Patrick Schwarzenegger tweeted Saturday that he received stitches after the accident, and he thanked doctors who he said cared for him in Sun Valley.
He provided a link to a photo he posted to the social networking application Instagram, showing a deep cut on his lower back. He did not provide details on how he got injured.
A request for further details late Saturday from Shriver's spokesman was not immediately returned.
In 2006, Arnold Schwarzenegger broke his leg in a ski accident at Sun Valley while he was governor of California.