Last updated:

25 July 2023

What causes the pump in your muscles? Most gym folks claim it’s blood rushing to the working muscles – but they’re wrong. Here’s the facts.

what causes the pump

Reading time:

2 min read

I can still remember the first time I got a pump in the gym.

It felt like that scene from Rambo III where a tank crashed face-first into a helicopter…

The carnage… testosterone flying… people screaming…

It’s a feeling every athlete and bodybuilder has come to LOVE, with Arnold Schwarzenegger even going as far as to say this:

“Like the feeling of cumming… so I am getting to cum at home, and cum in my workout, and cumming all day long. How lucky am I?”

(I hope you read that in his voice.)
what causes the pump bodybuilding


Most people in the gym will tell you that the pump is caused by blood rushing to the working muscles.

However, this is wrong.

It’s caused by a build-up of waste products inside the muscle cell as you push it towards muscle failure. These waste products (metabolites) are created as a byproduct of the body breaking down nutrients to use for fuel. The net result is that the fluid which usually sits outside the muscle cell (interstitial fluid) and fluid from the blood are quickly drawn inside the muscle cell, causing your muscles to temporarily swell up and look f**king awesome for about 60 minutes.

This whole process is called osmosis.

Interstitial fluid is a water solution which consists of water, salt, sugar, fatty acids, coenzymes, amino acids, hormones, neurotransmitters, white blood cells and cell waste-products. It makes up about a quarter of the water in the human body.

So there you go, the pump is actually water – and that gives you even more reason to drink plenty of it each day!

Who Is Russ Howe PTI?


Russ has been a personal trainer in the UK since 2002, and provided both training advice and full programs on this website since 2011.

His work has been featured in Men’s Fitness magazine, and the content on this website led to him being voted one of the world’s top 50 fat loss coaches by HuffPost.

Russ spends his time coaching men and women inside the legendary Powerhouse Gym, South Shields, and writing training tips for the 114,301 members of his popular free training e-mail (join it below).

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