I've been working out lately on an indoor spin cycle. As I've been training I've found that I can get into a 85%/90% zone at either a high RPM (110) with low resistance or the same zone at a low RPM (60) with high resistance...often times out of the saddle.
My question is what is the physical difference between those two extremes? I assume that from an aerobic/threshold point of view it doesn't really matter the RPM as long as the heart rate is at the right zone. Is it just a matter of excersising different muscle groups (or types..fast twitch/slow twitch)?