Originally Posted by
terrymorse
Then I'd guess your legs are just getting fatigued when standing. It takes training to build up fatigue resistance in your quads.
Standing drills should help: Stand for 10 revolutions, sit for 30, repeat. Eventually you should be able to stand for 30 revolutions without fatigue.
Thanks! I'll have to try this. I would think all the hill climbing I do would build up my quads, but I always climb seated and get super winded if I climb standing.