Senior Member
Join Date: Sep 2011
Location: Cincinnati, Ohio
Posts: 489
Bikes: '74 Schwinn Le Tour (x2), '83 Bianchi, '96 Trek 820, '96 Trek 470, '99 Xmart Squishy Bike, '03 Giant Cypress
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Your metabolism is up, and you're "feeling the burn".
Lactic acid build-up in the muscles.
I enjoy when I have a good solid run/ride that does that to me.