Just pondering on how much bigger my thigh muscles are going to get now that I've built up a loaded commute and grocery-getter bike. Now I have a pretty light road bike which seems to be good for going long distances with minimal effort and getting a nice sweaty, heavy-breathing cardio workout. And with the commute bike which now has a rear rack and Performance trans-it grocery bag type panniers, the bike makes my upper leg muscles work harder and seems to be more of a weight-training exercise.

Has anyone alternated between a light and heavy bike and seen a bigger improvement in overall training and endurance because of it? Also, does anyone load up their panniers with weight just to get more of a workout?

Also, if any of you out there are worried about the weight of a bike with groceries, it's not a problem at all even up hills. The weight is a non-issue unless you're not riding a few times a week.