Sometimes when I ride hard I feel limited by my legs, and when this happens I've noted that it's difficult to push my heart rate up toward my max. Other times I feel more limited by my cardiovascular system: my heart is more easily pushed toward the max and I'm breathing much faster/heavier. My legs feel like they could do more but my heart is pounding like crazy and it actually causes me concern to push the legs as hard as they'll go (am I being paranoid?).
What's probably changing in each circumstance to cause the difference? I'd guess that the legs just aren't as strong some days, and so they don't go through oxygen as easily.
