problem with Bicep Barbell concentration curl

[shrek]

All the stress shifts to my back and it hardly seems like a bicep exercise...has anyone tried it and gone through this problem? You can't perform the curl without bending your back and when I do that, I'm really lifting the weight with my back, hardly any stress on biceps...whats wrong here?

 

[Gazhole]

Correction - you cant lift it without bending your back because its too heavy.

If you cant keep good form with that particular weight, lower the weight untill you can keep good form.