Exercise in Heat and Heat Illness

Muscular exercise generates heat, and when the environment is also hot the body must dissipate a large thermal load while still supplying blood to working muscle. The competing demands of thermoregulation and exercise place the cardiovascular system under strain, and when heat gain outpaces heat loss, core temperature rises into the range that produces heat illness. This entry describes the physiology of exercising in the heat and the spectrum of heat-related disorders.

Definition

Heat illness comprises a spectrum of disorders — from heat cramps and heat exhaustion to heatstroke — that arise when exercise and environmental heat raise core temperature beyond the body's capacity to dissipate heat, heatstroke being defined by a high core temperature with central nervous system dysfunction.

Scope

The entry covers human heat balance during exercise, the cardiovascular challenge of partitioning blood flow between skin and muscle, the role of sweating and dehydration, and the continuum of heat illness up to exertional heatstroke as a tolerance failure. It is a reference account of mechanisms and how evidence is generated; it does not provide first-aid, cooling, or treatment instructions.

Core questions

• How is heat balance maintained during exercise in a hot environment?
• Why does exercising in the heat impose a particular cardiovascular strain?
• How do dehydration and reduced plasma volume worsen heat tolerance?
• What distinguishes heat exhaustion from heatstroke along the continuum of heat illness?

Key concepts

• Human heat balance and evaporative cooling
• Skin versus muscle blood flow competition
• Cardiovascular drift and reduced stroke volume
• Sweating, dehydration, and plasma volume
• Core temperature and hyperthermia
• Heat exhaustion and exertional heatstroke
• Heat acclimatization

Mechanisms

During exercise in the heat, skin blood flow rises to carry heat to the surface for evaporative loss, while exercising muscle continues to demand a large share of cardiac output; this competition, together with sweat-driven loss of plasma volume, reduces venous return and stroke volume and forces heart rate upward to defend cardiac output (González-Alonso, Crandall & Johnson, 2008). When metabolic heat production and environmental load exceed dissipation, core temperature climbs; sustained hyperthermia, often compounded by dehydration, can trigger a systemic inflammatory and cytotoxic cascade and progress to heatstroke with central nervous system dysfunction and multi-organ injury (Bouchama & Knochel, 2002; Epstein & Yanovich, 2019). Repeated heat exposure induces acclimatization — earlier and greater sweating, plasma volume expansion, and lower exercising heart rate and core temperature — that improves tolerance (Armstrong et al., 2007).

Clinical relevance

Heat illness is a recognized hazard of exercise and labor in hot conditions, and exertional heatstroke is a life-threatening emergency. This entry explains the physiology and the evidence base; recognition, on-site cooling, and medical management of heat illness are governed by current clinical and sports-medicine guidelines and are outside the scope of this educational reference.

Epidemiology

Exertional heat illness affects athletes, military personnel, and outdoor workers, with risk rising with high ambient temperature and humidity, high exercise intensity, inadequate acclimatization, and dehydration; it is among the leading causes of exercise-related collapse and death in young athletes (Armstrong et al., 2007; Epstein & Yanovich, 2019).

Evidence & guidelines

The physiology and clinical spectrum are summarized in major reviews (Bouchama & Knochel, 2002; González-Alonso, Crandall & Johnson, 2008; Epstein & Yanovich, 2019), and prevention and recognition during sport are addressed by a position stand on exertional heat illness (Armstrong et al., 2007). Specific thresholds and management protocols are set by current guidelines and not reproduced here.

History

Concern with heat illness in exercise grew from military and occupational medicine and from road-race and athletic experience in the twentieth century, which documented exertional heatstroke and motivated systematic study of heat balance, the cardiovascular cost of heat dissipation, and heat acclimatization as a protective adaptation.

Debates

Definition and field diagnosis of exertional heatstroke

Because core temperature and mental-status criteria can be difficult to assess in the field and may overlap with exercise-associated collapse from other causes, the precise definition and rapid recognition of exertional heatstroke remain points of discussion in sports medicine.

Key figures

• Lawrence E. Armstrong
• Abderrezak Bouchama
• José González-Alonso
• Yoram Epstein

Seminal works

• bouchama-knochel-2002
• gonzalezalonso-2008
• epstein-yanovich-2019
• armstrong-2007

Frequently asked questions

Why is exercising in the heat harder on the heart than exercising in the cool?

In the heat the body diverts a large blood flow to the skin to lose heat while muscle still needs blood for work; combined with fluid lost in sweat, this lowers the blood returning to the heart, so stroke volume falls and heart rate must rise to maintain output.

What separates heat exhaustion from heatstroke?

Heat exhaustion involves an inability to continue exercise with high but not extreme core temperature and preserved mental status, whereas heatstroke is defined by a high core temperature together with central nervous system dysfunction and is a medical emergency.