Move over, yoga and Pilates. A long-ignored form of Russian strength training is making a comeback, and disciples swear that two months of the exercise does more to lift the derriere (and improve balance and agility) than a years worth of sun salutations. KETTLEBELL TRAINING AT STUDIO LOTUS offers a departure from the mat classes and private instruction that are typically offered at the Morningside Pilates studio. Attendees of any of the seven weekly Kettlebell classes ($185 for a once-a-week, eight-week series) will be greeted with a cast-iron, bell-shaped weight with a handle on top, which is swung using both arms in various arcs and loops. In less than an hour, the low-impact exercise burns fat, sculpts muscles and offers a rigorous cardio workout. Expect to be sore to the point of not being able to sit.