Airlines prohibit police from transporting prisoners handcuffed. To keep prisoners under control they use a leg brace on the prisoner underneath his or her pants. They can also use a locking shock belt under the prisoner’s shirt remotely controlled by officers escorting the prisoner.
It’s highly unlikely that one or both of these devices were not used on John Mark Karr for the trip from Bangkok to Los Angeles.
Leg Brace Link
Shock Belt Link