At the top of a very long, steep climb I become bow-legged near or at the top. I have to make a concerted effort to pull my knees in to point 'em straight. It could be that my quads & hams are swelling being that I've been Powerlifting for 15 years causing me to be unable to pull my legs in that close but I wanna make sure this isn't weak hip muscles or another muscle group being a weak link.
I am 225lbs at 6' & can squat 3 times my bodyweight in competition. I box squat about every 2-3 weeks to keep my hip muscles strong. I don't feel one muscle group giving out or burning more that any other so I can't pin-point it.
When I got older and heavier I found my knees going outward to keep my belly and thighs from crushing my privates against the saddle. It also helped my breathing, as did raising my stem until my weight came down.