It's a class on an exercise bike. The bike is a bit different to the bikes on the gym floor in that it doesn't have the programs and the pedals are harder to stop and usually the seat is harder. The bike has a knob in front of the seat that you turn to increase and reduce resistance so your instructor will tell you when to increase resistance and reduce it, when to sprint and when to slow down and when to pedal standing and pedal sitting (there is a lot of interval training).
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