If the ground happens to shake at the intersection of Central Avenue and Lake Street, don't assume it's an earthquake. It may just be the trio of 83-lb. subwoofers thumping inside the Bassline Fitness studio. There, the custom sound system—accompanied by two 180'' video screens—pumps beats as groups pump weights in a variety of fitness classes led by energetic instructors. During Bassline's signature Mashup class, the brainchild of owner David Fox, members exercise their freedom to design their own workout, free to spend the entire hour on the spinning bike or mix it up by hitting the rowing machine or lifting a treadmill over their head. Meanwhile, in an adjoining space, smaller-sized groups sweat their way through a balanced blend of cardio and weight training known as Lean425.