Excessive exercise[edit]
Too much exercise can be harmful. Without proper rest, the chance of stroke or other circulation problems increases,[64] and muscle tissue may develop slowly. Extremely intense, long-term cardiovascular exercise, as can be seen in athletes who train for multiple marathons, has been associated with scarring of the heart and heart rhythm abnormalities.[65][66][67] Specifically, high cardiac output has been shown to cause enlargement of the left and right ventricle volumes, increased ventricle wall thickness, and greater cardiac mass. These changes further result in myocardial cell damage in the lining of the heart, leading to scar tissue and thickened walls. During these processes, the protein Troponin increases in the bloodstream, indicating cardiac muscle cell death and increased stress on the heart itself.[68]
Inappropriate exercise can do more harm than good, with the definition of "inappropriate" varying according to the individual. For many activities, especially running and cycling, there are significant injuries that occur with poorly regimented exercise schedules. Injuries from accidents also remain a major concern,[69] whereas the effects of increased exposure to air pollution seem only a minor concern.[70][71]
In extreme instances, over-exercising induces serious performance loss. Unaccustomed overexertion of muscles leads to rhabdomyolysis (damage to muscle) most often seen in new army recruits.[72] Another danger is overtraining, in which the intensity or volume of training exceeds the body's capacity to recover between bouts. One sign of Overtraining Syndrome (OTS) is suppressed immune function, with an increased incidence of upper respiratory tract infection (URTI). An increased incidence of URTIs is also associated with high volume/intensity training, as well as with excessive exercise (EE), such as in a marathon.[73]
Stopping excessive exercise suddenly may create a change in mood. Exercise should be controlled by each body's inherent limitations. While one set of joints and muscles may have the tolerance to withstand multiple marathons, another body may be damaged by 20 minutes of light jogging. This must be determined for each individual.
Too much exercise may cause a woman to miss her period, a symptom known as amenorrhea.[74] This is a very serious condition which indicates a woman is pushing her body beyond its natural boundaries.[75]
Mechanism of effects[edit]
Developing research has demonstrated that many of the benefits of exercise are mediated through the role of skeletal muscle as an endocrine organ. That is, contracting muscles release multiple substances known as myokines which promote the growth of new tissue, tissue repair, and multiple anti-inflammatory functions, which in turn reduce the risk of developing various inflammatory diseases.[76] Exercise reduces levels of cortisol, which causes many health problems, both physical and mental.[77] Endurance exercise before meals lowers blood glucose more than the same exercise after meals.[78] There is evidence that vigorous exercise (90–95% of VO2 Max) induces a greater degree of physiological cardiac hypertrophy than moderate exercise (40 to 70% of VO2 Max), but it is unknown whether this has any effects on overall morbidity and/or mortality.[79] Both aerobic and anaerobic exercise work to increase the mechanical efficiency of the heart by increasing cardiac volume (aerobic exercise), or myocardial thickness (strength training). Ventricular hypertrophy, the thickening of the ventricular walls, is generally beneficial and healthy if it occurs in response to exercise.
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