Supreme Court Justice Ruth Bader Ginsburg (RBG) wears a sweatshirt that reads “Super Diva!” when she works out. But she’s not talking diva in the Mariah Carey or Aretha Franklin sense. No, like many things RBG, her sweatshirt pays homage to the world of opera.

Ginsburg even opted to blast opera during her workouts in the days before TVs were in the court gym (she now prefers PBS NewsHour), according to her longtime personal trainer Bryant Johnson. While most people opt for something with more of a beat to sweat to, Johnson explained, “When you know opera like she knows opera it gets you going. It has a beat.”

Johnson, who’s been training Ginsburg twice a week since 1999, shared the above anecdote and more tidbits of the justice’s fitness routine at a talk for his new book, “The RBG Workout: How She Stays Strong . . . and You Can Too!” on Tuesday night.

A crowd of mostly middle-aged women, dotted with a few millennials, filled Sixth & I to hear Johnson’s method of keeping the 84-year-old, two-time cancer survivor in tiptop shape.

While the left has expressed concern about what would happen to the court if the liberal justice were to die during the Trump administration, Johnson says he doesn’t feel “pressure” to keep Ginsburg alive, only to keep her feeling her best.

“Exercise won’t make you live longer,” he said. “But it will improve your quality of life.”

And it seems to be working. Ginsburg’s exercises have only gotten harder as she’s aged. Wall push-ups turned to knee push-ups that eventually transformed into full push-ups. Johnson saves the most important exercise, squatting with an exercise ball against a wall, for last.

“If you can’t do this yourself then you won’t be able to get off the toilet and you’ll need a live-in nurse,” the trainer explained. Valid point.

RBG isn’t the only justice Johnson trains — three out of four liberal justices are now clients (Ginsburg, Justice Elena Kagan and Justice Stephen G. Breyer). However, he says he would happily train any justice regardless of political leanings.

“Exercise is one of the great equalizers,” he said. But everyone’s workout is different. While Ginsburg opts for weight-bearing exercises, Kagan prefers boxing and basketball, the favorite workout of one former president. Johnson even recalled trying to get Barack Obama to shoot hoops with Kagan.

Obama’s response? “Kagan can’t shoot no ball.”