Have you ever noticed how your back stiffens up on a lengthy plane ride. After taking two of them recently, I certainly did. While you sit for several hours, there is less blood circulating to your back muscles and legs. With less circulation, there is less oxygen getting to these areas, and muscles that receive less oxygen tend to tighten up and eventually cramp. A rule of thumb to prevent that stiff back while flying is to take a walk down the aisle every 2 hours. This will get the blood circulating to all of these areas and you will feel much more comfortable. Have a good flight.