The youngest son of Arnold Schwarzenegger and Maria Shriver is recovering in hospital after a body-boarding accident at a Malibu beach.

Christopher Schwarzenegger, 13, "is a brave boy and is expected to make a full recovery", the statement said.

The teenager suffered a collapsed lung and broken bones in the accident last weekend.

Shriver was composed when she alerted emergency workers in a phone call - apparently from the beach - at about 12.20pm on Sunday.

He "can't move off the beach - maybe like a broken ribs or something like that", she tells a Los Angeles County Fire Department operator, according to a recording released by the department.

"He's having trouble breathing."

Two of his siblings tweeted about their brother: Sister Katherine said "he's a tough little guy and getting better", and brother Patrick said "keep praying".

A statement issued by former California governor Schwarzenegger and Shriver, who are separated and in the midst of a divorce, thanked paramedics, lifeguards, doctors and hospital staff who cared for the injured teenager.