Health hazards of physical activity

Many disorders benefit from exercise (Pederson Saltin, 2005). These include asthma, cancer, chronic heart failure, coronary artery disease, chronic obstructive pulmonary disease (COPD), depression, type 1 diabetes melUtus, type 2 diabetes melUtus, hypertension, intermittent claudication, osteoarthritis, osteoporosis, rheumatoid arthritis and obesity. 

Exercise also promotes physical, mental and social wellbeing and delays the infirmities and disabilities that develop with age but these are effects that are difficult to study and to define (see Haskell, 1996 Pederson Saltin, 2005). Recommendations for beneficial exercise are given in Pederson Saltin (2005). 

One aspect of exercise that increases the risk of heatstroke is that the loss of heat by sweating requires a proportion of the cardiac output to be directed away from muscle to the skin. In this way, heat is lost from the blood as it circulates close to the skin. However, even in severe hyperthermia, the blood supply delivering fuel and oxygen to the muscles takes preference over that to the skin. In this way, symptoms of hyperthermia are overriden and physical activity is maintained so that the core temperature can rapidly increase to dangerous levels. (Neilsen et al. 1990). 

Exercise is not without risks these include sudden cardiac death, hyperthermia, hypothermia, hypoglycaemia, hypo-natraemia, a reduction in the effectiveness of the immune system, overuse injury and interference in the reproductive system in females. Whether severe physical activity affects the reproductive system in males is sometimes discussed but these are no reports in the scientific literature. 

During a marathon, the rate of heat production can be more than tenfold greater than at rest and sufficient to raise the core body temperature by 1°C every eight minutes, if no cooling occurs. The core temperature is normally regulated so precisely that it does not rise more than about 1°C. The main mechanism for cooling is evaporation from the skin. Endurance runners can produce one litre of sweat per hour which removes about 2.4 MJ of heat. The energy used and therefore converted into heat in a marathon is about 12 MJ 

---