Mark Wahlberg, star of the new football film "Invincible," says he's glad that he did his own stunts, since one hard hit fixed a previous back injury.

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The movie set in 1976 is based on the true story of Vince Papale, a 30-year-old diehard Eagles fan who tries out for the team and shocks everyone, including himself, by making it, CBS reported Tuesday.

Papale is played by Wahlberg, who told "The Early Show" that he knew the football action in the movie had to be as realistic as possible for the film to succeed, so he did all the contact scenes himself.

It turned out to be a great decision, Wahlberg said, one of the hits he took during production actually knocked something in his back into place, helping a longstanding back injury.

"Invincible" took in about $17 million in its first weekend in theaters.