Largely depends on the instructor and the bike. However, overall, if you can stand it, you are much better off doing Spinervals dvds (or other workouts) on your actual bike, on a trainer, than doing spin classes. Spin classes are a nice "addition" or occasional replacement every now and then to avoid burnout. Absolutely not a total replacement for the trainer.
At my former club we had 3 different instructors: (1) "spin guy;" (2) "fitness girl;" and (3) "tri girl."
Spin guy gave you a good overall workout, but, did too many "jumps" and low resistance sprints. Fitness girl followed the spin class material to a tee, tailored the workout to the lowest common denominator (which was pretty low) and did movements you would NEVER do on a bike (no, not kinky stuff, that would have actually kept me in class). Tri girl was a triathlete who did a lot of intervals and high-resistance drills that actually mimicked movements on the bike.
Spin guy and fitness girl were worthless for pure bike training (but ok for general fitness). Tri girl was good and worthwhile.
At my current club, spin classes are WAY TOO easy and have too many ridiculous movements (jumps, etc.) to mimic actual riding. I avoid at all costs.
Next, look at the bike. The LeMond RevMaster is, IMO, the most "realistic" and smooth trainer out there. They even have "aerobar attachments" now (I have one at home, but, rarely use it). You can get a good workout on this bike.
The "Jonny G Spin Bike" and the Schwin version are, IMO, worthless. I hate them. Not adjustable enough, does not fit well, rattles, and is not smooth. IMO, very little value.
The more time you can spend on your actual bike, the better, even if it is on a trainer. You are better off doing that. I compare spin class to elliptical trainers. They will give you a great workout, will help maintain fitness, and help prevent injury, but, it is no substitute for actual running.
If there are no dogs in Heaven, then when I die I want to go where they went. - Will Rogers
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