Recovering from a herniated disc, Metallica singer and guitarist James Hetfield sat out three shows on the band's Summer Sanitarium tour, but the band thundered on with help from their tour mates, including Kid Rock and Korn singer Jonathan Davis, a band spokesperson said.

"As far as we know, the tour is going to continue in Denver on the 12th as scheduled, and James is expected to participate," the spokesperson, who preferred not to be named, said. "Hopefully, he's going to make a quick recovery and everything will go back to normal. ... He is one tough motherf----r. He is all about getting a gig done."

Hetfield, 36, aggravated a chronic back injury while jet-skiing during a break after a July Fourth show in Baltimore, MTV News Online reported. Despite receiving anti-inflammatory cortisone shots, Hetfield was unable to take the stage for Friday night's show in Atlanta and could not make dates in Irving, Texas, or Sparta, Ky. Management representatives told MTV News that Hetfield was in Denver receiving treatment for a herniated disc and a bulging disc.

According to reports on the band's official tour site, the rest of Metallica — drummer Lars Ulrich, bassist Jason Newsted and guitarist Kirk Hammett — opened their regular-length sets into Summer Sanitarium jam sessions, inviting members of tour mates System of a Down, Korn, and Kid Rock's band, Twisted Brown Trucker, to help fill the void.

Newsted took the mic for "For Whom the Bell Tolls" (RealAudio excerpt), "Creeping Death" and "Seek and Destroy." Metallica covered Creedence Clearwater Revival's "Fortunate Son" with the help of Kid Rock in Kentucky, and Kid Rock sang Bob Seger's "Turn the Page," along with a medley of "Sad But True" and his own "American Bad Ass" (RealAudio excerpt), the studio version of which samples the Metallica tune.

Korn singer Davis sang Metallica's "One." System of a Down guitarist Daron Malakian and singer Serje Tankian pitched in on "MasterTarium" — a medley of "Master of Puppets" (RealAudio excerpt) and "Welcome Home (Sanitarium)." Twisted Brown Trucker guitarists Kenny Olson and Jason Krause also helped out.

Metallica plan to make up for the gigs Hetfield missed after the Summer Sanitarium tour wraps on Sunday in Queen Creek, Ariz. The band will return Aug. 4 to Atlanta and Aug. 6 to Irving, and on Aug. 2 will play the Buckeye Lake Music Center in Columbus, Ohio, to make up for the Kentucky gig. The free shows will be open to fans who retain their ticket stubs from the past weekend's shows.

The only other time Hetfield missed work was in 1992, when he suffered burns from incendiary devices used during the band's tour with Guns N' Roses, the band spokesperson said.

Elsewhere during a tough weekend for heavy metal, Iron Maiden canceled shows in Germany, Bulgaria and Greece after guitarist Janick Gers fell 10 feet from the stage at a concert Saturday in Mannheim, Germany. Gers was hospitalized for head and back injuries, but Iron Maiden still are expected to begin their North American tour with Rob Halford and Queensrÿche on Aug. 1 in Toronto.