Dehydration and hyperthermia impair performance by increasing cardiovascular and thermoregulatory strain, accelerating fatigue and reducing muscular and cognitive function. Their effects are closely linked because dehydration limits the body's ability to dissipate heat.
The Physiological Mechanisms
Dehydration is a reduction in total body water, commonly caused by sweating during exercise. As water is lost, blood plasma volume decreases. This lowers venous return and stroke volume, so heart rate must increase to maintain cardiac output, producing cardiovascular drift.
Reduced plasma volume also limits blood flow to the skin. Because less heat can be transferred from the body's core to the skin, heat loss through sweating and evaporation becomes less effective. Dehydration of approximately or more of body mass is commonly associated with a noticeable decline in prolonged endurance performance, particularly in hot conditions.
Hyperthermia occurs when core body temperature rises because heat production exceeds heat loss. A high core temperature increases central nervous system strain, perceived exertion and competition for blood flow between the working muscles and skin. It can also disrupt muscle activation and metabolic processes, leading the athlete to reduce exercise intensity or stop.
| Condition | Main mechanism impairing performance |
|---|---|
| Dehydration | Reduced plasma volume lowers stroke volume and increases cardiovascular strain |
| Hyperthermia | Elevated core temperature increases central fatigue and reduces neuromuscular drive |
| Combined effect | Reduced skin blood flow and sweating accelerate heat storage and exhaustion |
Both conditions can also impair concentration, decision-making and motor control, which is especially important in skill-based and team sports.
A common misconception is that dehydration only causes thirst. In fact, thirst is a warning signal; the main performance impairment results from cardiovascular, thermoregulatory and neurological changes.
Exam Technique
For an IB SEHS explain question, construct a causal chain: sweating causes fluid loss, fluid loss reduces plasma volume, stroke volume falls, heart rate and core temperature rise, and fatigue occurs earlier. Distinguish dehydration from hyperthermia, then explain how each can intensify the other within A.3.3 Fatigue and recovery.