Lower arm
Not all bodybuilders train their lower arm. Most of them do not require because during other exercises grabbing dumbbells burden lower arms. But to people who have not genetic makings that make possible to miss training lower arms, they need train and burden lower arms too. Some bodybuilder say that if they work with bigger load and retetition number, towards the end of training their arms begin to exhaust before muscles wanted to train. It can be visible during standing rows or bending arms and its reason is the good position of deriving and stricking points. Our lower arms always move and their resistivity is big againts mild effort, there are needed hard exercises done by big repetition number (20-30). But the most important to respect principle of pyramid during distribution of load.
Source: http://body.builder.hu