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THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: CLOSED CHAIN OUTSIDE LEG ROTATION

A recently published study on foot pressure data acquired during skiing (1.) recognized that compressive force pressure data acquired in skiing is underestimated by 21% to 54% compared to pressure data acquired on a force platform in a controlled environment.  The underestimation varies depending on the phase of the turn, the skier’s skill level, the pitch of the slope and the skiing mode. The paper states that other studies have stated that this underestimation originates from a significant part of the force actually being transferred through the ski boot’s cuff (to the ski). As a result, the CoP trajectory also tends to be underestimated along both the anterior-posterior (A-P) and medial-lateral (M-L) axes compared to force platforms.

In conclusion, these studies have highlighted a major contribution of different factors to the nGRF applied throughout a turn, such as the foot’s position during a turn (inside vs. outside), the CoP A-P (front to back) displacement, or precise loading of different foot sole regions.  Unfortunately, these results have been studied separately.

There is a lack of continuity across the various positions in skiing and, in particular, a lack of a model with which to explain mechanisms such as balance on the outside ski and open and closed chain internal rotation of the leg and foot in conjunction with progressive inclination and G force loading on it as the skier crosses the fall line in the bottom of a turn. The associated mechanics and biomechanics represent a new paradigm requiring new thinking and new insights. Existing text-book explanations are not sufficient to explain these mechanisms.

Open Chain Whole Leg Rotation vs. Closed Chain Rotation

Rotation of an unloaded (non-weight bearing) lower limb is relatively straight forward when there is a small angle at the knee. As resistance to rotation of the foot is progressively introduced with increasing weight imposed on it, the kinetic chain begins to close. As it closes, the points at which the foot transfers torque to the walls of rigid shell footwear such as ice skates and ski boots starts to emerge as an issue as does the loading of the foot created by the weight of the body imposed on it and the position of COM in relation to the foot.

In order to tension the biokinetic chain and trigger the two-phase Second Rocker, COM must be aligned over the foot as shown in the grahic below.  This alignment requires that the leg adduct (move towards the center of the body) approximately 6.5 degrees. To bring the 3 points of the tripod of the foot into contact with the ground, the foot must evert (sole turn outward) the same amount. Eversion is accompanied by a corresponding torque coupled 6.5 degrees of internal rotation of the leg as shown in the left hand figure in the graphic below (see my post – OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP). The bipedal figure on the right shows adduction, eversion and internal rotation as 0.0 – 0.0 – 0.0 for reference. The monopedal figure on the left shows the changes in adduction, eversion and internal rotation as 6.5 – 6.5 – 6.5.

 

The alignment of COM with the foot can be achieved by moving COM laterally as shown by the arrow emanating from COM in the Monopedal figure or by moving the foot medially as shown by the white arrow or through a combination of the two movements.  The act of positioning COM over the outside foot (Getting Over It), sets in motion internal rotation of the outside leg and eversion of foot into the turn. This engages an over-centre mechanism between the platform of the ski and the inside edge underfoot.

The over-centre mechanism results in an alignment of the resultant force R forming an angle with the transverse aspect of base of the ski that is slightly less than 90 degrees. In order to Get (COM) Over It (the foot), it is essential that the outside leg is not only able to adduct and rotate internally as the foot everts, but to achieve this configuration without delay in order to set up the over-center mechanism. The problem for the majority of skiers is that the objective of most boot fit systems and boot-fitting procedures is to support the foot in a neutral configuration. Eversion of the foot is a component of pronation. Impeding or preventing pronation, restricts or even prevents the required amount of eversion.

Closing the Kinetic Chain on Whole Leg Rotation

Open kinetic chain leg/foot rotation with the foot unloaded (not bearing weight) is relatively simple. But the mechanics and biomechanics begin to get complicated when resistance is progressively introduced that starts to close the kinetic chain as happens when the outside ski is rotated across the path of the skier in the fall line in the bottom of a turn.
The graphic below shows a foot supported on a platform with 2 points of resistance (FR) applied to the platform opposite the 2 points of application of the moments of force, ML (green) and MM (red). The forces tangent to the arc of the moments of rotation are shown as FT.
When the weight of the body is progressively shifted to one foot (i.e. Monopedal Stance) and the foot everts, the talus (shown in gray in the graphic above) moves inward towards the center of the body and shifts slightly rearward as evidenced by the corresponding movement of the inside ankle bone.  This is easily seen when moving from bipedal to monopedal stance on a hard, flat surface while barefoot. In order to effectively transfer torque from the foot to the platform, the forefoot and ankle and knee joints must be fascially tensioned. This requires that the big toe (Hallux) be aligned on the anatomical axis (dashed line) and the forefoot fully splayed. This stabilizes the heel and head of the 1st metatarsal (ball of the foot).  Torque from internal rotation of the leg will be transferred to two discrete points adjacent the Load Counters mounted on the resistance platform.

Removing the resistance force FR from the inner (big toe) aspect of the platform provides insights to what I refer to as the Turntable Effect that is associated with internal rotation of the leg and eversion of the foot that creates an over-center mechanism. The turntable rotation is shown in light yellow. The effect will vary for different structures of the foot depending on the location of the center of rotation of the platform under the foot.

The location of the blade of an ice skate on the anatomical center of the foot has been used to explain why it is easier to cut into a hard ice surface with a skate compared to the edges of a ski. But the real reason it is easier is because ice skaters use the Second Rocker, Over-Center, Turn Table Mechanisms as shown in the graphic below. The skate is positioned under COM. It can be readily seen that the skater is not using the inner aspect of the shaft of the skate to hold the skate on edge.

In my next post, I will discuss the progress of emerging CARV and NABOSO technologies after which I will continue with my discussion of the Mechanics of Balance on the Outside Ski.


  1. Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing: Published: May 4, 2017  – Thomas Falda-Buscaiot, Frédérique Hintzy, Patrice Rougier, Patrick Lacouture, Nicolas Coulmy
  2. http://wp.me/p3vZhu-29n

RACE BOOT SETUP: BODY ALIGNMENT CHECK

Since this is the time of year when racers tend to either make changes to their boots or change to a new boot brand, I will describe the initial steps in the process (and it is a lengthy process) that I follow in setting up ski boots for a racer. Although the process is similar for any skier, it may be less structured and less intensive depending on the desired end result.

As a general rule, the closer a racer’s boots are to creating an optimal functional environment for the feet and legs (lower limbs), the more critical any changes become. Optimal is a moving target in that ski boots have such a significant effect on racer/skier function that the body is constantly making small changes in an effort to maximize performance. In my experience, that the farther a racer/skier’s boots are from optimal, the more unlikely that any changes, even in the wrong direction, will create a noticeable impact on performance. But when the boot/binding/skis system is close to optimal, even small changes can have a large impact. In this situation, changes in boot board ramp angle of a tenth of a degree or changes in the thickness of an insole of a mm are usually readily perceived by an elite racer/skier.

Where to Start? The body

The process starts with a quick visual assessment of the racer’s posture to see if any obvious issue such as significant duck feet (toed out) of one of both feet are present. The ideal Plumb Line Alignment of the major body segments and joints is shown in reference books such as Muscles Function and Testing, Third Edition by Kendell and McCreary. The most mechanically efficient alignment occurs when the gravity line of a plumb bob as viewed from the side falls through the back of the ear lobe and passes through the center of the shoulder joint, just behind the center of the hip joint and just in front of the centers of the knee and ankle joints.

If any structural issues are obvious, I recommend that the racer/skier have alignment and kinesologic assessments done by certified medical professionals. This is especially important if a skier or racer has been injured. Often, full function has not been completely restored.

I am not talking about the static alignment usually done in ski or boot fit shops. I am talking about an assessment process that evaluates and corrects the processes responsible for the maintenance of dynamic alignment, generally referred to as balance. It is superior balance that gives elite racers and skiers the edge over others.

One of the several resources in Whistler that I personally use is Dr. Andrea Bologna, DC, CACCP of the Village Centre Chiropractic & Massage Centre. Dr. Bologna wrote the following as an overview of the process that she uses to assess Body Alignment (Structural).

Body alignment (structural) assessment gives a skier a baseline to determine any deviations from “normal” in terms of positioning and alignment of the structure of the body.  Correcting misalignments will give a skier the edge on not only skiing or any other activity pursued by taking stress off of joints and muscles, improving posture and allowing the body to move freely with the correct biomechanics.

The following components make up the Body Alignment (Structural) Assessment

Step 1: A complete history is taken that includes past injuries, activities, etc.

Step 2: Body posture is assessed to determine how the body lives in space.

Assess main postural alterations and compensatory changes.

Anterior-Posterior Posture:

  • The pelvis may show a high ilium on one side and/or rotational component to the sacrum which may stem from changes in the spinal structure or in ankle or knee alignment and biomechanics.
  • One shoulder may be elevated and/or a rotational component observed to the rib cage.
  • Head tilt and/or a rotational component may be observed.

Lateral Posture:

  • An increase or decrease in the lumbar lordosis and/or thoracic kyphosis may be observed.
  • Knees may be hyper-extended.
  • One or both shoulders may be rolled forwarded.
  • Head forward position may be observed.

Step 3: Two computerized spinal scans are performed (thermal and EMG or electromyography) to determine which areas of the spine have nerve irritation or interference and which muscles are working harder or pulled tighter due to physical stress.

Step 4: A 3D digital foot scan is performed to determine changes in the arches of the feet, compensating posture affecting the knees and pelvis, and weight imbalance between the right and left sides of the body.

Step 5: A palpatory spinal assessment will determine spinal misalignments causing altered structure and resulting aberrant biomechanics.

The body evaluation process is key to determine what changes need to be made to correct the body structurally to allow for ideal biomechanics during ski training and racing.  The evaluation will determine the most specific way to adjust the spine and related joints for lasting results in the shortest time possible.

Dr. Andrea graduated from Parker University in Dallas Texas with a doctorate of chiropractic in 2005. She completed a 180 hour certification in Chiropractic Pediatrics from The Academy of Chiropractic Family Practice and the Council on Chiropractic Pediatrics. She is Webster Certified through the International Chiropractic Pediatric Association.

Dr. Andrea specializes in pediatrics and pregnancy, and sees a variety of world class athletes as well as weekend warriors. She moved back to BC to work together with her brother Dr. Michael Bologna after living in Texas for 10 years, resides in Whistler, and enjoys downhill and cross country biking.

In addition to body alignment, it is also important to assess foot function. There are many excellent resources that I will discuss in future posts.

In my next post, I will discuss where I start the process of racer boot setup.