If you have the feeling that your body is tense a likely cause is stress.
However some of that stress may be physical: an incorrect seating position or height settings for a chair cause your muscles to be under constant strain. They will never have the opportunity to truly relax when you're sitting at your desk.
Here's a guide to help determine the proper measurements:
Ergotron Workstation Installation tool (cm units)
I picked up this link from LifeHack.org.