Non stop calistenics are an excellent way to improve a cyclist fitness. Using a 3 kilo weight ball will kick your butt. Most cyclist tend to have stiff backs and tight hamstrings from riding in flexion(hunched over). Spending time working on core strength and flexibilty will be more beneficial for your overall fitness and should make you more comfortable on the bike as well.

My physical therapist wife recommends incorporating rotational movement with any strength building exercise. It maintains flexibilty in your spine. Functional strength building does not include things like leg extensions. Where in the real world would you do a leg extension?