Joaquin Phoenix demonstrates what he did to propose to his girlfriend

Los Angeles - Joaquin Phoenix is engaged.

The 40-year-old actor proposed to his yoga instructor girlfriend, after she made him perform a complicated yoga move during a gym session.

The Her star revealed the news during his interview segment on The Late Show With David Letterman on Monday and he even got down on the ground to demonstrate his new found flexibility.

He shared: "We go out and at first we do these breathing exercises and I go, 'I can do this. It's pretty good.' And then she says, 'We are going to get into this first position, harnessing of the hog.' The harnessing of the hog, they have all these weird animal names."

"Before I can say, 'Who's the hog?' She grabs me."

Joaquin then proceeded to contort his body into an unusual shape as he shouted out to host David Letterman: "It's a compromising and vulnerable position."

"She gets a strap and puts it around my shoulder and she pulls me up.She's pulling and thrusting me and I go, 'Oh no stop, my back is in pain.' And she goes, 'No it's emotional back pain.'"

The three-time Academy Award nominated actor then dropped the bombshell as he revealed how he had proposed to the mysterious gym girl after their encounter.

He said: "This is the crazy part of the story ... we started dating. And I think she's the one. I proposed to her and she said yes!

"I don't know what that says about me. Nothing good, right?"