The muscular pump is caused by trapped plasma within the muscle. During intense muscular contraction this force inward occludes the vasculature momentarily backing up blood flow through that particular muscle group. A compensatory increase of blood pressure forces plasma from the congested capillaries into the interstitial spaces of the muscle cells.
Bodybuilders commonly perform pumping up exercises before appearing on stage.
For most of us there is no real benefit from achieving a pump except for boosting your ego. Although, inability to achieve a pump is one symptom of overtraining . Fluid volume in the muscle and possibly blood volume decrease when glycogen stores are low. Intense cycling or swimming can also bring about a pump. The burning sensation during certain weight training exercises or high repetition training is caused by an accumulation of acid in the fatiguing muscle.