Ruth Bader Ginsburg's Trainer Reveals How the Justice Stays Super Fit at Age 85

"While not her favorite, the justice understands that this movement is great for her core," Bryant said.

Here is how to do an elbow plank:

• Start on the floor, resting on your forearms and knees.
• Step your feet out one at a time, coming into a plank position.
• Contract your abs to prevent your booty from sticking up or sinking. Your spine should be parallel to the floor, with your abs pulling toward the ceiling.
• Hold for 30 seconds, and work your way up to one minute as you get stronger.