Well, I've just completed extensive cardio medical care and the Doc gave me a pamphlet. In it, is noted that sitting tends to crimp the blood flow from the legs to the body. Verbally, he added that there is a tendency over time for the ab muscles to sag while sitting and this is enhanced by the use of a chair back. When the abs sag, there is less support for the diaphram thus endurance falters. After prior surgery for an abdominal aneurism and my latest heart surgery my abs are a mess I know ... he didn't have to tell me. For me now exhaustion occurs quckly and I can feel the dampness of sweat in just a half hour doing any about any activity. This extreme and rapid loss of fluid demands replenishmment due to dehydration a severe problem due to my diabetes. I keep a gallon of water here in my office and more wherever !