With the size of New York City apartments most grown-ups have long since given up on having their birthday parties in their homes. For many that means celebrating at bars and restaurants, but for some proud few an answer has come in a surprising place: The gym.

Yup, years after gyms started opening their doors for bachelorette parties full of ladies eager to learn to dance on a stripper pole, some adults are now persuading their friends that the best way to celebrate their big days is to sweat together. After all, kid parties have been renting out areas at places like Chelsea Piers for ages, so why can't the over tween-set?

One man who recently held just such a birthday party on a Sunday morning explained the logic like this: "For me, it was the perfect mix of fitness, hangover cure and justification for guilt-free boozy brunching." Which is to say, he spent an hour sweating with friends before moving on to the more traditional birthday bars. Which meant he also "spent the next day in bed too hungover to move."

Of course, looking good with friends isn't cheap. Crunch, for instance, offers private classes (everything from Spinning to AntiGravity Yoga, to BOING with Kangoo) to anyone with $250 to spare. And for that quarter-grand you get an hour-long class for up to eight people. Beyond bachelorette and birthday parties, a rep tells us they also regularly are booked by offices looking for team building exercises. Exercises clearly being the operative word.