Originally Posted by dchan
Didn't say automatically.. Also I stated that the flexion at the knee is not the cause but a result of trying to stay balanced if we move the knee forward relative to the balance point under our feet. We can do this by tipping the boot forward or dorsiflexing, assuming you can bend your boot or have the range of motion. if you have taken up all your ankle range by preflexing using a lot of forward lean or used a steep ramp and an overly stiff boot, many will compensate with the classic back seat stance.
OK, you said "generally." This is the stand alone sentence I'm perplexed about: "in order to compensate for this COM change, the skier's body will generally flex the knee more (move the hips back) and flex at the waist an appropriate amount to stay centered over their feet."
So my problem remains. If you mean what many or most skiers will do, whether from delta or some other phenomenon, you're dead on, of course. Which is what I concluded vis-a-vis "backseat." But if you're meaning that this is a desirable or useful outcome, such as you imply with the word "appropriate," I'm still confused. As far as I know, it isn't good mechanics. Ever. Period. At best, it's a common response that keeps us upright at the cost of out of the frying pan and into the fire.
I give. You're clearly an instructor, I'm a doofus, and I'll assume I'm massively misunderstanding your grammar.
Originally Posted by cgeib
One variable/joint at a time, flexing:
Knee moves CM back
Ankle moves CM forward
Hip moves CM forward
Mix and match for the net effect, might be forward, might be aft. Common for the boots to limit the range at the ankle and greater movement needed on up the chain to knees, hip, spine, neck, arms,....
Flexed knee & hip is only classic backseat if you believe the hip=CM.
Appreciate the elaboration. This may get at my problem. I don't know a rat's ass about motion analysis, and I'm not a great skier. But I know some biomechanics. And I'm dubious about a chronological sequence like this. If that's what you mean. Or the implication it's a necessary biomechanical compensation. Or the comment about the hip and COM. The hip - if you mean the greater trochanter - is actually a decent estimator of the normal coronal location of the COM inside the pelvis. If your hip moves in space, for instance, your COM also moves. Now I can change my COM by say, bending forward from my head while keeping my hips stable. But I'll fall on my face pretty quickly. The COM is dynamic, but only within a fairly small area; it's not actually freely moving in space independent of the body even when we're falling. If my hip/trochanter moves backwards, say by doing a squat correctly, my COM stays put because of the weight ahead of it. I think this is what you may be after in terms of the knee?
Now about this deal with starting things with the knee. Again, I'm no great shakes, but I don't start a turn with my tibia (I say tibia because the knee's not the lever arm, it's a link, and unless you want to hyperextend it, or shear something, it doesn't handle much force.)
I start with my toes. I can drive my hip inside (and down the fall line) as I'm still dorsiflexing during initiation, and my COM never has to go posterior a whit. (It may, but that's mistake, not a necessary sequence.) If anything, my tibia's moving my COM laterally and forward after my toes and foot and ankle have done the early work. Yes, I'm sinking, and yes that involves the tibia and hips moving in space, but not necessarily backward. Otherwise, you're indicating every good skier in the world momentarily goes major backseat, then recovers. Granted racers do this sometimes, during recovery movements, and it looks spectacular in slo mo, but AFAIK it's not a mandatory first movement in terms of mechanics. In fact, I was under the possibly wrong impression that the only time backseating is a good thing is to accelerate in the last 1/3 of the turn. Shades of Killy. Again, prolly a doofus about this, and it's possible you're talking about mm of change, very rapid, and maybe you have a tetra of ultra slo mo stick figures to back this all up. I'll stand corrected.
But there are no analogies in other locomotion. For instance, we don't walk or run or climb stairs like this; our COM either stays where it belongs a cm or two behind the superior anterior pubic ramus. Or to help things out, it moves forward. So we're doing a sort of Groucho Marx walk or run, falling forward, whoosh, get that other heel out there in time to keep our face off the pavement. That's what the kinesthetic centers of our brain have been hardwired to do. If our COM goes back, we fall on our butts. And overuse the quads. And underuse those nice mechanics formed by the gluteal attachments. In fact, we have butts, alone among all primates, to help keep our COM forward.
Anyway, I'll stop the geeking.
Edited by beyond - 9/15/15 at 10:03pm