I doubt it's the shoe - it would seem to me that there are too many interface issues, not to mention the wear and tear that goes on at the bottom of your shoe. Maybe a piezoelectric load cell that fits between the crank arm and the the pedal.
Where else are you going to put it? Crank spider has been done twice. There's one for the bottom bracket. One at the hub. One that measures chain tension. And then there's the measure-everything-else-to-derive-the-power iBike unit.
The only thing left is the pedals.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter