dsh, I understand what you're saying too, but the OP wants to train, I would say that pretty much means he wants to get stronger and increase his cardio endurance. I don't know of any leg workout where strength isn't attained by using more weight or resistance, that said, a 20 lb bike isn't going to be as effective as a 30-40 lb bike on weight alone. Take going to the gym and doing a leg workout, the first few times, anyone will hurt after the workout and even see a drop , but as the human body repairs itself, the strength and endurance will increase. Everyone will hit a wall where there is no more to be gained. That was my experience this summer. I rode my road bike, attained a certain level of fitness and strength. Then I bought the SS/FG based on posts like yours and others saying that it would make me stronger and fitter. That never happened, the SS/FG didn't have the GI to torture me like the road bike's top gears, and I could find a bunch of gears that could get cardio work done on the road bike that I could get out of the SS/FG. But after riding both those skinny tired bikes for weeks on end each, I finally decided to get on the mountain bike and see what that experience was going to be like. Pushing myself on it and just trying to get similar times out of it, the mountain bike left me even more drained after the same route. It had nothing to do with being bored or going slower, I was trying to accomplish a time to distance milestone that wasn't attainable by me on the mtb. You'd think after weeks of riding the skinny tired bikes, that I wouldn't be spent ? But the mountain bike was the next level of fitness for me in the order of riding each bike. Even when I swapped back and forth amongst them after riding each for the entire summer. The SS/FG was the easiest of the bikes I have, because it simply doesn't have the GI the road or the mountain bike has, weight and or rolling resistance for that level of training.

As for you stating that a heavier load is less effective than spinning, I disagree there as well. There is only so much you can get from the spinning and cardio. The whole purpose of training is to improve. To me that means you want to be able to take the next level of GI and be powerful/strong enough to get the same cadence/rpms from it. To do that your legs have to be more powerful and your cardio has to improve as well.

All I can say is try training with the 3 types of bikes yourself. I did and I know that each ride I mentally prepared myself to attack the rides, the mountain bike was the most physically exerting ride of the 3 bikes. So much that I got off the bike after 15 miles and not only as others have described, my heart and lungs felt like they were going to explode out of my chest, but I'll add I didn't hold my breakfast down that day. And the key was training on the mountain bike as if it were a road bike.

I agree with that and depending upon what gearing/speed you're riding the mtb, going 15 mph on it is working harder than going 18-20 mph on a SS/FG. So what exactly is your point as it pertains to training on a cruiser vs SS/FG ?