As I was in the process of writing this post, a FaceBook group on skiing posted a link to an article From PSIA: Examining Transitions. The article is based on a presentation last fall by US Ski Team Head Men’s Coach, Sasha Rearick, in which he shed new light on transitions (1.).  While Rearick did shed light on some events associated with transitions, as with previous efforts by others on this subject, Rearick failed to shed light on the mechanics and physics associated with edge change.

As I explained in my last post, transferring the weight from the outside foot and ski of a turn to the inside foot and ski in the transition phase sets in motion what I call the Eversion/Internal Rotation Cascade that rotates the base of the ski into a transient moment of full contact with the surface of the snow between changing to the new (downhill) edge.

At the start of the transition leading up to ski flat between edge change, the center of pressure (COP) of the weight of the body applied by the sole of the inside foot will be under the heel where it is aligned on the proximate center of the ski. In this configuration, the force applied to the ski by the skier is working with gravity to rotate the ski.

The post left off by showing how rotational inertia will tend to make the ski continue rotating about the uphill edge past ski flat and penetrate into the snow surface on its downhill aspect as shown in the graphic below.

Rotational inertia will tend to make the inside edge of the new outside ski automatically rotate into the turn except for the fact that the force FW applied by the skier is on the wrong side of the new edge.

The graphic below has a dashed red reference that is parallel with the snow surface.

If the force FW applied by the skier is still aligned on the transverse center of the ski, it act will act to oppose edge change as shown in the graphic below. When the axis of rotation of the body of the ski changes with a change in edges, the transverse aspect of the base of the ski and the platform under the skier’s foot will tend to accelerate into an eversion translation. But this can only happen if the associated biomechanics are not interfered with by the structures of the ski boot.

The graphic below shows the change in the mechanics of rotation associated with edge change.

At the start of the transition, movement of the mass of the skier’s upper body is in phase with the downhill rotation of the ski and the force FW applied to it. But when the ski changes pivots at edge change and the mass of the skier continues to move downhill, the force FW applied to the ski will tend to rotate it back to ski flat; i.e out of the turn, unless the point of application of force FW changes during ski flat as shown in the graphic below and COM of the skier is aligned with force FW.

………. the angle between the platform and force you apply to it, the platform angle, must be 90 degrees or smaller.  – page 19, The Ski’s Platform Angle, Ultimate skiing; Le Master

The shift in center of pressure from the heel to the ball of the foot in a turn sequence seen in pressure studies of expert skiers is well documented (2., 3., 4). What the studies are really confirming is the use by expert skiers of the Two Phase Second Rocker mechanism to rock (tip) the outside ski on edge and control the edge angle during the load phase of a turn sequence.

Since the limit of the position of the application of force by the foot in relation to the inside edge of the outside ski is the center of the ball of the foot the effect of ski width underfoot and stand height should be obvious. Both rotational inertia and torque will increase as the width of a ski underfoot (profile width) is reduced and stand height increased. When Ligey says he creates pressure, he is creating far more than just pressure.

While LeMaster appears to recognize the importance of a platform angle less than 90° for edge control and, to some degree, the effect of stand height, the explanation offered for superior edging is that this can be attributed to waist width and stand height making skis more like ice skates.In my next post, I will discuss the role of Turntable Rotation in setting up a platform under the body of the outside ski for a skier to stand and balance on while maintaining edge angle.

  1. http://eliteskiing.com/2017/03/31/from-psia-examining-transitions/



One of the most important events in the turn sequence is edge change. Yet, it is rarely mentioned in technical discussions. One of the few references I was able to find on edge change is in the CSIA Technical Reference which states:

Edge Change = Balance Change: Changing edges requires a change of balance.

Edge change occurs during an unbalanced, controlled fall in the transition phase that leads to the development of a balanced position on the outside ski as it crosses the fall line in the bottom of a turn. Properly executed, edge change leads to the development of a platform under the outside ski for the skier to stand and balance on.

The edge change sequence starts in the transition phase when a skier begins to transfer weight from the outside (downhill) ski to the inside (uphill ski). At the start of the transition, the edges of the inside ski are uphill and on the lateral (little toe) side of the foot. From a perspective of the gait cycle, the base of the ski is inverted (turned inward towards the center of the body). This is the normal configuration when the foot is unweighted in the gait cycle. The foot strikes the ground on the lateral (little toe) side and rotates about it’s long axis in the direction of eversion to bring the three points of the tripod of the foot into contact with the ground. As the foot everts, the leg rotates internally through torque coupling in the subtalar joint. The normal kinetic flow from foot strike to the support phase in mid to late stance is one of inversion of the foot/external rotation of the leg to eversion of the foot/internal rotation of the leg. Put another way, the human lower limbs will naturally rotate into a turn so long as the biomechanics are not interfered with.

At the start of the transition leading up to ski flat between edge change, the center of pressure (COP) of the weight of the body applied by the sole of the inside foot will be under the heel where it is aligned on the proximate center of the ski.

The Eversion/Internal Rotation Cascade

Transferring the weight from the outside foot and ski to the inside foot and ski in the transition phase sets in motion what I call the  Eversion/Internal Rotation Cascade. When the cascade starts, the force F W applied to the ski by the foot  by the weight of the body will impart rotational inertia as the ski rotates about the pivot point formed by its inside edge.

For the sake of simplicity, the stack of equipment between the sole of the skier’s foot and the snow is represented by a rectangle in a 3:2 ratio where the stand height is 50% higher than the width (FIS maximum stand height = 93 mm – maximum profile width = 63 mm). Sidecut is also not shown.

The following graphics show the sequence of the Eversion Cascade. Note: Internal rotation of the leg is not shown in this sequence.

The first graphic below shows the moment or torque arm ma that is set up by the offset that exists between GRF from the firm piste acting at the inside edge and the point where the center of pressure of the weight of the body acts in the plane of the base of the ski. The large red arc shows the radius of rotation. The small red arc shows the radius of the moment of force. In this sequence, the ski is rotating downhill away from the pivot at the uphill edge.

When the base of the ski comes into full contact with the surface of the snow, rotational inertia, will make it want to continue rotating about the uphill edge and penetrate into the snow surface on the downhill aspect. If the force FW applied by the weight of the body is still aligned on the transverse center of the ski, it will oppose edge change.

In my next post I will discuss how the Second Rocker affects the mechanics of edge change at ski flat.



As a segue to my post on Turntable Power and how it cantilevers ground reaction force acting along the running surface of the inside edge of the outside ski, I have decided to post the discussion on the problems with existing ski boots from my US Patent 5,265,350 with associated international patents. The patent was issued on November 30, 1993 (24 years ago) to me as the sole inventor and assigned to MACPOD Enterprises Ltd. (Toronto).

The objective of US Patent 5,265,350 and subsequent patents filed and granted to MACPOD was to identify problems with existing ski boots and offer solutions and a functional criteria for advancing the state-of the art going forward. Some of the problems noted and solutions offered, apply to footwear in general.

The final paragraph raises the issue of the limitations of conventional ski boots in terms of accommodating and enabling biomechanically generated forces such as torque from the mechanical force transfer points of the foot to the structure of the ski boot.

The following material is verbatim from the text of US Patent 5,265,350.

Problems with Existing Ski Boots

Existing footwear (ski boot design) does not provide for the dynamic nature of the architecture of the foot by providing a fit system with dynamic and predictable qualities to substantially match those of the foot and lower leg. 

Although somewhat vaguely stated, a generally accepted theme has arisen over the years, one of indiscriminate envelopment and “overall restraint” applied to the foot and leg within the footwear. The stated position of various authorities skilled in the art of the design and fabrication of footwear for skiing is that the foot functions best when movement about its articulations is substantially prevented or restricted.

To serve this end, inner ski boot liners are usually formed around inanimate lasts or, alternatively, the foot and leg are inserted into an inner liner within the ski boot shell and foam is introduced into a bladder in the liner so as to totally occupy any free space between the foot and leg and the outer ski boot shell. The outer shell of the footwear is closed around this inner envelopment forming an encasement with which to secure and substantially immobilize the foot and leg. This is considered the optimum and, therefore, ideal form of envelopment. The perspective is that the physiologic structures of the foot are inherently weak and thus, unsuited for skiing. Enveloping the foot within an enclosure which makes it more rigid is thought to add the necessary strength with which to suitably adapt it for skiing. The reasoning being, that the foot and leg now having being suitably strengthened, can form a solid connection with the ski while the leg, now made more rigid, can better serve as a lever with which to apply edging force to the ski.

To some degree, the prior art (existing ski boot design) has acknowledged a need for the ankle joint to articulate in flexion. However, the prior art has not differentiated exactly how articulation of the ankle joint might be separated from the object of generalized and indiscriminate envelopment and thus made possible. Therefore, the theme of prior art (existing ski boot design) is inconsistent and lacks continuity.

The only disclosure known of a process wherein the separation of envelopment of the foot from articulation of the ankle joint is contained in U.S. Pat. No. 4,534,122, of which the present applicant is also the inventor. This material discloses a supportive structure (i.e Dorthotic) wherein restrictions to flexion of the ankle joint are essentially removed, support being provided from below the hinge of the ankle joint.

In keeping with the theme of indiscriminate envelopment and overall restraint, the following structures are generally common to all footwear for skiing disclosed by prior art (existing ski boot design):

(a) a continuous counter system which surrounds the foot and provides for the process of envelopment;

(b) an arrangement of pads or padding with which to envelope the foot;

(c) a substantially rigid outer shell which encases the structures employed for envelopment;

(d) an articulation of the ski boot lower outer shell and the cuff or cuffs which envelope the leg of the user, usually accomplished through a common axis or journal;

(e) a structure to brace and support the leg since prior art considers the ankle joint to be inherently weak and in need of support; and

(f) some form of resistance to movement of the cuff (shaft of the ski boot).

The prior art (existing boot design and boot fitting procedures) refers to the importance of a “neutral sub-talar joint”. The sub-talar joint is a joint with rotational capability which underlies and supports the ankle joint. The sub-talar joint is substantially “neutral” in bipedal function. That is to say that the foot is neither rolled inward or rolled outward.

If the foot can be substantially maintained in a neutral position with the arch supported and with a broad area of the inner aspect of the foot well padded, there will exist a good degree of comfort. Such a state of comfort exists because the foot is not able to roll inward (pronate) to a degree where significant mechanical forces can be set up which would allow it to bear against the inner surface of the boot shell. In effect, this means that initiation of the transition from a state of bipedal to a state of monopedal function, is prevented. This transition would normally be precipitated by an attempt to balance on one foot. If the foot is contained in a neutral position, traditional supportive footbeds (arch supports) are quite compatible with the mechanisms and philosophies of the prior art.

Problems arise when the foot is attempting a transition from a state of bipedal stance to monopedal stance. If the transition to monopedal stance or function can be completed without interference from the structures of the ski boot, all is fine and well. However, if the transition is allowed to proceed to a point where the mechanics associated with the monopedal function can establish significant horizontal forces, and the further movement of the foot is blocked before the transition can be completed, the skier will experience pain and discomfort at the points where the inner aspect of the foot bears against the structures of the footwear. This is the situation experienced by a majority of the skiers with prior art footwear. It is at this point where arch supports, if employed, also begin to cause discomfort. It should be noted that it is the normal tendency of the foot to pronate when weight bearing on one foot.

Footbeds (arch supports) may work in conventional boots (which traditionally do not allow natural biomechanics or movement of the foot to occur), but in a boot which accommodates and supports natural leg and foot articulation and function, arch supports can be detrimental.

When the foot attempts to pronate inside the ski boot, it is often the case that the ankle bone will come to bear against the inner surface of the boot shell. When contact of this nature occurs, pain and other related complications usually result. Since the consensus of those skilled in the art of ski boot design and modification is that pronation or the rolling inward of the foot is detrimental, and, thus, undesirable, provision is not made to allow for such movement. Rather, the structure of the footwear is intended to resist or even prevent it.

Thus, the problem with existing footwear arises due to the dynamic nature of the architecture of the foot. When the wearer is standing with the weight equally distributed between left and right feet so that the centre of mass of the wearer is manifesting itself in the centre between the feet, the architecture of the wearer’s foot assumes a specific configuration. As the wearer begins to shift his weight towards one foot so that the other foot bears proportionately less weight, the wearer’s centre of mass moves over the medial aspect of the weighted foot so as to assume a position of balance. In order for this movement of the wearer’s centre of mass to occur, the architecture of the weighted foot must undergo a progressive re-alignment. Existing footwear does not adequately anticipate this re-alignment of the architecture of the foot and thus such footwear inhibits the wearer’s ability to assume a balanced position.

A further problem with existing footwear is the fact that longitudinal relative movement between the foot and the footwear may occur. This happens, for example, when the forefoot/midfoot section of the foot is not adequately restrained under certain conditions, such as when flexion is occurring between the lower leg and the foot. Such longitudinal relative movement contributes to the disruption of biomechanical reference points associated with the dynamics of the ski and, in addition, results in a delay in the transmission of force between the leg and foot and the footwear.

Yet a further problem with existing footwear for skiing, in particular the rear entry type, relates to the obstruction of the leg in forward flexion. A relatively freely flexing gaiter or cuff (i.e. shaft) is necessary in order to permit the posterior muscle groups of the lower leg to modulate external force exerted on the footwear. This requires that the axis of the footwear be allowed to rotate so that small degrees of flexion/extension occur at the foot with the lower leg being relatively passive and that large degrees of flexion/extension occur as coordinated ankle, knee and hip flexion. The construction of the prior art requires flexion/extension to occur primarily at the knee and hip joints which is disadvantageous to the user.

While some types of rear entry boots do disclose gaiters or cuffs which provide a degree of relatively free flexion, there remains numerous problems, the most serious of which is the fact that the device employed to secure the foot of the user exerts, in addition to the downward directed force on the foot, a simultaneous rearward directed force on the leg which acts to resist forward flexion in spite of any free hinging action of the cuff. The result is an interference with the physiologic function of the foot and leg of the user.

Yet another problem resides in buckle or overlap type footwear. In order to provide for entry of the foot of the user and for resistance to flexion, plastic materials are employed for the outer shell which have flexural qualities. This is necessary in order to facilitate the aforementioned requirements. Plastic materials by their very nature tend to resist point loadings by a relaxation of the material at the point where stress is applied. This characteristic creates serious problems for two reasons. First, the teaching of this application is that force must be applied and maintained only to specific areas of the foot and leg of the user while allowing for unrestricted movement of other areas. The application and maintenance of such force by flexible plastic materials in the structures of prior art is necessarily difficult, if it is possible at all.

Second, the plastic materials in relaxing under the application of stress assume a new shape by moving into void areas. Thus, the probability is great that the plastic material will change shape so as to inhabit the very area required for the uninhibited displacement of the structures of the foot and leg. The result of these limitations is interference with the physiologic function of the user.

Top and rear entry footwear for skiing and skating necessarily have interior volumes greater than that required by the wearers foot and leg, particularly in the area over the instep, in order to accommodate entry. This additional volume makes the incorporation of structures designed to provide accurate and consistent support to specific areas necessarily difficult and ineffective. This results in reduced support for the foot and leg.

Another problem with conventional footwear relates to the flexion of the lower leg relative to the foot. It is desirable to provide a degree of resistance to such movement to assist in dampening movement of the mass of the skier relative to the ski resulting from, for example, a velocity change due to terrain changes and to assist the user in transferring energy to the ski. Adjustment of such resistance is desirable in order that the user may compensate for different physical makeup and different operating conditions. In present ski footwear, sources of resistance for such purpose are poorly controlled and often produce resistance curves inappropriate for the operating environment (i.e. temperature) thereby adversely affecting the balance and control of the user and creating a need for additional energy to be expended to provide correction. In many applications, resistance is achieved by deformation of shell structures thereby resulting in reduced support for the user’s foot and leg. If indeed provision is made for adjustment of flex resistance in the instances cited, it is very limited in terms of ability to suitably modify resistance curves.

Torque Transfer and The Turntable Effect

Yet a further problem relates to the efficient transfer of torque from the lower leg and foot to the footwear. When the leg is rotated inwardly relative to the foot by muscular effort, a torsional load is applied to the foot. Present footwear does not adequately provide support or surfaces on and against which the wearer can transfer biomechanically generated forces such as torque to the footwear. Alternatively, the footwear presents sources of resistance which interfere with the movements necessary to initiate such transfer. It is desirable to provide for appropriate movement and such sources of resistance in order to increase the efficiency of this torque transfer and, in so doing, enhance the turning response of the ski. 

In my next post, I will discuss Turntable Power in conjunction with the Over-Centre mechanism.


In June of this year, I posted on my beta testing experience with NABOSO surface science, small nerve, proprioception stimulating technology (1.).

Recently, I received the consumer version of NABOSO called NABOSO 1.0 shown in the photo below.

NABOSO 1.0 has a tighter grid than the NABOSO beta version I have been testing. The pyramid-like texture is also smaller.

The photo below shows NABOSO 1.o on the left and NABOSO beta on the right. The photo was taken before I trimmed NABOSO 1.0 to fit my shoes. 
Here is the information that came with my pair NABOSO 1.0 insoles.

I use both NABOSO 1.0 and NABOSO beta in my Lems Primal 2 and Xero Prio shoes. I immediately sensed better balance with the tighter grid of NABOSO 1.0. But I found it interesting after going back to NABOSO beta, after a period of time in NABOSO 1.o, that NABOSO beta felt more stimulating. Based on this subjective experience, I think there may be some advantage to switching back and forth between different texture grids. Hence my interest in the new NABOSO 1.5.

NABOSO 1.5 can be pre-ordered now for a reduced price of $30 US at orders@nabosotechnology.com

Disclosure: I do not receive any form of compensation from NABOSO or Dr. Emily Splichal. Nor do I hold any shares or have any financial interest in the company. The sole benefit I derive from NABOSO is to my feet and my balance and the efficiency of my movement.

I will be testing NABOSO insoles in my ski boots this winter in conjunction with toe spreaders starting with NABOSO 1.0. I will report on my experience in a future post.

  1. http://wp.me/p3vZhu-27v


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


Neither the Two Phase Second Rocker (heel to ball of foot rocker) described in THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: HEEL/FOREFOOT ROCKER (1.) or the Rotating Turntable Effect described in THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: THE ROCKER/TURNTABLE EFFECT (2.) are new. They have been the trademark technique of the world’s best skiers for decades. But the ability to engage the associated mechanics and biomechanics requires what amounts to a perfect storm that typically occurs early in the development of a young skier. More than raw athletic talent, discipline and dedication, the ski boot appears to be the critical factor that determines who acquires the ability to engage these effects.

In working with skiers and racers who are gifted natural athletes, it has been my consistent finding that a change in ski boots that compromises neuromuscular function will result in the body adopting compensatory mechanisms that can reduce competence on skis to survival reactions. Given sufficient time, the survival mechanism will become imprinted until a point is reached where it is accepted as normal by the body. Even after the cause is corrected, it can take years of retraining to erase and replace survival motor patterns. A good example of this is what happened to Mikaela Shiffrin at the start of the 2014-2015 World Cup after changes were made to her boots in the fall of 2014. Fortunately, she was able to revert to her previous boots over Christmas and quickly restore her former competitive competence.

Four synergistic mechanisms associated with the mechanics of edge change result in the creation of a platform under the outside ski that a skier can stand and balance on. These are:

  1. The Two Phase Second Rocker (heel to ball of foot rocker) Mechanism
  2. Impulse rocker loading that occurs at edge change
  3. The Over-Center mechanism, and
  4. Open and Closed Chain Whole Leg Rotation; The Rotating Turntable Effect.

The most critical and seemingly least appreciated and understood mechanism in skiing is the mechanics and biomechanics of whole leg rotation.

LeMaster recognized the role of whole leg rotation in skiing in his book Ultimate Skiing when he stated under Twisting Actions (p 13) that torques play important roles in turning skis and holding them on edge. In Chapter 7, Turning the Skis (p 107), LeMaster states, Rotating the leg inward generally rolls the ski on its edge, too, combining the increase in the edge and platform angles—often a desirable combination while acknowledging that leg rotation is powerful and can produce large torques through the whole turn. But LeMaster does not describe the mechanics associated with whole leg rotation in this context.

The Center of Rotation

Whole leg rotational force is applied to the femur primarily by the gluteus medius.

The most important source of rotational power with which to apply torque to the footwear (ski boot) is the adductor/rotator muscle groups of the hip joint. – US Patent 5,265,350 MacPhail

Rotation of the femur is transferred through the tibia where it is applied through its lower or distal aspect to the talus that forms the ankle joint with the tibia.

The graphic below shows a skeleton of the foot aligned on a fixed reference axis (dashed line).The graphic below shows the same skeleton of the foot rotated 15° medially (towards the center of the body) against the fixed reference axis (dashed line).

The graphic below shows the relative displacement of the heel and forefoot in relation to the fixed reference axis (dashed line).

The graphic below compares the displacements of the heel and limit of the forefoot at the end of the second toe with horizontal lines in the center of the graphic. The lines show that the end of the second toe displaces almost 4 times as much as the rearmost end of the center of the heel during whole leg rotation of the foot. Hence the advice in my post, THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: PRESS AND POINT THE BIG TOE (3.), to point the big toe in the direction you want to go.

Open Chain Rotation vs Closed Chain Rotation

  • Open Chain Rotation – occurs when the foot can rotate in the horizontal plane in conjunction with the rotation of the whole leg from pelvis. In ski technique, this is referred to as steering.
  • Closed Chain Rotation – occurs when the foot is fixed on its long axis and whole leg rotational force is applied to the foot from pelvis.

Open Chain whole leg rotation acting about the axis of the ankle joint in combination with a Two Phase Second Rocker induced Over Centre mechanism are prerequisites to the application of Closed Chain Rotation. The emerging profile created by the steering angle of the outside ski as it crosses the fall line below a gate yields important clues as to the technique of a racer.

In my next post, I will discuss Closed Chain Rotation applied to the outside ski in a turn and the transfer path of torques applied to the foot by the leg through the boot-binding interface to the ski.

  1. http://wp.me/p3vZhu-2at
  2. http://wp.me/p3vZhu-2bb
  3. http://wp.me/p3vZhu-25W


It has been known for decades that an unbalanced moment of force or torque will be present on the outside ski when the center of pressure of the load applied to the ski by a skier is acting along the center of the transverse axis of the ski where it is offset from GRF acting along the inside edge. Ron LeMaster acknowledges the existence of an unbalanced moment of force on the ouside ski in both The Skier’s Edge and Ultimate Skiing (Edging the skis). LeMaster states in Ultimate Skiing;

The force on the snow is offset from the center of the skier’s and creates a torque on it that tries to flatten the ski.

Ron didn’t get the mechanics right. But he correctly shows the unbalanced torque acting on the ankle joint. LeMaster tries to rationalize that ice skates are easy to cut clean arcs into ice with because the blade is located under the center of the ankle. While this is correct, ice skaters and especially hockey players employ the Two Stage Heel-Forefoot Rocker to impulse load the skate for acceleration. Hockey players refer to this as kick.

In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP, Robert Colborne said:

…..In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.

The load or weight of COM is transferred to distal tibia that forms the ankle joint. This is the lower aspect of the central load-bearing axis that transfers the load W from COM to the foot. What happens after that depends on the biomechanics. But the force will tend to be applied on the proximate center of the stance foot. This is a significant problem in skiing, (one that LeMaster doesn’t offer a solution for) when the ski is on edge and there is air under the body of the ski. The unbalanced torques will move up the vertical column where they will manifest at the knee against a well stabilized femur.

But this unbalanced torque creates another problem, one that is described in a paper published in 2005 by two Italian engineers (1.) that describes how this load deforms the base of the boot shell.

The Italian study found large amounts of deformation at mean loads of up to 164% body weight were measured on the outer ski during turning. The paper suggests that the ski boot flex index is really a distortion index for the boot shell. The lower the flex index, the greater the distortion potential.

For the ski-boot – sole joint the main problem is not material failure, but large amounts of local deformation that can affect the efficiency of the locking system and the stiffness of the overall system.

Values of drift angle of some degree (>2-3°) cannot be accepted, even for a small period of time, because it results in a direct decrease of the incidence of the ski with the ground.

My post GS AND KNEE INJURIES – CONNECTING THE DOTS (2.) cites studies that found that knee injuries are highest in GS in the shortest radius turns where peak transient forces are highest.

As shown in Figure 2a FR (sum of centrifugal and weight forces) and F GROUND (ground reaction force) are not acting on the same axis thus generating a moment MGR that causes a deformation of the ski-boot-sole system (Figure 2b) leading to a rotation of the ground reaction force direction. The final effect is to reduce the centripetal reaction force of the ground, causing the skier to drift to the outside of the turn (R decreases, causing the drift event).

An imperfect condition of the ski slope will emphasize this problem, leading to difficulties maintaining constant turning radius and optimal trajectory. The use of SGS ski-boot in competitions requires a particular focus on this aspect due to the larger loads that can be produced during races.

I have added a sketch showing that the moment arm M R created by the offset between the F Ground and F R is in the plane of the base of the ski where it results in an Inversion-lateral rotation torque.

The importance of sole stiffness is demonstrated with a simplified skier model…..…ski boot torsional stiffness with respect to ski longitudinal axis in particular is very important as it deeply influences the performance of the skier during turning…. A passage over a bump or a hollow may generate a sudden change in ground reaction force that may lead to a rapid change in the drift angle delta. The ski boot must be as stiff as possible going from the lower part of the boot to the ski (i.e. lower shell-joint-sole system)

As explained in the method section using the simplified model, values of some degree cannot be accepted, even for a small period of time, because the skier stability and equilibrium could be seriously compromised especially when the radius of curvature is small. A non perfect condition of the ski slope will emphasize the problem, leading to big difficulties for maintaining constant turning radius and optimal trajectory.

This excellent paper by the two Italian engineers concludes with the following statements:

Authors pushed forward the integration of experiments and modeling on ski-boots that will lead to a design environment in which the optimal compromise between stiffness and comfort can be reached.

The possibility of measuring accurately the skier kinematics on the ski slope, not addressed in the presented study, could represent a further step in the understanding of skiing dynamics and thus could provide even more insightful ideas for the ski-boot design process.

I first recognized the shell deformation, boot board instability issue in 1980, at which time I started integrating rigid structural boot boots into the bases of boot shells I prepared for racers. The improvement in ski control and balance was significant. The instability of  boot boards associated with shell/sole deformation with 2 to 3 degrees of drift at modest loads of up to 164% body weight has significant implications for footbeds.

  1. AN INNOVATIVE SKI-BOOT: DESIGN, NUMERICAL SIMULATIONS AND TESTING – Stefano Corazza 􀀍 and Claudio Cobelli Department of Information Engineering – University of Padova, Italy – Published (online): 01 September 2005 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3887325/
  2. http://wp.me/p3vZhu-zx