The former professional bodybuilder has been moonlighting in a California gym as instructor "Howard Kleiner"
Would you work out harder if you knew Arnold Schwarzenegger was your coach?
Disguised as Howard Kleiner, a "fitness instructor who had become regional manager for Gold's Gym", Schwarzenegger was filmed approaching clients mid-workout to give them tips on their training.
Standing over one member doing sit-ups, the star of Terminator tells her: "you have elbows and knees, so touch them."
He then encourages another member to include "ten reps of water sipping" between each set of exercises.
Schwarzenegger's appearance was a stunt to promote and raise funds for After-School All-Stars, a nationwide programme that provides free mentoring, homework help, tutoring and fun activities to students from low-income families.