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EpicSki › Performance Articles › Effects Of Rotary Movements On The A Frame

Effects Of Rotary Movements On The A Frame  


by Bob Barnes, Training Director

Keystone Ski Resort, Colorado


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An A-frame, by definition, represents "knee angulation" of the outside leg. "Knee angulation" deserves quotation marks as it is technically a misnomer. It can result from an anatomy/equipment issue--specifically a setup that leaves the skier "under-edged" and trying to compensate. Or it can be a sign of a technical error.  In either case, the A-frame is a symptom of a problem--not the problem itself.


The A-frame/abstem typically appears at the end of a turn, but the problem is fundamental, and particularly obvious at the initiation. It relates directly to the "rotary mechanism" that a skier employs, which in turn affects stance, which dictates movement options.



Rotary Mechanisms (commonly but quite wrongly referred to as "turning forces") are the movements and mechanical principles we use to produce or regulate "torque" (the "twisting force" that is the essence of the Rotary Skill). Fundamentally, there are four basic rotary mechanisms and within these four are numerous sub-categories.


  1. Rotation
  2. Counter-Rotation
  3. Blocking Pole Plant
  4. Independent Leg Steering  


For those who aren't clear, try this experiment: Put a piece of paper on a carpeted floor and stand on it with both feet. Now twist and turn it. How can you do this?


  1. One way is to turn your upper body and arms first, then "yank" the paper around with your feet.  This "1-2" movement is classic ROTATION, also known as "upper body rotation," "stopped rotation," and others. It is the fundamental, defining movement of the venerable "Arlberg technique," epitomized by the exaggerated "Christiania Legere." The upper body movement can involve the entire upper body, the arms, the hips, the shoulders, the head, or any combination, but the fundamental principle is the same -- and it exemplifies the principle of "conservation of momentum."
  2. A second way to turn the paper is to do "the Twist." Here the upper body and lower body twist OPPOSITE directions, simultaneously.  This mechanism exemplifies Newton's Third Law (equal and opposite motion).  This is COUNTER-ROTATION.
  3. A third way is to grab something solid with your hand and twist against that. You could do this with a ski pole (if your significant other isn't watching) if you plant it firmly at an angle and push yourself around with it. Reaching out to the side and pushing something forward or back is the very definition of "torque" ("a force applied through a lever arm"). Like a wrench, your arm becomes the "lever arm" and you become the "nut." In skiing, this mechanism can take many forms, but most commonly it takes the form of the "BLOCKING POLE PLANT" so this is what I call it. I've had instructors in clinics who decided to call it "wrenching"--I like it!
  4. Is there any other fundamental way to turn that piece of paper? I can't think of any! Rotation, counter-rotation, and "wrenching"--note that all three of these mechanisms involve the upper body in some form. Now try this: put a second piece of paper on the floor, a foot or so away from the first. Put one foot on each sheet. You now have another option for turning that first sheet of paper, one that does NOT involve the upper body: just turn your foot! Using only your leg, twist the paper left and right. Try it with the other one, too. Make sure that ONLY your leg moves, rotating in the hip socket. Your pelvis and everything above it remains stationary. This is the mechanism I call "INDEPENDENT LEG STEERING." It has also been known as the "fulcrum mechanism."


You can, obviously, turn either foot/sheet of paper separately with this mechanism.  Now try turning them both at the same time. Look in a mirror, or have someone watch, to make sure that your legs turn beneath your pelvis, and that no "rotation" or "counter-rotation" of the upper body takes place.


  • Note that, unlike the other three mechanisms, with Leg Steering you can turn your feet quickly or slowly, smoothly or jerkily, continuously or "stop-and-go," and, most importantly, you can "aim" them with extreme precision.  And your upper body has absolutely no role in it--so it is free to accomplish other tasks, like balancing.  With this versatility and precision, Independent Leg Steering represents the defining movement of contemporary skiing.  It alone is the rotary mechanism that allows us to steer our skis through carved turns, to shape our turns continuously, and to guide them through any desired amount of skid. Not entirely intuitive, it is the steering mechanism we must introduce to beginning skiers, whether in a wedge or a parallel stance.


As a further aside, note that "leg steering" entails a couple things. There must be two separate pivot points (two sheets of paper) with some space between them! And to turn one foot, the other must be on the ground. Weight does not need to be equal, but the moment we lift the foot off the ground, we eliminate this mechanism as an option. (Try it with the sheets of paper--twist one sheet left and right with both feet on the floor, then lift the other foot and do it again. Your upper body necessarily comes right back into play.)


Please note that, while I assert that "leg steering" is the essential rotary mechanism of contemporary skiing, expertise still requires mastery of all of them. They all have their uses, and seamless, skillful, tactical blending of all possible skills and movements remains the hallmark of the true expert!



Each of these rotary mechanisms clearly affects the skier's stance. "Rotation" results in a "rotated stance"--upper body twisted into the turn, facing uphill at the end of the turn. "Counter-rotation" results in a "countered stance"--upper body facing toward the outside of the turn, or downhill at the end of the turn. "Leg steering" also results in a "countered stance," although typically much less so than counter-rotation, and the seperation between upper and lower body occurs in the hip sockets, rather than the abdomen and spine. "Blocking Pole Plant" does not entail any specific twisting of the body, and can combine with any stance.



With an understanding of Rotary Mechanisms and their effect on stance, we can now explore the next link in the cause-effect chain: how stance affects other movement options--specifically edging/angulation movements.


We control edge angle with ANGULATION MOVEMENTS--various combinations of ankle, knee, hip, and spine movements.  Let's see how the rotated, countered, and "slightly-countered" (what I like to call "properly aligned") stances affect these options--particularly knee and hip angulation. Stand up again....

First, let's just play with hip and and knee angles.  Bend sideways at the hips--explore the whole range.  Careful with those knees--remember that knee angulation means bending the knee "normally" while rotating your leg inward at the hip socket.  If it hurts, it's wrong!


Now, let's "rotate"!  (Please exaggerate, to highlight the point.) Twist the the upper body left, into the position it would assume if we used rotation to initiate a left turn. Holding this rotated position, try creating hip and knee angles again. Knee angles are easy, right?  But the rotation pulls our hips out over our feet--thereby precluding hip angulation!


Let's "counter-rotate" (exaggerate again). Just the opposite occurs, right? From a highly countered stance, strong hip angles are easy--as easy as simply "sitting down," bending at the waist. But knee angles are now impossible! Why? The required rotation in the hip sockets is all used up, effectively locking the knees.


Finally, let's turn both feet using "independent leg steering." To confirm that you've done it correctly, note that imaginary lines across your toes, knees, pelvis, hands, and shoulders will all remain parallel, unlike the "rotation" and "counter-rotation" examples above. Eureka!  Finally, all options remain available!  Hip angles, knee angles, and of course, ankles and spine--all movements are still possible!

Summary Principles: Rotation allows knee angulation, but precludes hip angulation. Counter-rotation allows hip angulation, but precludes knee angulation. Leg steering allows all important angulation options.


Still with me?


Now, we can see where that A-frame and abstem come from--and more importantly as instructors, we can see the solution!


"Rotation" is a highly intuitive rotary mechanism. It's usually the first instinct of any of us when we try to "force" something we're standing on to turn. Perhaps this explains why so many skiers rely on it as their fundamental turn-initiating movement--especially those who first learned on very long, seemingly "hard to turn" skis. Then there are those who learned from these people--which, I'm sorry to say, includes many of us instructors and a whole lot of patrollers. Regardless, it's a common sight on the slopes today. Let's look at it.


The skier rotates his/her upper body down the hill, then "yanks" the skis into a (skidded) turn. Of course, the first part of this movement--the upper body rotation--requires that the lower body/feet be stabilized by a solid "platform" (that piece of paper in our exercise above had to have some resistance to allow the upper body to rotate). "Rotation" typically takes the form of the "rotary pushoff"--literally a push with a twist from the platform of a solid edge set on the downhill ski, followed by an unweighting to allow the skis to twist more easily.


As the turn progresses, the skier skids through the "control phase," body rotated into the hill, hips consequently pulled out over the skis. To end the turn, and create the platform for the next turn, the skier must set an edge. Hip angulation isn't an option, so the outside knee moves in--creating the a-frame. The rotation in the hip socket (from this knee angulation), combined with the relatively flat ski at first, contributes to a washout of the tail of the downhill ski until it finally attains sufficient edge angle to bite and end the turn. There's the abstem!


We see this package of movements all the time, don't we? I hope I've clearly described the chain of causally linked events that lead to this a-frame and abstem syndrome.


It should also be clear that we can't solve the problem where we see it--at the end of the turn. We can't tell this rotated skier to "create some hip angles" at the end of the turn. We must solve the problem where it starts--at the start of the turn. The problem may manifest itself as an Edge-skill problem, but its cause lies in the Rotary skill!


So how do we "solve" an A-frame? If it's a technical, rather than equipment/anatomy, problem, we must usually focus on rotary skills. We have to help the skier identify the "rotation" that causes the problem and help him/her substitute "leg steering." This alone may solve the problem, by keeping the skier in a stance where other, more effective, edging options remain viable. Or we may have to further help the skier learn to exploit these new-found options. Either way, this Edging problem at the end of the turn will only be solved by focusing on Rotary--at the beginning of the turn!

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