By now it is pretty well accepted both that physical inactivity is bad — recently linked to 6 percent of all cases of heart disease worldwide and 11 percent of all premature deaths in the United States — and that exercise is good, because it tends to increase life span and protect against heart disease. Yet few studies actually show that exercise reduces the unhealthful impacts of too much sitting. People who work out but also sit for long hours — active couch potatoes, you might say — may often share the same elevated risks for disease and early death as their less active peers.