It’s not spinning, Kim Gottlob explains, it’s indoor cycling. See, a regular stationary bike, she said, is good for cardio, and for toning the muscles in your legs. But, riding in place, speeding up, slowing down, does little to strengthen your core, and even less for your upper body. Here, at Hammer House on Broad Street, when she tells her class to turn right, they turn right. Left, and they do the same, their bikes following their bodies as they continue to pedal and start to lean. When she ratchets up the resistance and tells them they’re riding a mountain path, the bikes sway back and …