Anecdotally, I can sit in a chair for hours, but sitting on a rock or reclining on the ground (things I do often) seem to cause my (45+ year old) body to "set i

Anecdotally, I can sit in a chair for hours, but sitting on a rock or reclining on the ground (things I do often) seem to cause my (45+ year old) body to "set in" and it becomes difficult to move afterwards.