One day removed from her controversial induction into the Rock & Roll Hall of Fame, we caught Madonna leaving the Reebok Sports Club in New York after a workout yesterday afternoon.

But don't go thinking a vigorous exercise regimen's the only secret to Madonna's eternally youthful look - according to Justin Timberlake, the Material Girl's not afraid to suffer for her health!

According to the Sun, JT told a crowd of onlookers at a Hall of Fame ceremony over the weekend the story of how, while recording with Madge, he'd started feeling under the weather - only for Madonna to come through with the cure!

“We had a recording session in London," the singer told his audience. "I wasn’t feeling well. She said, ‘Would you like a B12 shot?’ And so I’m thinking that we’re going to call a doctor and he’s going to make a house call."

Guess again!

"[S]he proceeds to reach into her designer handbag and pulls out a Ziploc bag of designer syringes out. She looks at me and says, ‘Drop ’em’. I don’t know what to say to that, so I drop my pants. She gives me a B12 shot in my ass, and then she looks at me and says, ‘Nice top shelf’."

Man - why didn't we think of that one?

Of course, Justin was a good sport about the whole situation, claiming that the event marked "one of the greatest days of my life. That’s what she will continue to be for all of us — the shot in the ass, when we really need it.”

Good to see that he's not holding a grudge - some stars would get all butthurt in a situation like this one!