another cadance exercise

another cadance exercise ( more like squats on the bike really): I like to call this one Lucky Thirteens
-- spinning along in a 32X16 at 13mph (roughly 90rpm) shift up to the big ring and continue grinding along at 13mph (roughly 60rpm) until the gear gets to be too tall, then back down to the middle ring and spin along again at 90rpm, then back up to the big ring, then down to the middle, repeat ad nauseum