A number of different conditions can cause difficulty in maintaining any one position for any length of time. The most common cause of this problem is low back pain caused by degenerative disc disease in the lumbar spine. You could also have this problem due to severe knee arthritis. If you have a lumbar disc problem, and in particular if there is any neural impingement associated with the lumbar disc disease, you might very well have pain shooting down your leg that makes it painful for you to put weight on your leg. At the same time, if you attempt to sit in a regular upright chair, your low back pain will prevent you from sitting for more than a few minutes. You might get up to walk around but then need to sit down again due to your leg pain. This is why people who have lumbar disc disease often need to change positions frequently throughout the day.
Unfortunately, there are very few jobs that allow you to get up and down and walk around whenever you want to all day. Even if you had a job like that, your production pace would be greatly affected by the time you spend away from the work station. Even if your lumbar disc disease is not severe enough to meet the listing requirements for that condition, you could still be considered disabled due to your need to change positions and be away from the work station so frequently. Your doctor would need to support you by saying that the condition of your spine is consistent with having these limitations. Be sure to share with your doctor how your pain affects you in your daily life.