I never have problems with cramping in my legs. I don't know if it's because I just can't sprint long enough or what -- but if I push myself hard, it's not my legs that stop me but my chest. Once I've caught my breath again (a few minutes of low exertion or rest), my legs are ready again.

Some back of the envelope calculations based on how fast I go up hills of a known grade and my weight (plus bike) suggests that I can sustain about 120 watts for over an hour (with no problems), and can sprint at about 400 watts for about 30-45 seconds before I'm close to death.

If I sprint while in the drops, I get sick to my stomach (well, my lower abdomen -- it's hard to say exactly what hurts.) If I'm more upright it can still happen but it takes longer. I'm guessing that this is largely due to my weight -- being in the drops let's my gut puts pressure on things.

I guess I really need to just force myself to ride out of the saddle occasionally, get used to it, see what my knees can tolerate. For now, it just feels so weird that I don't want to do it. But it works for other people ...