Sit down, you're rocking the boat
My office recently employed the services of an ergonomics consultant & physical therapist to evaluate our workstations. I signed up to make sure that I had all my heights correctly set, and heard some disappointing news.
It turns out that all or nearly all of the research & buzz about standing desks recently has been rather one-sided (go figure, trends being what they are), and standing all the time carries its own risks, including an elevated risk of hardened arteries. The takeaway from the visit was that standing all the time is almost as bad as sitting all the time, and the trick is really movement rather than one static position.
This desk was (not) made for walking
There was a pretty sharp distinction made between "movement" and using a walking or biking desk. She said that those are very rarely recommended and tend to cost a lot of productivity over time. They are mostly a weight loss gimmick rather than an ergonomics win. She also claimed that there is no science backing up the idea of sitting on an exercise ball being helpful. They tend to be unstable and lack back support.
Her recommendation for me at least was to alternate between sitting and standing (my desk adjusts to either height) throughout the day. Standing all the time can put a lot of strain on the postural muscles (the same ones I was trying to strengthen), and slouching in your chair (as I'm prone to do) stretches them out and makes them weaker. Using a foot rest can help relieve strain from standing as it lets you alternate leg positions without requiring you to lean on your desk. As a side note, I was also using too little arm movement when mousing (you should engage more than just your wrist), so that may be another thing to watch for.
Alternate between sitting and standing during the day if possible. Work movement into your routine, such as stretching, going for a drink of water, or walking around the office. If you stand, get an adjustable, locking foot rest so you can periodically change your position (think of the brass railing at the bottom of a bar).
While I'm sad that I hadn't found the magic cure-all button for work positioning, it was naive of me to think that such a thing existed. I wanted to feel as if I'd found "the right answer," but it appears that, like many things in my field and in life, the answer is "it depends."
Note that the evaluation has been done but the results haven't been sent back to us yet. When they are I'll have more links to share, including the recommended foot rest.