The "steroid pump" is a hyperemia state where the sacoplasm of the muscle, that's the jelly matrix inside the muscle, increases. This volumizes the muscle. At high doses of steroids this causes a problem with athletic activity because the pump caused by increase blood flow restricts blood and oxygen exchange. So cardio output suffers. Think of Matius Putinowski when he first started MMA. He would turn red from increased blood volume, high red cells and hyperemia. He would gas out in one minute. Another issue is hypertonicity of the cardiovascular endothelium. This is a thin layer of smooth muscle that lnes all blood vessles. Normally it pulses and helps the blood move through he circulatory system. These cells also have androgen receptors. Under high androgen concentration the cells "stiffen" thus recducing their pulse rythms. This further complicates circulatory functions. So at high AAS levels you have a situation where red cells can be very high thickening the blood, sarcoplasm is enhanced volumiing the muslceature, added pump from increased blood volume and AAS associated fluid transport, and stiffened endothelium. The end result is reduced cardiac efficency.In the end, this is the reason, although they probably don't realize it, that MMA fighters and other performacne athletes use low end doses.