Heres where we get the idea...
Here is an article by Stuart McRobert.
To build muscle mass, you must increase strength. It?s that simple. You will never get huge arms, a monstrous back, a thick chest, or massive legs without lifting heavy weights. I know that probably doesn?t come as a revelation to anyone.
But despite how obvious it seems, far too many people (and not just beginners) neglect power training and rarely make increasing the weights lifted in each successive workout a priority. You must get strong in the basic mass building exercises to bring about a significant increase in muscle size.
One of the biggest mistakes typical bodybuilders make is when they implement specialization routines before they have the right to use them.
It constantly amazes me just how many neophytes (beginners), near neophytes, and other insufficiently developed bodybuilders plunge into single-body part specialization programs in the desperate attempt to build big arms. I don?t fault them for wanting big arms, but their approach to getting them is flawed.
For the typical bodybuilder who is miles away from squatting 1 ? times their bodyweight for 20 reps (if you weigh 180 lbs., that means 20 reps with 270 lbs.), an arm specialization program is utterly inappropriate and useless.
The strength and development needed to squat well over 1 ? times bodyweight for 20 reps will build bigger arms faster then focusing on biceps and triceps training with isolation exercises. Even though squats are primarily a leg exercise, they stress and stimulate the entire body.
But more importantly, if you are able to handle heavy weights in the squat, it logically follows that the rest of your body will undoubtedly be proportionally developed. It is a rare case that you would be able to squat 1 x times your bodyweight and not have a substantial amount of upper body muscle mass.