Originally Posted by LiquidFeet
If by pulling the feet back one is seeking to adjust where the feet are in relation to the CoM, you are most likely going to move the knees back too. That's the job of the glutes - hip extension. Foot pull-back as I know it involves opening both the hip and the knee.
First, Skiatansky's comments are very much focused on airborne skis, the situation is different under load. If you're sitting on a chairlift the easiest way to get your ski tips down is to pullback the feet and bend your knees more WITHOUT dorsiflexing your ankle at the same time. If you are in a weightless transition that is similar. You have any video of dolphin turns? that is what is happening.
Under load, its a different matter, as under load foot pullback is very much about dorsiflexing the ankle and pulling the foot back at the same time, which may or may not involve flexing the knee more. Under load the dorsiflexing of the ankle and reaction with the snow provides a way for the skier to pull their CoM forward...without changing the knee or hip one iota.
Secondly, the foot pullback concept is nothing more than that. Its not always about "getting the CoM ahead of the BoS". Its about minimize the BoS getting too far ahead of the CoM. See the difference? I think as you rightly point out LF, it will be very difficult to get the CoM completely ahead of the BoS with nothing but isolated foot pullback...but not always impossible. If your knee is not bent that far to begin with, then just a little bit of movement in the ankle flexing forward will move the CoM ahead of the BoS without changing anything in the knee or hip.
Obviously, in a more more squatty stance with highly bent knees, there is not generally enough ankle RoM to be able to get the CoM completely ahead of the BoS by pulling back the feet and flexing the ankle alone. But that is not always the immediate goal. It may be the goal in a few milliseconds in the future however. You can still minimize the loss to aftness by pulling back on the feet, meanwhile your knees and hip do whatever they do.
This involves hamstrings to pull and it also involves dorsiflexing the ankle, but recruitment of muscles higher up the chain can be used to leverage against the ground/snow to accomplish both. Dolphin turns can help to isolate some of the muscles used, but there are other muscles that matter, as under load we use the ground as leverage for all of that and dorsiflex the ankle at the same time also! Under load it is DEFINITELY not only about bending the knee, though that is very likely involved in the pulling action also.
Pulling the feet back does not require the the knee or hip to be extended, nor flexed. it only requires the feet to be pulled back, or at the least for pulling tension to keep them back. Its not a requirement that the CoM gets completely ahead of the BoS, its only about managing the foot's position the least amount ahead as possible.
Its also quite a bit more difficult, but not impossible, to pull your feet back under load while also extending your knee at the same time. In fact its quite a bit easier to do this while flexing your knee at the same time. This is because of the muscles recruited and the way in which they are recruited. But its not actually a requirement to bend the knees more as skiatansky described for dolphin turns, nor is it a requirement to extend the knee. You could be flexing or extending your knee or not changing it at all...and effecting foot pullback. Its quite a bit easier to do it while flexing though.