According to Scientific American, veins bulge on the arms and hands as well as other body parts, such as the legs, during and directly after a workout because of the increased arterial blood pressure's physiological mechanisms. WebMD identifies other possible causes of bulging veins in various areas of the body, including blood clots, varicose veins and peripheral venous disease.
Systolic blood pressure increases at the onset of exercise as the heart's rate increases to pump more blood through the arteries, explains Scientific American. Increased blood pressure in the arteries forces plasma fluid resting in the capillaries into the compartments surrounding the muscles. The process of the plasma fluid passing through the vessel walls is called filtration. This filtration results in a swelling and hardening of the muscles during exercise, which in turn pushes cutaneous veins toward the surface. The veins appear to bulge. Bulging veins are especially prominent on persons with little subcutaneous fat.
According to Muscle Mass Magazine, bodybuilders achieve vascularity, the prominent appearance of veins, through attaining a body fat percentage less than 10 percent, increasing muscle mass, retaining less water and elevating arterial blood pressure. Some bodybuilders use vasodilators, usually applied topically, to increase blood flow to the surface of the skin.