Originally Posted by Stu

the problem most people have when they do curls is that they curl there wrist at the end of the concentric motion, this causes blood to become trapped in the forearms which can results in the painful pump that alot of guys experience. My first guess would be that this is what you are doing. As mentioned previously lower the weight and concentrate on keeping your wrist straight throughout the entire curl.

The reason hammer curls will not cause this pain is because your wrists are in a position where they cannot flex outwards.