The loudest voice warning about the dangers of endurance exercise may be that of James O'Keefe, a sports cardiologist and former elite triathlete.

WSJ: The Exercise Equivalent of a Cheeseburger?

At 70 knots a female voice sounded in our headsets: ''Warning.

FORBES: Magazine Article

In fact, if you try to, say, call up a Web page while on a voice call with the Verizon model, you get an error message warning the two things can't be done simultaneously.

WSJ: New Verizon Service Bests AT&T in Call Reliability for iPhones