question about the right bicep

[Fixxxer]

When i start to palm mute, my bicep flexes. And after about half hour, i feel as if i've be lifting weights. Is this supposed to happen? Or is it the angle at which my arm is at?

[furiousnewf]

How hard are you pressing down :)

[Fixxxer]

im not pressing down that much at all, when i strum normally it doesnt flex that much, its when i just place my "palm" on the bridge that it flexes quite a bit