Prolonged immobility has long been known to increase the risk of blood clots forming in veins in the legs (the medical term for which is deep venous thrombosis).  Blood clots in the legs can be quite painful and debilitating but they can also travel to the lungs which can be life threatening.  So doctors use medicines or inflatable leg squeezing devices to prevent blood clots in hospitalized patients who are bed-bound.  But there is a much more common time when we all are fairly immobilized – tr...