I don't know about you, but when I fly in today's airplanes I feel like Colonel Nicholson (played by that wonderful actor, Alec Guinness) in the movie "Bridge Over the River Kwai." If you have seen that movie, you will remember the scene where Colonel Nicholson is captured by the Japanese during World War II and placed in a small metal box in the boiling hot sun as a punishment. When he gets out, he can barely walk because he is so cramped. Flying in those small seats on airplanes (17 inches across) for any length of time gives me the same feeling. I get up at the end of my flight and my back and feet ache as I try to retrieve my bag from the overhead compartment. Fortunately, there may be a way to avoid those feelings: Yoga!. Yes, yoga.
As discussed in a recent article in The New York Times, "Striking a Pose Above the Clouds," May 5, 2013, there are several yoga poses you can do while flying and even sitting in your suit. These are basically simple stretches that will improve your circulation and maybe prevent that aching back. One of the easiest is to stretch and twist your body to the right and then the left. I suggest that you exhale as you twist to each side and inhale as you sit upright. Of course this move may be difficult if you are sitting next to the window or in the middle seat.
Another simple move is to place your ankle on top of the opposite knee and hold it there. Again, given the narrow space between your seat and that in front of you, this move may be difficult to do unless you at least partially recline your seat. While these exercises can help with the lower back, a simple hug to yourself can provide relief for the upper back. Just hug yourself, trying to hold your hands against your shoulder blades.
I find two other stretches to be particularly helpful on long flights; One is simply to bend forward as far as you can in your seat. The other is equally easy: raise your hands in the air as high as you can. Do this several times while doing deep, diaphragmatic breathing and you will feel better overall.
One helpful hint is to stand up as often as you can to avoid swollen ankles or worse yet, phlebitis an inflammation of the veins in the leg and which can cause death if blood clots are formed). Another way to avoid those is to simply lift both feet up on your toes several times/ You can do this exercise sitting or standing.
These are just a few poses and exercises that seem to help. It is also important to drink lots of water to keep hydrated and avoid alcohol. Try this approach and see if you feel better after a long flight.