For decades, bankers crunched numbers and counted money at 207 Fifth Avenue. Today, the building is full of exercisers doing crunches and counting reps, but the family behind Body Reserve has chosen to maintain the building’s incongruous prewar charm throughout its quarter century as a fitness emporium. Lights glow from below onto a latticework balcony railing, and stately square pillars support an enormous wood-framed clock built right into the wall.

In addition to weight benches and—on a mezzanine up close to the Greek Revival–style crown moulding—rows of spinning machines, the gym hosts barre, TRX, and spin classes and personal-training sessions taught by energetic instructors and the gym’s canine mascot. Boot camp pairs participants with a fitness coach for a high-energy session that blends military workout routines with cardio and strength training to burn up to 500 calories per session. Spinning burns calories quickly aboard easy-to-master bikes, whereas Hatha yoga takes a slower pace, combining breathing and slow, purposeful movements to release stress and tone limbs.