i have the same problem whether sitting on stand or not after about an hour or so i have to stand up because my knees start to hurt and my legs get sore, but it only started last year, and it also does it durin school and when we have our block day which we have an double class periods of 1 and a half instead of 45 minute, and on wensdays i am basically siting al day long and it is torture, Oh and by the way i am only 16, course being 6'1" and 270# doesnt help much.