I've always had strong legs. But because of injuries I've had to give up weight bearing exercises almost entirely. That means I can't lift heavy when I work my legs in the gym for most things. But cycling has improved the muscle tone in my legs and I have not lost any strength (based on how easy it is to lift what I can challenge myself with, other sports I can do, etc.). Some of the smaller muscles in the legs (hip flexors, anterior tibialis, peroneals, for example) are stronger. My hams, calves, and glutes are also stronger because cycling makes me focus on proper technique which uses those muscles versus how quad-dominant my body likes to be in most things. I started noticing these changes when I was only spinning indoors (which I did for quite a while waiting to "heal" and to see if I wanted to ride outdoors at all).

The thing about raw strength of elite cyclists is true..they tend to not have as much fast twich muscle nor do some of them need to produce very high power with their legs. What makes them great is that the power they do produce they can maintain for much longer than the untrained person, which is where those slow twich muscles come into play.

Slow twich muscle is smaller in cross-sectional mass than fast twich muscle, so by training those predominantly by cycling won't bulk you up or increase your strength much.

But if in your training you are not only working on producing and maintaining power but producing MORE power than you currently can, then you'll be working in a way that challenges all of your muscles.

So on a spinner, don't just spin at what's easy for you to do at 90rpms. And don't just struggle to push along at 60rpms. Use your heart rate zones and perceived exertion to train your body in different ways to get fitter.

Example: Start out with a warmup at an easy resistance where 100rpms puts you at maybe 50%. Work up to 55%, not going over 110 or so. Then do a long set where 90-100rpms puts you up at 65% by increasing your resistance. From then on, increase resistance and/or speed for long sets (well over 5 min each) until you're just shy of your anaerobic threshold. Then bring yourself back down to 75% then 70% then 60%, holding each for a while. There, you're building endurance.

For speeding cardiovascular recovery time and working on muscular strength, do shorter intervals where you're really cranking up the speed (you should still feel the resistance and not be bouncing out of control) or cranking up the resistance (not dropping below 60-65, because that's bad for your knees).

There are other theories, and you'll find other fun cardio stuff in spin classes. But this is a place to start. But it'll do you the most good once you learn some about form and technique, and that's where a good spin instructor can help you the most.

It's a lot of fun and a good workout without being so hard on the joints. Have fun with it!