If you really wan to build up your muscles quickly, try Tapping.
4--7--12 etc... on all the strings
Move em around like 5--8--12 and then alternate
5--8--12, 5--7--12, 4--7--12 etc ....
Keep going until you can't anymore... break for a few minutes then do it again.
After many months you'll notice the muscle on the outside of your palm on the left hand just below the pinky is larger than the one on your right hand.
It'll also build up your speed and accuracy.