The infamous question in any spin class:
Is my bike set up correctly?
Use this as your guide:
The height:
Adjust the seat on your exercise bike so that when you fully extend your leg, your knee will be almost straight but not locked out. You dont want your knee to be too bent or you wont get the benefits f the full range of motion in your movement.
The fore-aft (How close the seat is to the handle bars):
When your feet are in the 30/90 position (parallel to the floor and in the toe gages), your knee should form a vertical line to the top of the laces in your sneaker (imagine what that would be if you are wearing cycling shoes). If your knee reaches the front of your shoe (too far forward) or falls in by your ankle or behind (too far back) then you need to adjust this position.
Handle Bars:
Higher puts less pressure on your lower back. A general rule of thumb is to have the handle bars at a similar height to your seat, but this is really your choice.
And, always feel free to ask the instructor to double check! We like to help!