The upright row is a weight training exercise. This exercise involves the trapezius, the deltoids and the biceps. The narrower the grip the more the trapezius muscles are exercised, as opposed to the deltoids.
Deltoids (shoulders) and Back
Seated rows mainly work your rhomboids and biceps, and a little bit of your latissimus dorsi.
Never, you always have working muscles. Eg: your core muscles work to keep you standing/upright.
They don't hold you upright, more like sideways...
They both stand tall and upright .
Most of the work is being done by your leg muscles, but the upper body sees some activity as it keeps you balanced and upright.
No, they stretch different muscles.
It does in some cases, Say your benching one day. Your maxing out and your sore, you want to take a day break, so maybe work out your legs or core. If your doing a "Minor" exercise, like: upright rows, push ups, or pull ups it doesnt take in effect that much.
Piloerector muscles
The gluteus muscles are used when walking. They support the upper body in an upright posture.
The muscles are called arrector pili and they pull the hair upright for both of those reasons and a few others.. let me know if you want to know more :)
Pectoralis Major
There aren't any antigravity muscles in the human body. But as long as you're upright there are plenty of muscles that are working against gravity.