Bicep question...

[Twin]

I do the curls standing.. like in this video

http://www.youtube.com/watch?v=ZawrdYA0WXU


My preacher curl weight goes up. But my curls dont..

then i did to the dumbbell curl first and i noticed it was easy with 30 lb's...

so i came to the conclusion that the preacher curl wares me down or hits the same part of my bicep... so perhaps i need to change dumbbell curl with forced negativity... but idk.


edit: also, again, i usually can do more reps with my preacher curl then the previous week. and increase the weight sometimes... so my bicep is getting stronger, but its just that im so tired and used all my strength from the preacher curl when it comes time to do the dumbbell curl...


but thanks guys. i will mix it up and do standing barbell curl every other week instead of seated. every single time..