Stunt man Josh Headford was flying through the air at the Shrine Circus when his motocross bike hit a cable.
The impact sent the 20-year-old rider crashing to the ground- breaking a leg, wrist, elbow and shoulder.
Headford said the cable should have been lowered or removed by circus staff.
"There was a lot of shock," said audience member Katrina Vanwell. "You could hear everybody just, 'oh my gosh' immediately because it was insane. I can't even explain how hard it was to watch that, it was horrific."
An investigation is under way to find out what went wrong. Meantime the stunt man says despite his injuries, he plans to ride again after he recovers.