Look at these two images. The first shows ankles closed. That means the lower leg, the shin, is tilted forward from ankle up to knee.
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This next shows ankles open. The lower leg rises at 90 degrees to the ski. This is our default stance position, learned on dryland from years of practice standing around. It needs to be unlearned for skiing, because it leads to backseat skiing.
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You can train yourself to keep ankles closed, but it takes paying attention to what your ankles are doing while skiing. You'll lose that attention, and need to bring it back when you notice that - whoops! - you lost it and the ankles are open. This is the first thing I work with clients on to get them to stop sitting back using their quads.
This next image has a line drawn upward from the foot, perpendicular to the surface of the ski. Notice how this skier, with closed ankles, has about equal body mass behind and in front of the line. He's a guy with broad shoulders and narrow hips. If he were a woman with narrow shoulders and slight chest, but wide hips, he might need to be even more "forward" to have his body mass and weight centered on his skis. Closing the ankles would do it.
Now imagine this guy (Warren Smith who runs Warren Smith Academy in Europe) not changing anything about his stance, but now he's on a tilted slope. His skis are tilted downhill, and his body is tilted more than it appears in this image. He will feel like he's waaay forward over the fronts of his skis, even though nothing has changed and he's pretty centered over the skis. In either case, his quads probably won't be working all that much because he's using his skeleton to hold himself up; he's fore-aft balanced, not back.
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Here's a skier, not a confident nor aggressive one, who has open ankles and is dramatically sitting back. Those quads are getting quite a workout, similar to do doing wall-sits. You, a confident and aggressive skier, may be doing a version of this at speed and not know it. Ask yourself, are your shins rising up at a 90 degree angle from your skis? Or are they tilted forward?
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Wall sits. I can't believe this woman is smiling. Oh, maybe that's a grimace
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Usually open ankles are accompanied by sitting back or leaning waay forward at the waist. Your lower back doesn't hurt, it's your quads that hurt, so I'm
guessing you were doing some sitting back on those blacks, keeping up with hubby.
@Skisailor has an alternative solution to sitting back. It involves going up and down, opening and closing the ankles, with each turn. You might find this easier to do since that up-down movement makes itself known more dramatically than keeping the ankle closed does. The up-down movement will draw attention to itself more than the closed ankles, you'll notice that you've lost it faster, and so you'll be able to put it back into your skiing more thoroughly.
If doing the necessary repetitions (best done on nonintimidating terrain) to embed the closed ankle thing is too boring or time-consuming for you, try skisailor's approach. Perhaps she will post here.