate away from the body.
Sweating achieves evaporative cooling. As sweat evaporates from the skin, it carries heat away. When you stop running, internal heat production drops rapidly, but cooling mechanisms, especially vasodilation and wet sweat on the skin, continue for a period. This lag means the body keeps shedding heat at a high rate even as the heat source diminishes.
This continued rapid heat loss often leads to an “overshoot” in the cooling process, causing a sudden drop in skin and core temperature below the set point. The body interprets this drop as a threat to its thermal stability and responds involuntarily with shivering. Shivering is a rapid, rhythmic contraction and relaxation of muscles designed to generate heat through movement. This acts as a corrective measure to raise the core temperature back to its normal operating range.
Energy and Fluid Depletion
While thermoregulation explains the mechanism of the chill, a lack of proper fuel and hydration can exacerbate the response. Prolonged running relies heavily on stored carbohydrates, or glycogen, which are broken down into glucose for energy. If these stores are depleted, your blood sugar level can drop, a condition known as hypoglycemia.
Glucose is necessary for the brain’s ability to coordinate temperature regulation. When blood sugar levels are low, the body’s thermal control system can become impaired. This impairment can lower the temperature threshold at which shivering is initiated, making you more susceptible to chills even with a smaller drop in core temperature.
Dehydration interferes with temperature control. Exercise-induced sweating reduces overall fluid volume, which can lead to hypovolemia, a decrease in blood plasma volume. Since blood transports heat from the core to the skin, reduced blood volume makes it difficult to maintain blood pressure and efficiently regulate temperature.
Dehydration can impair the body’s ability to regulate temperature during the run. If the body’s fluid balance is compromised, the subsequent rapid shift in temperature after stopping can be more pronounced, heightening the chill sensation. Insufficient fluid replacement limits the body’s ability to stabilize its internal environment, making the runner more vulnerable to rapid temperature fluctuations.
Recognizing When Chills Are a Warning Sign
Post-run chills are generally transient, mild, and resolve quickly once you change into dry clothes or consume a warm beverage. However, chills accompanied by other severe symptoms should be treated as a warning sign of a more serious condition, such as heat exhaustion, severe dehydration, or hypoglycemia.
Chills accompanied by persistent confusion, severe dizziness, or an altered mental state warrant concern. A lack of sweating during the run, despite intense effort, signals that the body’s cooling system has failed, which can indicate heat stroke. Persistent shivering that does not resolve with rest and warmth, or the presence of a fever spike, also warrants medical attention.
Preventing uncomfortable or unsafe chills involves managing the thermal transition and addressing potential fuel and fluid deficits. Implementing a proper cool-down, such as walking for 10 to 15 minutes, allows the body to gradually reduce heat production and blood flow to the skin. Changing out of sweat-soaked clothing immediately prevents excessive evaporative cooling and helps maintain a stable temperature. Consuming fluids and carbohydrates immediately following exercise helps replenish reserves, supporting thermal stability and recovery.