I suppose it's possible, but I doubt it. I have no problems keeping up until my legs get tired, which seems to tell me (although admittedly uneducated) that my legs are either using up their glycogen stores or are full of lactic acid. In either case, I take this to assume that 1. I'm mashing when I should be spinning. 2. my cardio-vascular system isn't keeping up enough to allow me to spin and/or carry away the lactic acid.

in both cases, it seems like working to increase my cadence will help. please let me know if I've made incorrect assumptions. I'm just trying to improve.