At almost 70, a problem that I've been fighting in recent months and years is an increasing varicosity in my lower calves. A bit of blood seeps out and this has created a reddish brown rash. When I stand for several hours, the sag in the lower end of the circulatory system reduces blood pressure in the brain, and I lose some higher functions. The blood vessels have become stretched, and, now thinner, they have become leaky.
Solution: Put the legs up on the desk, when I sit at the computer, and put the keyboard on my knees. Putting just the feet up on the desk isn't good - it can strain the knees. But crossing my feet and resting my calves on the desk, with the keyboard on my knees, slid to the right four inches so it's centered and kept from sliding down to my tummy by a notebook - this has me feeling better than I have in ages.
This position may also help prevent carpal tunnel syndrome. I sit at a 60 degree angle to the desk. The right mid-forearm rests on the edge of the desk and the hand floats over the mouse.