Enlarge By Chris Pizzello, AP Jewel fractured her tibia in both legs.  Enlarge By Evan Agostini, AP Nancy O'Dell has two knee cartilage tears.

Singer Jewel, 34, has fractured tibias in both legs. O'Dell, co-host of Access Hollywood, has two knee cartilage tears. Their injuries are the latest in a series to hit celeb and pro competitors in recent seasons.

"I originally thought it was tendinitis or ligament strains," Jewel said before a concert appearance Thursday night in Highland, Calif. — which she would do sitting down. "It was painful enough for a doctor to put me on cortisone. When I went in for an MRI, it turned out to be something else. I had no idea something like this was possible."

Jewel's stress fractures are just below her knees, which had been swelling from a practice regimen of up to eight hours a day. They'll heal in six to nine weeks, says Jewel, who is using crutches to get around.

"Hopefully I'll get to sing on the show and come back and (compete) again," Jewel says.

Jewel's rodeo champ husband, Ty Murray, is still competing. "It's a big bump for us," he says. "Being together was a big part of doing the show."

O'Dell, 43, says she had felt twinges in her leg a few days ago and was diagnosed by a specialist Wednesday.

"Beyond disappointment doesn't even being to describe how I feel about not being able to continue," O'Dell says.

Dancing executive producer Conrad Green says no replacements will be revealed until Monday's premiere (8 ET/PT). "It's really unfortunate," he says. "When it boils down to it, this is an intensely athletic activity. Both (injuries) seem to be related to over-training."

Green says the celebrities, who are competitive by nature, often put in too many hours on the practice floor in the four weeks before competition.

"We'll re-look next season to limit the training in the early weeks, because I don't want people to get injured," he says. The 15-week-long show "is a marathon, not a sprint."

Among DWTS celebs injuried in prior seasons: Olympic volleyball player Misty May-Treanor (torn Achilles tendon); comic Jeffrey Ross (eye injury); actress Susan Lucci (fractured foot); and actor Cristián de la Fuente (torn arm tendon).