McKayla Maroney says the "body glitches" that Simone Biles is currently experiencing could end the gymnastics legend's career ... explaining "a lot of gymnasts actually quit because of these mental blocks."
The 2012 Olympic gold medalist spelled it all out in a powerful video on her social media page Tuesday ... saying she's been in Biles' shoes before, and it's not fun at all.
"I've had a lot of experience with these mental blocks," Maroney said. "They are miserable. They are a gymnast's worst nightmare and they can end your career."