"See it's like this..."
He curls his finger at me and I shuffle forwards, avoiding eye-contact with the rest of my sweaty classmates. When I'm within punching distance he starts the routine. He grabs my hair, and then mimes jerking it down on his knee, and then slamming it in the floor. It's hard to know what to do with my face. I'm not used to being cast in the role of the fearsome attacker.
We all nod grimly, even me, his vanquished adversary.
"Thanks." He flashes me a smile as a stumble back to my place, and then, as if wanting to add a personal touch, "And remember to keep those fists up."
I used to go to a Body Attack class in London, run by a friendly (but presumably deadly) girl my age who always got us jabbing and kicking in formation to retro tunes, like a chorus of ineffectual fembots. This time round it was a sample class at a health and vitality expo, and the testosterone factor was pumped up to the max.
Which was more than can be said for me. Thirty minutes in and I'm panting like a bitch and fantasising about the cooldown. The girls flogging anti-aging skin care and flax health bars are watching us sweat and kick and grunt, presumably grateful for some entertainment after a hard day of guarding their samples and scrutinising their fine lines.
Our ripped leader, though, shows no signs of tiring.
"Rule no.1: Stay on your toes." He starts weaving to the hip hop beat, fists guarding his face. (That's rule no.2.) It's obvious he's a pretty good dancer - light on his feet for someone of his build.
"Say someone's coming up on ya. You wanna be all like, 'I don't want to no problems man' but still. You want them to know... to know, right? To know they ain't gonna be messin with you."
I nod eagerly, muttering "True dat!" under my breath, forgetting for a second that I punch like a girl.
But he's not finished with the demonstration. This time be picks on a tiny Asian teenager to his left. She looks pretty fragile next to him, especially when he gets her in a headlock and starts demonstrating the places on her body where he's aiming his kicks. Blissfully unaware of the uneasy spectacle he's presenting - muscles bulging as he shows us how to pummel her slender frame - he turns to us with a grin.
I just concentrate on my slams, and try not to choke up my green tea energy drink.