Want Big Arms? Squat.

There are still some people out there who don't work their legs. For a long time, I thought this was a mythical breed, being to sketch comedy what the unicorn, dragon, and wildebeest are to fairy tales. But it's true, and they're very stubborn. They say they're not interested in the real world strength that only comes with leg and lower-back work. They say they don't wear shorts anyway. They say they're little wimps who can't handle the vomit-inducing beauty of a twenty-rep set of rock-bottom squats. However, there's always one thing that gets them: leg day makes the rest of you bigger.

“How can this be?”
“Are you serious?”
“Why do regardless and irregardless mean the same thing?”

It's true, squats, deadlifts, and leg press will make the rest of you grow, too. Tests have been done in which people only did squats, and their arms grew. The squat is a great exercise, but unlike in deadlifts, the upper arm is totally uninvolved. So how does it happen?

1. Heavy lifts release hormones
Hormones that make muscles grow. Your legs can move so much weight, it basically makes your body freak out. Your body doesn't want to let itself be crushed under hundreds of pounds, so it releases hormones to let you build the muscle that will assure that doesn't happen. You will never do a heavy enough lateral to get this effect.

And

2. CNS power
The large muscles of the glutes, hams and quads require a lot of nerve signal to fully activate. When your body gets used to activating a huge amount of muscle at once, it builds up your central nervous system. This will benefit every exercise, even your laterals (why do you like laterals so much?).

Anything you lift in the real world you need to be able to support with your legs and lower back, but even if you aspire to a martini glass figure, know that not going through the sickening, brutal pain of leg day is going to limit your growth.