snow reaction force


There appears to be a widely held perception within the ski industry, even among coaches and trainers at the World Cup level, that skiing like Hirscher and Shiffrin is simply a matter of observing and then copying their movements. There also appears to be a widely held perception that strength training and training on BOSU balls, wobble boards, slack lines and thick foam pads will transfer to improved balance on skis.

In a recent article, Nailing the Coffin Shut on Instability Training Ideas (1.), trainer, Bob Alejo, cites 59 papers on the topic of instability training in support of his position that not only are the assumptions about instability training improving balance in a specific activity incorrect, instability training may actually have a negative effect on performance.

As far back as 1980, I had found that an immediate improvement in skier performance after ski boot modifications was a reliable indicator that the modifications were positive. Sometimes this was evident in the first few turns. I had also found that equipment modifications or equipment changes that had a negative effect did not become obvious right away. I didn’t understand the reason for the immediate and sometimes dramatic improvement in skier performance following ski boot modifications. But I suspected it had something to do with improved skier balance.

By 1990, I had hypothesized that elite skiers are able to create a dynamically stable foundation under their outside ski and foot in a turn to balance on by rotating the edged ski against resistance from the sidecut and that this has the effect of extending ground reaction force from the snow out under the body of the ski. But even after the Birdcage studies of 1991 validated my theory, I still didn’t fully understand the reason for the dramatic improvement in skier performance in the Birdcage tests or following modifications made to conventional ski boots. Strain gauges fit to the Birdcage showed forces and the sequence of loading. But the strain gauges could not measure the magnitude of the forces.

It was Dr Emily Splichal’s (2.) that answered my question when she said;

It doesn’t matter how physically strong you are. Without a foundation of stability, you are weak. With a foundation of stability, you are stronger and faster than anyone.

In his article, Nailing the Coffin Shut on Instability Training Ideas (1.), Alejo supports Dr. Splichal’s position:

The predominant theme of the training data analysis under unstable conditions is the striking reduction in force and, subsequently, power. It would be of no surprise then that the speed of motion, as well as the range of motion, were negatively affected under unstable conditions, as cited in the literature.

Reduced Force Outputs Result in Less Power

Essentially, even though both groups improved in some instances, the stable surfaces group outperformed the unstable group in all categories. So much so that it led the authors to conclude that the results of their study affirmed—what was a criticism then and now is fact—that unstable training does not allow for enough loading to create strength and data.

Simply put, athletes can handle heavier weight under stable conditions versus unstable conditions.

Dynamic Stability is critical for a skier or skater to assume a strong position from which to generate force while maintaining control and initiate precise movement from. A key marker of dynamic stability in ice skating and skiing is the magnitude of impulse force, especially peak force.


Impulse is a large force applied for a short duration of time. Peak force is the highest force applied during an impulse force.

If superior dynamic stability is the reason for the dominance of racers like Hirscher and Shiffrin then pressure data obtained during skiing should show higher impulse and peak forces than generated their competition. While the technology to measure these forces is readily available I don’t have access to this data even if it does exist. So I’ll use data generated from hockey skate study I was involved in 2012 that compared data captured from competitive skaters performing in their own skates to skates I had modified using principles from my patents and modifications described in this blog.

The first step was to capture baseline data from the test subjects own ice skates (OS). The bar graph below shows the peak force in Newtons applied by each of the four test subjects. Peak force has a very short duration.

Subjects 1 and 3 applied a peak force of approximately 800 Newtons. A pound is 4.45 Newtons. So 800 Newtons is approximately 180 lbs.

Test subjects #1 and #3 are almost identical. But test subject #1 has a very slim edge over test subject #3.

Test subject #2 is 3rd in ranking while test subject #4 is last.

Assuming this was a study of competitive skier test subject #1 appears to have a stability advantage over the other skiers. This would translate into quicker more precise turns (hairpin turns) and less time on their edges.

In my next post I will show what happened when the same test subjects used the skates I prepared.

  1. Nailing the Coffin Shut on Instability Training Ideas –



Turntable rotation generated by the powerful internal rotators of the pelvis (the gluteus medius and minimus) in combination with second rocker mechanics can create a platform under the body of the outside ski and foot that a skier can stand and balance on using the same processes to balance on solid ground. The associated mechanics creates a platform under the body of the outside ski by extending  ground reaction force acting along the portion of the inside edge in contact with the snow, out under the body of the ski.

In order to understand the mechanics, we need to start with a profile through the section of the body of the ski, binding and boot sole under the ball of the foot. The graphic below is a schematic representation of a ski with a 70 mm waist and 100 mm shovel and tail with an arbitrary length of 165 mm. The total stack or stand height from the base of the ski to the surface of the boot that supports the foot is 80 mm. The uppermost portion of the schematic shows the shell sidewalls of a 335 boot in relation to the 70 mm width of the stack. A ski with a 70 mm waist will place the center ball of the foot of skiers with US Men’s 10 to 12 feet close to over the inside edge. The heavy black line at the bottom of the stack shows the projection of the sidecut width beyond the waist.The schematic serves as a base on which to overlay a free body diagram showing the forces acting across the interface of the inside edge with the snow. This is where the rubber meets the road.

There are two possible scenarios in terms of the axis on which the center of pressure W of the skier will act. Unless the foot can sufficiently pronate and especially generate impulse second rocker loading, W will lie on the proximate anatomic center of the foot and transverse center of the body of the ski as shown in the graphic below. In this location, W will create a moment arm due to the offset with the GRF Pivot under the inside edge at the waist. The resulting moment of force will externally rotate the ski and foot under load out of the turn while simultaneously rotating the leg externally.The graphic below shows the second scenario where the center of pressure W lies directly over the GRF Pivot under the inside edge. In this position, W will load the inside edge under the ball of the foot and assist edge grip. But in this configuration, rotating the ski onto its inside edge necessitates overcoming the moment of force created by the moment arm resulting from the offset between the GRF Pivot and GRF acting at the limits of the sidecut. This requires a source of torque that acts to rotate the ski into the turn about the pivot acting at the inside edge at the waist of the ski.An obvious source of torque is to use the leg to apply force to the inner aspect of the shaft of the foot; aka knee angulation. But this will not create a platform under the body of the outside ski. Applying a load to the vertical wall of the shell opposite the ball of the foot will apply torque load to center at the GRF pivot as shown in the graphic below. The moment arm is formed by the point at which the Turntable Torque is applied to the boot sidewall (green arrow) to the center of rotation at the GRF Pivot.


The torque applied to the vertical sidewall of the boot shell is the Effort. The sidecut of the ski is the resistance. What effect will this have on the body of the ski under the foot? There is a lot more to this subject that I will begin to expand on in my next post.


Comments made by followers of my blog suggest that significant confusion exists 0n the meaning of terms and representations of mechanics, biomechanics and physics used in typical explanations of ski technique and ski mechanics. In particular, there appears to be confusion between pressure and the representation of point forces.

Pressure is a physical force applied to an object that is distributed over the surface of the object.

Center of Pressure or COP is the point center of ground reaction force opposing a corresponding center of applied force acting on a object supported on the ground or a stable surface that acts in the capacity of ground in terms of providing a source of reaction force.

Torque or Moment of Force results from an offset between the centers of opposing physical forces acting on either side of an object.  This offset results in a torque or moment arm that tends ti create rotation about a center. When one force has a greater magnitude than the other force, rotation of the object will occur around the point of rotation.

Why typical balance explanations of skier balance are wrong

Balance in skiing is often depicted as a simple alignment of opposing point forces, usually a resultant force R acting in opposition to a snow reaction force S. The mechanics that make the edges of a ski grip are often shown as a simple alignment of opposing forces acting a single point on the edge. Explanations of this nature are physically impossible. What the authorities in skiing seem to conveniently be ignoring is the fact that pressure is applied by the snow along the entire running surface of the edge in contact with the the snow while an opposing area of pressure applied by the weight of the skier is acting on the body of the ski with an offset between the two centers of pressure. The authorities in skiing also seem to conveniently ignore what is arguably the key even in establishing a platform under the outside ski for the skier to stand and balance on, edge change.

Mikaela Shiffrin’s Get Over It drill on the Burke Mountain YouTube site makes a good segue to an explanation of the Mechanics of Edge change in the my next post –

Bridget Currier is the model every skier should aspire to. She perfectly executes what I call the skimove. The skimove engages the external forces at ski-flat/edge-change to drive multi-plane torques acting about her outside ski into the turn while setting up a solid platform under her outside foot for her to stand on. Magnificent! This video should have at least a million views.

My comment from 2 years ago

Note carefully Currier’s stance in balance on her new outside ski, in particular, the angle of her torso with the snow. This is key to loading the ball of her outside foot.

Note carefully Shiffrin’s comment to move forward onto her new ski and how she used to think the movement was a lateral (sideways) move.

Most important of all – Patient Initiation. The reason? Shiffrin and Currier, don’t tip their outside ski on edge. They rock it on edge with a rocker impulse loading mechanism. The sequence is Rock, Roll n’ Rotate then Rotate the outside leg.


The cornerstone of an effective ski technique is the ability of a skier to apply a force to the outside ski that is perpendicular to the transverse aspect of the base and aligned in opposition to the Snow Reaction Force acting at the inside edge. This configuration of forces is essential to make the edge grip and act as a pivot for rotation of the sidecut of a ski in terms of penetration into the snow surface. The ability of a skier to apply a vertical force to the inside edge must co-exist with the ability to apply and control forces acting across the inside edge. This is fundamental to balance and control of the skis.

The post SKI LEVERS describes how the sidecut of a ski creates what amounts to a dual pivot, offset lever system with the edges underfoot acting as pivots for each of the two lever systems as shown in the graphic below.

Screen Shot 2014-11-18 at 11.10.41 AM

But the sidecut of a ski results in much more than a simple offset lever. As the sidecut increases ski width beyond the minimum width underfoot, the amount of lever offset progressively increases as sidecut approaches the tips and tails of a ski. As sidecut increases the width of a ski, the magnitude of torque acting on ski increases until the maximum offset and torque load is reached at the maximum width.

The graphic below shows how the load from the weight W of COM is transferred from the proximal femur to the distal  tibia by the central load-bearing axis. The default position of the centres of the loads WL and WR is on the proximate anatomical center axis of the foot (the proximate transverse centre of the calcaneus).

Central Axis 3

Unless it can be conclusively demonstrated that the load W transferred to the outside foot of a turn has been transferred to the proximate centre of the head of the first metatarsal and is applying a force aligned with, and in opposition to, the Snow Reaction Force at the inside edge of the outside ski, the load W should be assumed to have been transferred to the anatomical center axis of the foot and from there to the transverse centre of the base of the outside ski.

The graphic below is a schematic representation of a cross section of the ski as a  dual pivot, offset lever system. The inside edge underfoot P is acting in the capacity of a pivot for rotation of the side cut on the left hand side of the schematic into the snow surface. The ability to apply a force aligned in opposition to the Snow Reaction Force acting on the inside edge is literally the pivotable issue. In the schematic below, the load W is acting on the default load transfer axis and is offset from the Snow Reaction Force P acting at the inside edge which is serving as the pivot for rotation of the ski. The force Fs is the reaction force of the snow that opposes rotation of the sidecut of the ski.

Ski Lever A

The graphic below depicts the consequences of the absence of a force acting in to opposition to W. An unbalanced moment of inversion force will result from the offset of W and P that will rotate the ski until it is in either in compliance with the snow surface or inversion reaches the physiologic limits of the subtalar joint. As the ski rotates in inversion, shear forces will be set up that cause the ski to lose its edge and slip out of the turn.

Ski Lever B

The first consideration should always be to ensure that that on groomed and especially hard pistes, the foot can apply a force (Fe) in opposition to the Snow Reaction Force acting at P. This is essential to edge grip and the role of the inside ski as a pivot.

Ski Lever Fe onlyEnsuring that a skier is able to apply a force to the inside edge of the outside ski that is perpendicular to the transverse aspect of the base and aligned in opposition to the Snow Reaction Force acting at the inside edge in concert with the ability to apply and control forces acting across the inside edge should be the highest priority. An inability of a skier to effectively apply force to the inside edge of the outside ski and especially an inability to control forces acting across the edge, will create an unbalanced inversion moment of force about long axis of the ski and foot that can create a state of inversion stress in the affected lower limb. In skis with a Width Profile under foot of 100 mm or greater, a serious condition called Fat Ski Syndrome can result if Fats are used on groomed and especially hard pistes.





The science of the study of human balance is well established. Studies of balance use two key metrics; COM (Centre of Mass) and COP (Centre of Pressure). The following text is excerpted from Human balance and posture control during standing and walking – D A Winter PhD, P. Eng. – Gait & Posture: 1995; Vol 3: 193-214, December. (1)

Centre of Pressure (COP) is the point location of the vertical ground reaction force vector. It presents a weighted average of all pressures over the surface of the foot that is in contact with the ground. It is totally independent of COM. If one foot is on the ground, the net COP lies within that foot. If both feet are in contact with the ground, net COP lies somewhere between the two feet depending on the relative weight taken by each foot.

The location of COP under each foot is a direct reflection of the neural control of the ankle muscles (my emphasis added).

Increasing plantarflexion activity moves COP posteriorly (ergo, toward the back of the foot). Increasing inverter activity moves COP laterally (ergo, towards the outside of the foot). COP is often mistakenly equated with COG (Centre of Gravity). COP is calculated with software from pressure data obtained from a force plate or in-shoe pressure insole. (my emphasis added)

Because it is calculated COP can reside in the arch of the foot even though it may not be in contact with the ground.  – my comment

“Centre of Mass (COM) is a point equivalent of the total body mass in the global reference system (GRS). It is the weighted average of the COM of each body segment in 3-dimensional space. It is a passive variable controlled by the balance control system. The vertical projection of COM onto the ground is often called the Centre of Gravity (COG).

“Balance is a generic term describing the dynamics of body posture to prevent falling. It is related to the inertial forces acting on the body and the inertial characteristics of body segments.  The CNS is totally aware of the problems of controlling a multisegment system and interlimb coupling that can facilitate balance control.

“In the literature there is a major misuse of the COP when it is referred to as ‘sway’, thereby inferring that it is the same as the COG. Unfortunately some researchers even refer to the COP directly as the COG.”

In the mechanism of balance control, COP is the equivalent of the Balance Police. It keeps COM from breaching the limits of there base of support by outpacing COM in the race to the limits of the base of support within the foot or feet. In quiet standing, the force of gravity disturbs equilibrium by pulling COM forward. This causes the ankle to dorsiflex. As COM moves forward, it starts to overtake COP. In order to prevent a forward fall, the CNS signals muscles that plantarflex the ankle to increase their contraction. This increases the force of COP and pushes COM rearward. As COP shifts rearward, the CNS reduces the contractive force of plantarflexion so that COP passes COM in the race to the rear of the foot.

A similar process is employed by the CNS to prevent a sideways fall. Here, the force of gravity disturbs equilibrium towards inner or medial aspect of the foot. This causes the foot to pronate. To oppose the disturbing force, the CNS signals muscles to contract that invert the foot.

It is important to recognize that it is the external forces that disturb equilibrium  that cause the foot to pronate.

The same process is at play in skiing. However, since the sideways balance strategy involves inverter muscles, it is only possible to establish a balance platform (DOT 4: PLATFORM) on the outside foot of a turn and only then under specific conditions. In the skier/ski equipment system, COP is the point where the Resultant Force acting on a skier at ski flat that pulls COM downward towards the snow is opposed by muscles that the CNS recruits to oppose the pending collapse of the skeletal system and prevent a fall.

COP is calculated from pressure data obtained from a force plate or in-shoe pressure insole such as  the Novel Pedar system or Tekscan. Since COP reflects neural control of ankle muscles when a foot (the whole foot) is in contact with the ground or a stable source of (ground) reaction force, the use of the term COP is not technically correct in a situation where a ski is on edge unless a platform exists as described in DOT 4: PLATFORM. Until the ski lies flat on the snow between edge changes and there is full foot contact ground reaction force the appropriate term to describe the force applied by the foot to snow through the stack of ski equipment is centre of force or COF.

In a turn, COP is a good COP when it is on the right side of the law: ergo, when COP lies under the head of the 1st metatarsal and R is aligned between the inside edge underfoot and the limits of sidecut. The sketches below show the progression of COF at ski flat that moves COP to the head of the 1st metatarsal. If COP arrives at the head of the 1st metatarsal before the outside ski has attained a significant edge angle and COP remains in this position through the turn COP is a good COP.

Sketch 1 below shows the 2 key mechanical points in skiing (red cross)

Centres of key pts

Sketch 2 below shows the Centre of Force (COF) under the heel of the inside foot at the start of the transition between turns. The red dashed line shows the approximate trajectory of COF as it moves forward and becomes COP at ski (foot) flat between turns as the external forces cause the foot to pronate.



Sketch 3 below shows the forward progression of COP towards the head of the 1st metatarsal.


Sketch 4 below shows the successful transition of COP to the head of the 1st metatarsal where it lies over top of the inside edge of a ski of appropriate width.



Sketch 5 below shows axis on which COP and R must align in order to engage the external force R to drive edging and turning mechanics.


Sketch 6 below shows R on the same axis as COP.  In this configuration the alignment of R described under DOT 4: PLATFORM will enable multiplane torques generated by pronation to be directed into the turn.


In sketch 7 below COP has failed to make a transition to the head of the 1st metatarsal. When COP fails to make the transition to the head of the 1st metatarsal at ski flat between edge change before the new outside ski attains a significant edge angle, a moment arm will be setup between the inside edge and COP that will create an inversion moment of force or torque with an associated external vertical axial rotation of the whole leg.



In sketch 8 below COP has reversed direction. Once an inversion moment arm has been set up on the outside ski there is no way to undo it. The odds are great that COP will revert to its default position under the heel because it is under the mechanical line of the lower limb.


When this happens COP becomes a bad COP.

1. You can obtain a copy of David Winter’s paper at the following link: