Crossfit may have just been one-upped as the most intense workout of all time. The new pain-inducing fitness trend? Taking hits straight to the abdomen.

Earlier this week, Joe Jonas shared an Instagram video of the workout he's doing while on tour in Tokyo. In the clip, Jonas does a series of leg lifts beside a personal trainer wearing boxing gloves. At the end of each rep, she clocks him right in the gut. (Ouch, right?)

The DNCE frontman isn't the only celeb getting punched as part of a training tactic. Mia Kang, Sports Illustrated Swimsuit model and pro Muy Thai fighter, also posted a video of herself being socked in the stomach: every time she extends her legs while doing squats, fighters on each side of her kick her in the belly.

What's the point of voluntarily taking a beating? Here's one payoff: Boxers, MMA fighters, and other martial arts athletes do it during training because getting walloped is part of their job, and they need to practice absorbing the blow, says exercise physiologist and personal trainer Tom Holland.

That explains why Kang was enduring those kicks. As for Jonas, he might simply be trying to strengthen his abs. While an external hit to muscle won't make it tighter or stronger ("if this worked, we'd punch our biceps and our leg muscles," Holland says), contracting your abs just before a punch or kick hits the stomach can create stronger muscle fibers.

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"When I used to teach, I would cue clients by telling them to imagine that their child is getting ready to hit them in the stomach," says Holland. "It teaches people to better engage in ab exercises and use the core muscles throughout." The internal muscle contraction, not the outer trauma of a punch, is what helps create an enviable six-pack. 

Of course, a blow to this sensitive area can be seriously harmful for average Joes (that includes you, Joe Jonas). Since this area of the body contains many vital organs, a punch or kick can result in bruising and internal damage, adds Holland.

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Regular gym-goers can take advantage of this (while avoiding a trip to the ER) by having a partner perform the punch or kick motion and stop short; you'll reflexively tighten your core as you see it coming. Or during crunches, start each crunch by visualizing someone about to punch your midsection or drop a medicine ball on your abs, recommends Holland. Your natural reaction will be to pull in your ab muscles as you complete a rep.

Holland also suggests breathing through your ab exercises for the best results. "It's a real challenge, but breathing while doing pretty much any ab exercise engages the core even further," he says.