[qwerta]

My father-in-law works (and stands) on street market for past 40 years. He is in similar shape as other man in his age. So I think there is no miracle here, just some physical work and exercise.

[keithpeter]

I'll bet your father-in-law moves about a fair bit on his stall assuming the market has barrows or stalls. I think the issue might be simply not moving much in whatever stance.

I'm a teacher and in term time in classes (6h a day usually) I'm not sitting much at all, but then I'm not standing in one place much either. No issues as yet...