in order to gain advantages in endurance events. Abuse of EPO, however, increases RBC counts beyond normal levels (polycythemia) and increases the viscosity of blood, possibly leading to hypertension and increasing the likelihood of a blood clot, heart attack or stroke. The natural secretion of EPO by the human kidneys can be increased by altitude training, but the body has limits on the amount of natural EPO that it will secrete, thus avoiding the harmful side effects of the illegal doping procedures. Other mechanisms Other mechanisms have been proposed to explain the utility of altitude training. Not all studies show a statistically significant increase in red blood cells from altitude training. One study explained the success by increasing the intensity of the training (due to increased heart and respiration rate). This improved training resulted in effects that lasted more than 15 days after return to sea level. Another set of researchers claim that altitude training stimulates a more efficient use of oxygen by the muscles. This efficiency can arise from numerous other responses to altitude training, including angiogenesis, glucose transport, glycolysis, and pH regulation, each of which may partially explain improved endurance performance independent of a greater number of red blood cells. Furthermore, exercising at high altitude has been shown to cause muscular adjustments of selected gene transcripts, and improvement of mitochondrial properties in skeletal muscle. In a study comparing rats active at high altitude versus rats active at sea level, with two sedentary control groups, it was observed that muscle fiber types changed according to homeostatic challenges which led to an increased metabolic efficiency during the beta oxidative cycle and citric acid cycle, showing an increased utilization of ATP for aerobic performance. Due to the lower atmospheric pressure at high altitudes, the air pressure within the breathing system must be lower than it would be at low altitudes in order for inhalation to occur. Therefore, inhalation at high altitudes typically involves a relatively greater lowering of the thoracic diaphragm than at low altitudes. See also Effects of high altitude on humans References Exercise physiology Sports medicine