What is "the pump" exactly?

Arnold didn't invent the pump, but nobody did more to make it famous...

Ever since then, people have spent their lives “chasing the pumps”. Exercise equipment, training programs, and supplements have all been built and sold with the sole purpose of creating that mythical pump.

But what exactly is the pump?

Why does it happen?

How important is it really?

Does it lead to lasting growth or is it just a cruel taste

The scientific term for the pump is “cellular swelling”. It happens when sustained muscle contraction occludes the veins leaving the muscle, so more blood begins to go in than out.

If we know our fluid dynamics, the pressure in the capillaries will force water into the space between the cells (called the interstitium).

Now we have more water and fewer solids outside the muscle cell than inside, so muscle cells begin to take on water.

Meanwhile, as the muscle contracts and fatigues, metabolites like lactate and phosphate ions accumulate in the cell. These molecules draw more water into the cell to reestablish a balance between intracellular to extracellular water.

The product? A muscle that is hyper-hydrated. But as everyone knows, the effect is only temporary. Once the tension is gone, the veins are no longer occluded and  metabolites can be cleared and water begins to leave the muscle.

You might be asking yourself right now- but wait, if its temporary, why does everyone seem to be so interested in it? Is the pump even necessary for hypertrophy?

We actually know surprisingly little about what cell swelling means for hypertrophy.

We know mechanical tension is a central driver of hypertrophy, and metabolic stress complements mechanical tension at low and moderate loads as long as we do enough reps to create fatigue.

This is also where cell swelling tends to occur. Some researchers speculate that cell swelling might create additional pressure against the cell membrane, augmenting the mechanical tension signal, but it's not clear how. 

Since all these things happen at the same time in a working muscle, it's hard to parse out a specific effect from cell swelling relative to all the other things that happen when we get a pump.

So if the science isn't crystal clear that the pump specifically is doing much to help with hypertrophy, that doesn't mean you shouldn't train in a way that gives you a killer pump. Training with moderate loads and consistent tension to create fatigue is the gold standard for hypertrophy training. It maximize the training effect relative to recovery demands. 

Go for that killer pump, but be careful my friends. I will leave you with this passage from the broscience bible...

“A pump is seeing your future. Once you see yourself with a pump you want to be that big all the time. Then once you get that big, your pump gets even bigger. No matter how hard you try you'll never be as big as your pump. A pump is like tomorrow, and you can never reach tomorrow because once it is tomorrow, it is today again. But you will never stop chasing tomorrow because you always want to live another day and tomorrow never dies.”

-Stefi