I can understand folks getting stiff and uncomfortable sitting for five hours in an airliner's seat. The answer, of course, is to get up and visit the loo a couple of times (even if you don't want to). Walk to the far end of the cabin - you have to time your walk for when there's no food trolley blocking the way, unless you've been really uncomfortable in which case the trolley gives you an excuse for standing up for a few minutes. This will also combat any risk of DVT.