Why do muscles 'pump up' when working out?

[6(SIC)6]

I've been working out for a while now and i've noticed how my muscles go like, twice as big as normal while i'm lifting, and they go very hard. I'm pretty sure this is what you big guys call 'a pump'.

Why does this happen? And what causes it? Is it just the muscle fibres themselves going hard and making the muscle swell, or are the muscles filling with blood or water and thats whats makes them swell?


Thanks.

 

[Mr BC]

I am not an expert but in my opinion 'the pump' is caused by increase in blood in a muscle.I also find that if you eat a diet high in protein the your muscle seem harder and stay pumped longer.

 

[JustinM]

yes it is because of blood going to the muscle to give it more "food/energy" and it also is taking away the lactic acid and CO2 i think

peace