Gains for dropping cadence?

For the last couple of years or more I've tried to religiously stick at 90rpm or higher as it seemed 'the' way to ride. I started it as I had niggling knee injury and less torque seemed appropriate so up went the cadence and in came 165mm cranks.

Anyway a few months ago I read something saying high cadence may be wasteful and started riding by feel in legs again and basically ignoring the bike computer. My knee didn't give me any grief last year so I assumed it was sorted and it still seems to be despite dropping cadence by a least 10-15rpm.

The big surprise this year is I've been setting loads of PRs which given my age I thought I'd never see again and most of them have been without bike computer at all. My backside also appear to have grown somewhat more muscular as well?

Are well all spinning for no reason? Is a bit of grinding good for you? 

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