The former California governor stumbles forward after the kick and the man is immediately restrained by a security guard.
The attacker was later handed over to police officers, event officials said.
Schwarzenegger later took to Twitter to thank fans for their concern, but insisted there was "nothing to worry about".
The Terminator later told fans to focus on the athletes at the event instead of the attack.
"We have 90 sports here in South Africa at the @ArnoldSports, and 24,000 athletes of all ages and abilities inspiring all of us to get off the couch. Let's put this spotlight on them," he wrote on Twitter.