Photo: Dave Hogan/Getty Images

As the stunned world mourns the loss of Michael Jackson, who died at the age of 50 yesterday at U.C.L.A. Medical Center, speculation about the King of Pop's cause of death is everywhere. We're wondering too -- could his rigorous training for an upcoming tour be part of the puzzle?

"It is believed he suffered cardiac arrest at his home," said brother Jermaine Jackson at a hospital news conference yesterday. Others are talking about the stress that consumed the superstar's life, his frail and thin body, his pre-tour workouts with "The Incredible Hulk" star Lou Ferrigno, and painkillers -- allegedly taken to cope with performance-related injuries.

No doubt, Jackson's rehearsals and performances over the past 40 years were grueling (he'd once broken a vertebra and leg from a fall on the stage). For his upcoming 50-show world tour, Ferrigno was reportedly making secret visits to Jackson's Los Angeles mansion to help him work on his fitness for dance routines. But some sources report Jackson -- who once rehearsed the famous moonwalk dance sequence for four hours until it felt like second nature -- would not lift weights.

Regardless of the outcome of his autopsy, and whether or not he was pushing himself too hard in workouts, one this is clear: Jackson inspired millions with his music and his moves. And doesn't that just make you want to get up and dance?