Hold a bowling ball close to your body, and it would be no problem to keep it there a while.  However, hold it straight out in front of you and you’d feel an exponential increase in the effort required to hold the ball up

Similar to a bowling ball, your head weighs about 8-12 pounds.  With proper posture it would be balanced directly above your shoulders. Muscles on the back of the neck are responsible for keeping the head from falling forward. Prior to computers, driving, and factory work this was a light-duty job for these muscles. However, it is very common for people today to spend many hours a day in a poor posture position with the head out in front of the shoulders.  Over time, this stresses muscles on the neck leading to pain and dysfunction.  Further, it can lead to permanent changes in the neck and upper back and cause ongoing neck pain, upper back pain, and headaches.

I found this video helpful in improving your posture while sitting at your desk.  Try this out today.  I just did and I feel fantastic!