The debate rages on. Cardiologists in an article published in the journal Heart on Friday say that exercising vigorously for more than 50 minutes per day could harm your heart. Researchers Dr. James O'Keefe and Dr. Carl J. Lavie compared modern endurance athletes to the original Greek marathoner, a courier who dropped dead after running the 26 miles from a battlefield to Athens. "Even though chronic extreme exercise may not kill you, it may erase many of the health advantages of regular moderate exercise," they wrote. A study of 416,000 people found that 40 to 50 minutes of daily exercise reduced participants' risk of death by about 40 percent over eight years, but that beyond 45 minutes, longer sessions didn't seem to have any benefit.