History of the Pump
Arnold made it famous in the first Bodybuilding documentary Pumping Iron. Ever since then every weightlifter has been chasing the pump all over their gym.
The greatest feeling you can get in a gym, or the most satisfying feeling you can get in the gym is… The Pump. Let’s say you train your biceps. Blood is rushing into your muscles and that’s what we call The Pump. You muscles get a really tight feeling, like your skin is going to explode any minute, and it’s really tight – it’s like somebody blowing air into it, into your muscle. It just blows up, and it feels really different. It feels fantastic.
What happens during your workout
During an intense chest workout the muscle will eventually become starved for oxygen. The endocrine system will be triggered from the fight or flight mechanism. Unnecessary biological functions will be stopped to divert blood into the muscle. As the workout progresses more and more blood will be diverted into the muscle to feed the muscle oxygen because without it muscle contraction would halt to nothingness.
At the deepest part of the muscle each tiny fiber, called myofibrils are banded with other myofibrils surrounded by connective tissue called endomysium, making it a fascicle.
Multiple fascicle are surrounded by more connective tissue called perimysium making those fasciculus. Multiple fasciculus are banded together by more connective tissue called the epimysium to make up a full working muscle. As you can see, muscle is contained by quite a bit of connective tissue that we’ll simply refer to as fascia.
So after explaining both of those, putting an abnormal amount of blood into a muscle would be like overstuffing feathers in a pillow, your fascia has one option. To stretch. All of the connective tissue surrounding the muscle stretches to compensate for the massive amounts of blood that aren’t normally in that muscle.
Since the fascia surrounding the muscles are made of a stretchy material called collagen, once stretched it won’t revert back to its normal size for quite some time. While the pump will go away usually within a half hour, this leaves plenty of time and space for muscle growth to occur.