Wednesday, October 31, 2012

The Kinesiology of Two Alpine Ski Racing Turns







(click on the photo above to see the whole thing)

The purpose of this study is to categorize the movement patterns, skills and requisite elements of alpine ski racing; to dichotomize the two most effective techniques; and to break down the aspects of skeletomuscular movements in each technique. While I am not a professional kinesiologist, I believe my analisis will be accurate enough for this study.

We begin by defining the “requisite elements” of alpine ski racing. The athlete we study here is one with two arms and two legs, normal range of motion in all joints, and a mind capable of grasping and demonstrating movement drills and concepts. The requisite equipment for this study shall be limited to ski boots, bindings, and alpine skis.

Next, we determine the nature and objective of alpine ski racing. The objective is to move the body and the requisite equipment as quickly as possible from the starting position, through each of a sequence of two-dimensional gates, and finally through a one-dimensional light beam at the end of the course; this performance is recorded by a clocked time, and compared to the clocked times of the other competitors.

While the skier can get himself out of the start with specific movements, and can “skate” to a certain speed, the vast majority of acceleration and speed are derived from gravity, which is used to pull the skier down the slope. Since there is a limit to how fast an athlete can travel and still complete the requirements of a racecourse, maintenance of speed is critical; paramount to the maintenance of speed is the manner of turning. The speed a ski racer is able to maintain as he or she turns left and right through the gates is chiefly determined by how well the skis are able to slide (denotes a flat ski) and carve (denotes moving longitudinally along its edge), and how he can keep the skis from “skidding” (denotes lateral sliding with the ski laterally angulated).

Because an alpine skiing racecourse has a specific beginning and end point, we say it is discreet. Because environmental factors are unpredictable and affect the athlete’s performance, we say alpine ski racing is an “open” skill.

Alpine ski racing is a dynamic skill set, requiring isometric (static), isotonic (dynamic), concentric (positive) and eccentric (negative) contractions of varying muscles and muscle groups. The mechanical principals cover a broad range: centrifugal force, acceleration, impulse, torque, gravity, mass, volume, and more. There is much science to alpine ski racing; however, modern competition dictates that a champion does not need to be a scientist.

From a historical point of view, alpine ski racing was originally based on turning very long, very stiff skis that had only marginal torsional strength (resistance to twisting), and almost no sidecut (the hourglass shape of a ski, as viewed from above). In order to turn, the athlete had to angulate the ski onto its edge medially and bend it (reverse camber) i.e. the “effort” in this case was the athlete’s center of gravity dropping down with an eccentric (negative) abduction and adduction (unilateral angulation) of the femurs; the resistances were the athletes' eccentric leg-muscle contractions, and the lengths of both skis-- because in its normal state the ski is arched up under the foot (cambered). The fulcrum was at the hip, and because the fulcrum was between the effort and the resistance, we call this a first-class lever. Historically, this need to bend the ski became known as “pressure.” (Pressure has been separated into a thousand parts, but few are able to define it, describe it or measure pressure) Because the skis were so stiff and offered so little sidecut, it was nearly impossible to bend both skis, so the focus was on bending the downhill ski.

The preparatory phase of this technique began at the turn initiation. The athlete had to first bias his weight to the downhill ski. (The uphill or inside ski was, for the most part, kept parallel, and only utilized to a small extent; therefore, for the sake of this analysis, we will suffice to say that historically the uphill ski “followed” the downhill ski, and was kept out of the way; sometimes, the uphill ski was even lifted off the snow for part or most of the turn). Then, the skier had to focus on slight lateral angulation of the ski (tipping) by some combination of flexing the knee and rotating the hip of the downhill leg, and abducting the hip of the downhill leg. Once the ski was angulated slightly (control being a major factor), the skier had to “pressure” the tip of the downhill ski by extending his waist, moving the pelvis forward, and extending the downhill hip and knee so as to bring his downhill foot behind his lateral plane (and center of gravity).

Once the ski began to “edge” or bite into the snow, the skier could begin the power phase: this was a controlled drop of his center of gravity via relaxation. Though different athletes performed varying techniques of the same skill sets, the movements were a combination of continued extension of the downhill knee; continued abduction of the downhill femur and slight pelvic tilt toward the downhill side; medial rotation (inward twisting) of the downhill hip and femur; flexion of the downhill hip; pelvic rotation toward the downhill side (counter-rotation); increased pronation of the downhill foot; and stabilization of many muscle groups to keep the upper body “quiet.” Control of this center-of-gravity drop was critical, because as the ski began to bend and turn, the lateral angle of the ski would increase and the first-class leverage would increase, which synergistically turned the ski more and created more edge angle and leverage. At the point of greatest edge angle and leverage, the skier’s feet moved down the hill relative to his center of gravity. This forced his waist, knees and ankles to flex under the massive centrifugal force.

If the ski’s edge held, the skis changed direction from down hill to across the hill. The simultaneous “release” of the downward effort and extension of the hips, knees and ankles marked the end of the power phase. The bent ski would return to its natural arch (camber), and the energy from this reflex could project in almost any direction. A skilled athlete could use this reflex to project himself down and across the slope to optimally align for the next turn.
Immediately following the release point, the skier would enter the return phase. This is termed “cross over” or “cross under,” depending on the way the torso relates to the lateral movement of the skis and feet. This phase utilized movements to re-align the skis down the slope and set up for the next turn initiation. Weight bias was more equal between the skis, however, the bias was changed to the other ski during the return phase. The chief movements involved rotation in order to “steer” the skis back the other direction. Again, this could vary from skier to skier, but in broad scope, the movements covered rotation of the femur of each leg, rotation of knee (pivoting of the feet), slight flexion of the waist and knees, return from pelvic rotation, and return from pelvic tilt from the previous power phase.

This age-old technique focused mainly on the use of the abdominal muscles for stability, the quadriceps and abductors of the legs for eccentric resistance (to take up the slack for skeletal posture deficiencies), and the gluteus maximus muscle to aid in the extension of the waist at the end of the power phase. This technique involved very little concentric (positive) contractions of muscles, as the body’s leverage to bend the ski was merely controlled with submaximal eccentric contractions (less than total negative effort) as the center of gravity moved downward. The exceptions to this were when the athlete made an error (for whatever reason) and had to “recover,” meaning regain his balance, course alignment, etc.; these recoveries could take almost any form and use almost any combinations of muscles, bones and joints in the body (many times, athletes put a hand or an elbow down on the snow to keep from falling completely).

The modern ski technique utilizes skis that are softer, shorter and provide far more sidecut. This kind of ski allows the athlete to more evenly distribute his weight between skis, and turn using both edges. In the specialized “waist steering” technique, the athlete does not need to focus on pressuring or bending the ski, but rather steering the skis toward the direction he wants to go.

The outdated Leveraged Reverse-camber technique begins with an “athletic stance;” the athlete focuses on neutral balance, but this leaves him quite stiff. The Waist Steering technique begins with “posture,” or proper stacking of the bones so that the athlete is more relaxed, comfortable and upright. The back should look flat as viewed from the side, because the vertebrae are stacked vertically. The pelvis is drawn up in front, which moves the center of gravity lower and more to the anterior than with an “athletic stance.” The chin is pulled back and the head is held very straight. The muscles of the legs coordinate to twist the knees outward at the same time both thighs rotate medially; this rests the weight of the body on the outsides of the knees where the tendons are very strong. When biased, each foot should distribute the body’s weight equally along it's surface (tread). Each foot twists outward with the knees, but the athlete should keep pressure on the big toes, the ball of each foot and the ankle. This outward twisting and drawing forward and up of the pelvis naturally keeps distance between the feet, the knees and the upper thighs.

The preparatory phase also begins at turn initiation. The athlete should first bias his weight to the uphill ski, but only slightly—this technique uses both skis, so both should be weighted. Then, the athlete should focus on slight lateral angulation of both skis by some combination of flexing the knees, supinating the uphill foot, pronating the downhill foot, adducting the uphill leg, and abducting the downhill leg. Once the skis are angulated slightly, the skier can begin to turn his waist from the direction of downhill to across the hill—in other words, he turns his waist the direction he wants to go. This waist turn should be slow at first so that both skis edge into the snow, but so that the uphill ski stays “advanced” or ahead of the downhill ski. This waist turning rotates the pelvis (coordinated with the spine and shoulders) on the hip joints. The uphill hip joint remains mostly static, while the pelvis rotates outward.

Once optimally aligned with the gate (which is a different discussion), the skier enters the power phase by continuing the waist turn through a full range of motion. This rotates the pelvis outward from the uphill hip joint. As a result, the downhill hip, leg, and ski advance relative to the uphill hip, leg and ski. By advancing the downhill ski with both skis on edge, the downhill ski will begin to (load at the tip) change direction from running down the hill to across the hill. With gravity still pulling the athletes center of mass down the hill, the skis synergize to angle more, bite deeper into the snow and cross the fallline. I liken this waist turning and forward sweep of the downhill leg to a speed skater rounding the end of the track: when he balances on the inside foot and sweeps his back leg forward.

During this synergy, the torque from the waist turn transmits through the legs and ski boots, down toward the tips of the skis. Even though it is rotational, the torque in this case is the effort; the ski boots become a rotational fulcrum; the resistance comes from the fronts of the skis they “cut” into the snow. A similar leverage system would be that of a lawnmower: the rotational force comes from the engine which spins the center fulcrum; this causes the lever or cutting arms to swing and cut the grass. Because both skis lever into the snow on the plane of the angle of the ski combined with the active advancement of the downhill foot, the turning synergy of Waist Steering has twice the effort of traditional, angulation leverage turns.. In Waist Steering, the uphill ski becomes more of the balance platform, while the downhill ski pushes the whole body uphill.

Once the skis are traveling across the hill, the skier can enter the return phase by simply turning his waist back down the hill. This eliminates the torque on both skis, causing them to flatten out, and making them easy to steer toward optimal alignment for the next gate. Here, the slight weight bias changes to the opposite foot. On steep slopes, this release reverse waist turn can be done quickly enough so that the lateral momentum can be maintained, but so the skis point down the hill and slide laterally (pivoting).

The following is an excerpt on balance from "Manual of Structural Kinesilogy," by Clem W. Thompson, Eleventh Edition:
1. A person has balance when the center
    of gravity falls within the base.
2. A person has balance in the direct
    propotion to the size of the base. The
    larger the base, the more balance.
3. A person has balance in proportion to
    the weight (mass). The greater the weight,
    the more balance.
4. A person has balance depending on the
    height of the center of gravity. The lower
    the center of gravity, the more balance.
5. A person has balance depending on where
    the center of  gravity is in the base of support.
    The balance is less if the center of gravity
    is near the edge of it's base.
6. Rotation about an axis aids balance.
    A moveing bike is easier to balance than
    a stationary bike.
7. Kinesthetic physiological functions contribute
    to balance. The semicircular canals of the
    inner ear, vision, touch (pressure), and
    kinesthetic sense all provide balance information
    to the performer.

My take is this:
1. The old ski racing technique moves the center
    of gravity well outside the base. The center of gravity
    actually becomes a pivot point, relating to # 6,
    but the center of gravity gets farther outside the
    base the lower one gets to the snow.
    Waist Steering brings the CoG near the edge
    of the base (relating to #5); it offers more
    balance because it is kept just outside the base.
2. I believe Waist Steering can offer a wider base
    than the old technique, although this requires
    practice and stretching of the hips/waist muscles
    and ligaments. I believe one issue not addressed
    with # 2 is that supination of the feet at the base
    is correct, while pronation is incorrect. The old
    way relies on pronation. Waist Steering relies on supination.
3. While # 3 may be accurate, I believe that greater
    greater weight (mass) is a liability when the CoG
    is outside the base AND there are strong rotational
    forces, such as in the old technique. Waist Steering
    should theoretically reduce the advantage of greater
    mass if compared to a larger skier using the old   
    technique.
4. I believe that if you look at two people with equal
    skill level in each of the techniques, the old technique
    will offer a lower CoG; however, because the rotational
    forces are extended on a far greater radius, and because
    the CoG is outside the base, the old technique offers
    considerably less balance.
5. My response to # 1 covers this well.
6. I would offer this metaphor for rotation between
    the two techniques: the old technique is like
    swinging a bowling ball around on a four-foot chain.
    The forces are immense, and any mistake can
    cause great damage to the bowling ball and other
    things. Waist Steering is like spinning a bowling
    ball on a vertical axis; if a mistake is made, the ball may
    not even change its spin axis, and is far less likely
    to damage anything.
7. Because Waist Steering is based on Tai Chi,
    which is far more effective at strengthening equillibreum
    and physiological functions such as tactile sense,
    balance is much greater and mistakes become
    far less common.

9 comments:

  1. The article is very interesting and informative, especially for people who would want to learn about alpine ski racing. However, I find it to be very text heavy. When I was teaching private ski lessons in Morzine, instead of making my students read text after text of the techniques of skiing, I incorporated some charts, figures and photos to illustrate these. It helped a lot in retention and made learning more fun.

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    Replies
    1. I will take your comment under advisement and add the images I have to further describe the differences. I can only tell you that I threw this writing onto the blog because there are people out there trying to take credit for my original ideas.

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  2. Or do you need a bike that is built for road racing? Depending on the kind of race you will need a special bike.motorcycle shipping

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