Deconstructing “The Pump” – What’s The Science Behind Muscle Pumps?

Ah, the muscle pump – in the annals of bodybuilding, is anything more sacred?  For the uninitiated, the pump is that sensation of muscle fullness after performing a lot of work for a given muscle group (whether directly or indirectly).

Ah, who am I kidding.  Everyone knows Arnold explained it best, way back in ‘77:

Of course, if you mention “Arnold” and “muscle pumps” in the same sentence, it’s usually this clip from Pumping Iron that springs to mind:

So we know bodybuilders like muscle pumps. But there’s an implied significance attached to achieving a pump that even casual exercisers know. It just instinctively feels ‘right to do.’

But does the pump deserve its vaunted status? Just what’s going on physiologically that causes a pump? And is it even necessary to achieve a pump to make good gains?

The pump is a traffic jam. (Warning: Boring science ahead)  What’s the mechanism that causes a pump?  Would you believe it’s nothing more exciting than osmosis?  Most trainers in the know dismiss the pump as mere fluid buildup, and on a macro scale, they’re right. Here’s what’s happening, in science-speak:

You’re embroiled in a difficult set, your muscle tissue is screaming for more energy (read: glycogen), and hydrogen ions are building up inside muscle cells (using that familiar burning sensation).  Blood rushes into the capillaries surrounding muscle cells and fluid and nutrients (oxygen, etc.) diffuse from the capillaries into the muscle cell.  On the other end, fluid (minus nutrients) exits the muscle cell by diffusing back into capillaries (osmosis).  When the hydrostatic pressure pushing fluid into the cell is greater than the osmotic pressure pushing fluid out of the cell, fluid builds up and a pump occurs.

Here’s the rub: The mechanism pushing fluid in is much more effective than the mechanism pushing fluid out.  In freeway terms, it’s a 6-lane highway entering the muscle cell, but a 1-lane street exiting the cell.  The end result?  The muscle pump – the body’s version of a traffic jam.  And just like all traffic jams, they eventually clear; pumps only last about an hour or two, and as fluid concentrations in the cell return to normal, cell volume too retreats back to normal.

What possible effect could this have on improved muscle growth? Only one, really: The “traffic jam” briefly results in a higher concentration of lactic acid, which has been shown in studies to correlate to growth hormone release (more lactic acid = more growth hormone).  Since growth hormone is a key hormone in muscle recovery (and consequently, hypertrophy), this premise shows some merit.

What does the science have to say about muscle pumps and hypertrophy (muscle growth)? Surprisingly little.  While some studies have shown a relationship between flow restriction and hypertrophy, it’s worth noting that they induced this flow restriction by literally occluding blood vessels (i.e., they wrapped an inflatable sleeve around the subject’s arm and cut off blood supply a la taking your blood pressure).   Even the researchers don’t agree on what exactly causes the muscle-building effects seen in the studies – one theory is that the occlusion of blood vessels results in greater mechanical load on muscle fibers (and we know that increased load results in greater muscle growth).  Whether or not all this translates into “pumps stimulate growth” depends on how liberally you read into the literature (and if you believe that a pump is occluding blood vessels to the same degree as a inflated sleeve cuff).

It’s also instructive to note there’s an alternative method to increase load on muscle cells – lift something heavier.

What about all the anecdotal evidence? Bodybuilders, who are the biggest (muscular) beings on the planet, prize the pump above all else, workout wise. Surely they’re onto something. Until you look at other populations of muscular athletes: Olympic weightlifters, powerlifters, gymnasts, sprinters; one black swan is usually enough to derail a theory.

But back to bodybuilding. One thing you may have failed to consider: The pump didn’t magically appear because you thought about it. Nor (generally) did it result from one workbout. It takes a lot of work to achieve a pump. So perhaps it’s not that the pump triggers anything special; it’s the work required to achieve the pump that’s the magic formula.

Certainly gymnasts work hard, fairly often.  Oly lifters too. Powerlifters can frighten small children with their training ferocity.

Hard work, the magic formula. Imagine that.

There’s no need to hit your arms with dumbbells, exotic bars, and cables from all different angles until you dislocate your aorta trying to achieve an incredible biceps pump.  It’s safe to say you can stimulate muscle growth without it.