Because you can get the exercise you need from walking your dog vigorously, running with your dog, doing some push-ups at home, or just playing.

WHITEHOUSE: President's Council on Fitness, Sports & Nutrition

Critics of the newer research say that the idea that running can harm the heart is based on research showing only an association meaning that exercise may not be the cause of the problem.

WSJ: Endurance Sports: Studies on Older Endurance Athletes Suggest the Fittest Reap Few Health Benefits