Activity and exercise can be important steps toward preventing muscle tension, but you also have to make sure you get enough rest. Some people overdo it at work, others might go to the gym and push their bodies to extremes. Either way you look at it, muscle tension can come from working yourself too hard and not providing the muscles with the rest that they need.
If you work a physical job, then muscle tension could be one of the side effects. Having a strong work ethic is not a bad thing, but you have to listen to your body. If you are working long days, you need to take breaks from the activity. When you have a day off, you should try to spend much of it relaxing.
As for exercise, you want to make sure to keep it reasonable, and this is especially true if you have a busy work schedule on top of the exercise. You have to know your limits and avoid pushing your body to extremes. When you are done working out, give your muscles time to rest and heal. You can only push your body for so long before it catches up to you.