How Can I Make My Pitbull Muscular?

Well what i did and my dog loved it, i tied a large long rope on a strong tree branch and tied a knot at the end of it and my dog would jump in the air and hold on to the knot at the end of the rope and she  would swing back and fourth trying to get the knot out of it and she would hold on for a while lol about a good 10 Min's at a time and her back muscles were so strong from her swinging back and fourth

You have him go screw your mother!!

There is NEVER a need for that unless you are a scumbag wanting to impress and or fight that poor dog unlucky enough to get a looser owner

Tie wights to a harness and have him/her pull them around everywhere. When it gets too easy for the dog, increase the weight...

Along with good exercise you need to feed him/her right good food i used rice and scraps of beef from local meat market healthy and beefs up your dog.

Put them on a treadmill with weights strapped t her harness and have the dog run increase the weights and decrease the speed of tread mill increase weight and slow down treadmill and feed the dog beef or chicken or tripe it will be massive i have a pit its a girl 1yr and weighs 45 pound

Also agreeing with the harness and weights, they are a sporting kinda dog so they like a challenge and it's good exercise not 2 mention healthy for them. My dog use to love doing it and she was very muscular however, if it is for the purpose of fighting or anything like that I also highly disagree with your intentions.

Plenty of regular exercise!

Run run run run run.  And also big solid chew toys made of rubber or bone will beef up his neck and chest.  But running and tug of war are BY FAR the best ways.  Oh and feed him food that is specifically for large breeds.