For a person to obtain prominent blood vessels on the forearms and other parts of the body, it is necessary to decrease his body's subcutaneous fat supplies. The International Business Times reports that this condition is not a result of bulging veins, but of elevated blood pressure during weight-lifting.
The International Business Times quotes Mark A.W. Andres, a Lake Erie College of Osteopathic Medicine professor, telling Scientific American that prominent blood vessels on bodybuilders are due to a spike in blood pressure in the arteries, which are responsible for transporting oxygenated blood from the heart to the muscles. This increase in blood pressure forces plasma fluid from the thin capillary walls into surrounding muscle, which then becomes firmer and swollen and presses subcutaneous veins closer to the skin's surface.
Stephen Ball, University of Missouri associate professor of nutrition and exercise physiology, told the International Business Times that the effect is much more noticeable on those with lower body fat percentages.
According to Iron Man Magazine, judges in bodybuilding contests refer to the extent of prominent veins on competitors as "vascularity." While judges often favor competitors with greater vascularity, that tendency is controversial as some say high vascularity is a sign of steroid use, since steroids cut subcutaneous fat levels considerably and quickly.