After announcing his participation in the San Antonio Rock 'n' Roll marathon a couple of months ago, Godsmack frontman Sully Erna has been forced to pull out of the Nov. 11 race as the result of an unspecified injury.

Back in August, Erna expressed his enthusiasm about the marathon by writing on his Facebook page: "Started officially training 2 weeks ago for the 26.2 mile Rock N Roll Marathon in San Antonio, Nov. 11th 2012! Hoping to get sponsors and fans to help support me and raise money for children’s hospitals around the country."

Now, a new post on Godsmack's official site reports, "Unfortunately because of an injury, Sully has chosen to back out of the San Antonio Marathon on Nov 11. He plans on continuing his training as soon as he is feeling better and appreciates all the fan support thus far. Fortunately, there will be other marathons in the near future and he looks forward to participating in one of them. He may have to take a little break for now, but he WILL be back in action very soon!"

While the injury was not specified, Erna did have knee surgery within the past year and was seen playing shows with a knee brace. However, he tweeted, "Knee is holding up great so far!" earlier this year.

Godsmack are set to play this year's ShipRocked Cruise with a massive lineup that includes Korn, Five Finger Death Punch, P.O.D., Sevendust and many others. Check out full details of the cruise here.