The general principle is simple: whenever the body has been in one position for a while, it is good to briefly stretch it in an opposite position.

I don't attend services when I am there, and I have never meditated, there or anywhere; I just take walks and read and lose myself in the stillness, recalling that it is only by stepping briefly away from my wife and bosses and friends that I will have anything useful to bring to them.

Ronald Lee, director of the Centre on the Economics and Demography of Ageing at the University of California, Berkeley, puts it briefly and clearly: "We don't really know what population ageing will be like, because nobody has done it yet.