As kewlrunningz stated, it is the whole package, heart, lungs and muscle endurance rather then muscel size and strength. In a place like CO. I would much rather have smaller legs muscles because they would not use up as much O2 and being lighter would make it easier to climb. The only time muscle strength becomes a big factor would be for short quick sprints, but for long distance their endurance is more important.
Slainte
