Arnold was a big proponent of training to achieve a pump. The pump is a phenomenon whereby muscles become engorged with blood following resistance training. It's primarily achieved by performing multiple sets with moderate to high reps.
During a moderate-rep set, the veins taking blood out of working muscles are compressed by muscular contractions. However, the arteries continue to deliver blood into the muscles, creating an increased amount of intra-muscular blood plasma. This causes plasma to seep out of the capillaries and into the interstitial spaces, the areas between muscle cells and blood vessels.
The buildup of fluid in the interstitial spaces along with the osmolytic properties of lactate creates an extra-cellular pressure gradient, which in turn causes a rush of plasma back into the muscle. The net result is blood that pools in your muscles, causing them to swell. Researchers call the pump "cell swelling."
Many consider the pump a temporary condition that's strictly cosmetic. This is shortsighted. Studies have demonstrated that a hydrated cell stimulates protein synthesis and inhibits proteolysis (protein breakdown).
Muscle hypertrophy (growth) is ultimately a function of protein balance – synthesize more muscle proteins than you break down and you'll pack on size. The fact that cell swelling simultaneously increases synthesis while reducing degradation is a muscle-building win.
What drives this swelling-induced hypertrophic response? Increased fluid in muscle fibers leads to a stretch of the cell membrane. It's like an overinflated water balloon. The muscle, in turn, perceives this as a threat to its integrity and responds by initiating an anabolic signaling cascade that ultimately serves to reinforce its ultrastructure.
Usually targeted movements that keep constant tension on the muscle induce the pump. The occlusion and hypoxia created from "chasing the pump" leads to a long list of hypertrophy-boosting mechanisms. One of these includes increased satellite cell activity, which also increases the muscle cell's ability to continue expanding.
So if muscle growth is your goal, don't be afraid to incorporate some pumping sets into your routine.