THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 13

The  article that follows was published on June 18, 2010 on an internet group called EPICSKI.  I have revised the article to improve clarity and consistency with the technical terms used in the THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE series of posts.

The Birdcage Experiments

 by David MacPhail

In the summer of 1991 a science team Steve Podborski and I had assembled to develop a new ski boot conducted pioneering studies on the Blackcomb summer glacier with a device we affectionately named the “Birdcage.” The purpose of the studies was to test my hypothesis of the mechanics and biomechanics of platform angle as it pertains to skier dynamic stability and the basic premise of my hypothesis that explains how  GRF acting on the inside edge of the outski is extended out under the platform of the ski. The Birdcage is shown in the photo below.

Birdcage

The Birdcage was fit with 16 sensors each with its own channel as shown in the legend below.

Specific mechanical points of the foot, in particular the ends of the eccentric torque arm, connected to specific points of the rigid structure of the Birdcage while leaving the remaining areas of the foot substantially unconstrained. The object of the experiments was to study the effects of specific forms of constraint applied to key mechanical points of the foot we had previously identified on skier balance as it pertains to steering and edge control. The experiments also included tests that studied the effect of interfering with specific joint actions. The experiments were designed in accordance with a standard scientific protocol; one that standardized conditions from test to test while varying one factor at a time.

For example, to study the effects of cuff forward lean angle on specific muscles, the range of rotation of the cuff was kept the same from test to test while the initial angle at which the cuff was set was varied from test to test. The cuff was fit tightly about the leg so as to reduce to a minimum any effects of movement of the leg within the cuff. Other aspects of the test such as position of the heel and ball of the foot in relation to the centerline and inside edge of the ski were kept the same.

By using such test protocols the firing sequence of specific muscles and their effect on dynamic stabilty could be studied. This data could then be used to determine the sequence of events and relationship steering to edge platform angle control. It was discovered that by varying the conditions that affected the firing and effectiveness of the soleus muscle, it could be played like a musical instrument. For example, if the cuff angle were set too erect the soleus muscle would make multiple attempts at the start of each loading sequence to try and get COG over the head of the first metatarsal.

Our primary tester for the experiments was Olympic bronze medallist and World Cup Downhill Champion Steve Podborski. Steve is shown in the photos below having the Birdcage adjusted to his foot and leg.

The cable coming from the rear of the device is connected to a Toshiba optical drive computer (remember, this is 1991) that Toshiba loaned us in support of our program. The biomedical engineer and the Toshiba computer are shown in the photo below.

Since telemetry was too costly and less positive we used a 1200 ft cable that linked the Birdcage to the Toshiba computer set up in a tent. Although the technician could not see the skiers being studied within a short period of time he could easily analyze their technical competence in real time by assessing the incoming flow of data from the sensors fit to the Birdcage. This was even more remarkable considering that the technician had no background in skiing, ski teaching or coaching.

The testers wore a harness to keep the cable from interfering with their movements. A chase skier ensured that the cable remained behind the testers and did not pull on the testers. Of interest is the fact that I was unable to elicit any interest in the results of the Birdcage study

As far as I know a study of this nature had never been done before and to the best of my knowledge a similar study has never been repeated since the Birdcage experiments. The Birdcage remains one of the most sophisticated analytical sports devices ever conceived even by todays’ standards. The Birdcage research vehicle is the barefoot minimum standard for the ski boot.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 12

At this point my discussion of the mechanics and biomechanics of platform angle is at what I can appropriately call the moment of truth. Moment in the context of the mechanics and biomechanics of platform angle means moment of force or torque; platform angle involves the ability of the CNS of a skier to control torques across the inside edge of the outside ski so the skier can stand and balance on the platform.

What is Balance?

That balance is the single most important factor in human movement, especially movement associated with athletic performance, is undisputed. In complex activities like skiing that involve movement in 3 dimensional space in a dynamic physical environment, optimal balance is critical. But what constitutes balance? In order to know if a skier is has optimal balance or is even in balance one has to know what balance is and what factors enable or compromise balance (i.e. postural) responses and  especially the factors that enable optimal balance.

The Balance Zone

A skier is in balance when the CNS is able to maintain the position of a skiers’ COG within the limits of a narrow band close to the inside edge of the outside ski during the load phase of a turn. The load phase of a turn occurs in the bottom of a turn when the force exerted on the platform by the COM of a skier must be balanced against the external resultant force of gravity and centrifugal force. In the load phase, the CNS must maintain COG within the forward limit of the Balance Zone within close proximity to the ball of the foot. When balance is challenged COG must not exceed the rearmost limit of the Balance Zone that lies just in front of the ankle joint. The Balance Zone and its limits are shown in the graphic below. If COG exceeds the limits of the Balance Zone shown in pink, the skier will lose their state of balance and with it dynamic control of the platform underfoot.  They will also suffer a lose of dynamic stability in the joint system of the lower limb.

The Balance Plane

In the ski system platform the plantar plane under the plantar aspect (sole) of the foot is the interface of CNS mediated balance activity. When the coordinated, concurrent forces are applied at the main force transfer point of the foot that I call the Center of Control, shown in the preceding graphic, the applied forces will manifest in more than one plane as shown in the graphic below.Force Fa applied under the head of the first metatarsal will be distributed over an area around its center.  When the force applied in the plantar plane is transferred through the structure of the platform to the base plane the center of force will maintain its position. But when the force area of distribution will increase as shown in the pink zones under the head of the first metataral and the base plane. In free rotation of the ski, resistance from the force of friction Ff will be minimal as will any force applied in the torque arm plane by the eccentric torque arm. Rotational force will be largely confined to the base plane.

The Missing Force Factor: Sidecut

In the free rotation, the effect of the sidecut of a ski is not a significant factor in terms of a source of resistance. But as the transverse aspect of the base plane of the ski acquires an angular relation with surface of the snow the resistance created by GRF acting at the  limit of sidecut at the shovel sets up an interaction between the rotational force applied to the inner wall of the boot shell adjacent the medial aspect of the head of the first metatarsal with the resistance created by GRF at the limit of sidecut at the shovel. In the graphic below I have connected the  2 dots of the platform ground effect problem with a line drawn between the two points.The graphic below shows a schematic of the mechanical aspects of the opposing moment or torque arms between the two dots that I connected in the preceding graphic. The inside edge below the head of the first metatarsal acts as a pivot in conjunction with the Center of Force applied 90 degrees to the transverse aspect of the base plane for the plaform to rotate about as the ski goes on edge.

As the base plane of a ski acquires an angular relationship with the snow the torque arm rotating the ski goes into what cane best be described as turbo torque boost. Whole leg rotational force continues to rotate the whole ski but the eccentric torque arm engages and applies a high torsional load that winds the body of the platform about the shovel. This mechanism has to be considered in the perspective of the of the inertia from the movement of the skier driving the cutting action of the shovel.  The graphic below shows the opposing how opposing torsional forces at the limit of sidecut and applied by the application of for by eccentic torque arm to the vertical shell wall by the medial aspect of the head of the first metarasal act to apply a upward force that extends to the outboard end of the plantar plane of the platform.  This is the mechanism that enables elite skiers to balance on their outside ski and initiate precise movement from from a dynamically stable platform.I first solved basic mechanics and biomechanics of the outside ski balance problem 30 years ago. The degree of difficulty was not great. Solving the problem took diligence and persistence in researching all the relevant aspects and identifying all significant forces and associated planes.

I’ll let the readers ponder the informaton in this for a while after which I will be happy to respond to questions and comments.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 11

In my preceding post I said that after a thorough investigation and analysis of the forces associated with platform angle mechanics I reached the conclusion that rotational (steering) force should be applied to an isolated area of the inner shell wall of the ski boot by the medial aspect of the head of the first metatarsal. The reason I conducted a thorough investigation and analysis of the forces is that as a problem solver this is my standard protocol. Protocol aside, the need for a thorough investigation of every aspect affecting athletic (skier) performance was known as far back as 1983.

….. quality teaching – coaching of neuromuscular skills in physical education should always be preceded by an analytical process where the professional physical educator synthesizes observations and theory from scientific and technical perspectives……

There are many sports skills which require that sports objects, implements, equipments, and apparatus be utilized. (implements such as ski boots and skis)

All factors must be studied in terms of the skill objective. If problems are noted in the performance of the skill, where did they originate? Within the performer? Within the sport object? Both? What precise changes must be made to obtain the skill objectives?

The directions for improvement given to the performer must be based on scientific and technical analysis of the total skill.

Analysis of Sport Motion (May 1, 1983): John W. Northrip

Planes of Forces

The ability to conduct a thorough investigation and analysis of the forces associated with platform angle mechanics and biomechanics requires as a minimum, a basic understanding of the engineering aspects of the associated forces. In the case of platform mechanics and biomechanics, knowing the plane or planes in which a force or combination of forces are acting is essential.

The Force Plane in the Perfect Fit

The objective of achieving a perfect fit of the foot and leg of a skier is create a perfect envelopment of the foot and leg of a skier with the rigid shell wall of a ski boot so that force is applied evenly to the entire surface of the foot and leg to create a unified mass with the ski so that the slightest movement of the leg will produce edging and steering forces. In this format force(s) applied to the base plane by the leg will be distributed to a broad area with limited ability to apply coordinated forces to a specific area of the ski. Sensory input is also limited by the uniform force applied to all apects of the foot by the perfect fit format creating what amounts to the skiing equivalent of the Bird Box.

Platform Planes

In the mechanics and biomechanics of platform angle there are potentially three horizontal planes in which forces can be applied as shown in the graphic below. The left hand image shows the rotational force applied to a torque arm plane elevated about the base and plantar planes. In the perfect fit format in the right hand image the leg is shown as a rigid strut extending to the base plane where rotational force is applied.When the foot and leg of a skier are perfectly fit within to the rigid shell of a ski boot any force applied by the leg can only applied to the base plane of the ski where the force is distributed over a broad area. Steering and edging forces applied to the ski by the leg lack precision because they cannot be applied to specific areas or applied in a coordinated manner.

In the above graphic the whole leg rotational effort applied to the base plane by foot in the two examples is shown with no resistance. In my next post I will discuss what happens when resistance is added that opposes the rotational force applied to the base plane.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE – PART 10

In THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 8,  I stated that after a thorough investigation and analysis of the forces associated with platform angle mechanics I reached the conclusion that rotational (steering) force should be applied to an isolated area of the inner shell wall of the ski boot by the medial aspect of the head of the first metatarsal as shown in the graphic below.Applying rotational or steering force to the medial (inner) aspect of the head of the first metatarsal requires the application of an effort by the skier that attempts to rotate the foot inside the confines of the ski boot. The application of rotational effort to the inner aspect of the vertical wall of the boot shell opposite the head of the first metatarsal will result in a reaction force that pushes the lateral (outside) aspect of the heel bone against the outer corner of the vertical shell wall as shown in the graphic below. The robust structure of the bones of the first metatarsal, midfoot and heel bone serve as a structural element in transferring rotational force to opposing aspects of the shell walls in an eccentric torque couple.The outline of the boot shell in the above graphic was generated from a vertical plane photo of an actual ski boot. The interference created by the inner wall with the localized application of rotational force to the shell wall by the medial aspect of the head of the first metatarsal should be obvious.

The radius of the moment arm acting on the outer aspect of the heel area of the shell is much smaller than the radius of the moment arm acting on the inner aspect of the shell opposite the head of the first metatarsal and many times shorter than the length of the moment arm acting at the shovel of the ski. The result is that rotational force applied to the eccentric torque arm couple by rotation applied to the ankle will attempt to rotate the torque arm and the axis of rotation at the ankle joint about an axis of rotation at the lateral aspect of the heel as shown in the graphic below. This mechanism enables a skier to  apply much greater rotational force into a turn at the center of the ski than can be applied at the shovel. This has signficant implications for platform angle mechanics. In addition to the above, the plane of the rotational force applied by the medial aspect of the head of the first metatarsal and lateral aspect of the heel bone to the shell wall is elevated above the plane of the rotational force at base of the ski below.

In my next post I will discuss what happens when the reaction force from the snow that opposes the 180 degree force applied to the base plane of the ski becomes sufficient to arrest rotation of the ski about its axis of rotation at the ankle joint.

COMMENTS ON SUPER PETRA VLHOVA

With the Mens’ and Ladies’ GS and SL World Championship events starting tomorrow I am republishing my March 28, 2017 post on Petra’s performance in the Aspen Slalom where I said,

This was not the same Vlhova I had analyzed earlier in the season. Vlhova has definitely changed and it is for the better.

The degree of neurobiomechanical function permitted within the structures of a ski boot, more than any other factor, can literally determine who stands on the podium. The GS and SL World Championship events will show which racer has the functional edge.


COMMENTS ON SUPER PETRA VLHOVA

As time permits, I analyze the movement and loading patterns of elite skiers such as Mikaela Shiffrin, Lindsey Vonn, Ted Ligety, Tessa Worley and others. Occasionally, a source sends me video of these racers training.

I have identified a specific movement and loading sequence pattern that I use to analyze technique. This requires decent quality video and specific camera angles. In a future post, I will describe the process, the key metrics I look for and what they indicate.

Up until I saw the video of Vlhova, that is the subject of my post, SUPER PETRA VLHOVA’S EXPLOSIVE IMPULSE LOADING IN ASPEN SLALOM, I rated her as one of the better technical racers on the World Cup circuit. But I did not consider Vlhova to be in the same class as a Shiffrin or a Worley.

When someone posted a link on FaceBook to Vlhova’s winning run in the Aspen slalom, I was stunned by what I saw in the first few gates. This was not the same Vlhova I had analyzed earlier in the season. Vlhova has definitely changed and it is for the better.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 10 – SUPPLEMENTAL INFORMATION


Because of the complex issues I am about to start discussing in the next series of posts I am providing supplemental reference information to assist the reader in understanding the issues associated with platform angle mechanics and biomechanics and underlying process of dynamic stability.

Background of events leading up to the outside ski platform ground balance solution

In late 1989, after gaining valuable insights from the medical textbook, The Shoe In Sport, I had formulated a hypothetical model that explained the macro details of the mechanics and biomechanics of platform angle and the mechanism of user CNS postural balance control.

Insights from The Shoe in Sport:

Correct positioning of the foot is more important than forced constraint and “squeezing” the foot.

Forward sliding of the foot should not be possible. 

From a technical (skiing) point of view, the ski boot must represent an interface between the human body and the ski. This implies first of all an exchange of steering function, i.e., the skier must be able to steer as well as possible, but must also have a direct (neural) feedback from the ski and from the ground (snow). 

The comment about the importance of correct positioning of the foot and the ski boot  representing an interface between the human body and the ski gave me insights that led to the discovery of key mechanical of the foot whose position in relation to the inside edge and X-Y axes of the ski affects the transfer and control of steering and platform forces to the ski and control.

When I wrote the application for US Patent No 5,265,350 in late 1991 and early 1992 I described the mechanics and biomechanics of plantar angle in great detail knowing this information would be freely available to the entire world to use once the patent was published. The only exception was the information covered by claims. Known mechanics and biomechanics are not in themselves patentable.

Patents and Research

It is important to note that patents, even when granted, do not apply to the use of a patented device for the purpose of pure research. Knowing this at the time I wrote the patent, I described the Birdcage research vehicle in sufficient detail with many figures to enable the device to be constructed at minimal cost so research could be conducted by others as soon as possible for the purpose of advancing the knowledge base and science of alpine skiing.

The following unedited text is excerpted from the patent.

……. the teaching of this (patent) 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 performance of such mediums (skate blades and skis) is largely dependent on the ability of the user to accurately and consistently apply forces to them as required to produce the desired effect.

In addition, in situations where the user must interact with external forces, for example gravity, the footwear must restrain movements of the user’s foot and leg in a manner which maintains the biomechanical references with the medium with which it is interacting.

Precise coupling of the foot to the footwear is possible because the foot, in weight bearing states, but especially in monopedal function, becomes structurally competent to exert forces in the horizontal plane relative relative to the sole of the footwear at the points of a triangle formed by the posterior aspect and oblique posterior angles of the heel, the head of the first metatarsal and the head of the fifth metatarsal. In terms of transferring horizontal torsional and vertical forces relative to the sole of the footwear, these points of the triangle become the principal points of contact with the bearing surfaces of the footwear. 

The most important source of rotational power with which to apply torque to the footwear is the adductor/rotator muscle groups of the hip joint. In order to optimally link this capability to the footwear, there must be a mechanically stable and competent connection originating at the plantar processes of the foot and extending to the hip joint. Further, the balanced position of the skier’s centre of mass, relative to the ski edge, must be maintained during the application of both turning and edging forces applied to the ski. Monopedal function accommodates both these processes. 

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 skiing, the mechanics of monopedal function provide a down force acting predominantly through the ball of the foot (which is normally almost centred directly over the ski edge). In concert with transverse torque (pronation) arising from weight bearing on the medial aspect of the foot which torque is stabilized by the obligatory internal rotation of the tibia, the combination of these forces results in control of the edge angle of the ski purely as a result of achieving a position of monopedal stance on the outside foot of the turn. 

The edge angle can be either increased or decreased in monopedal function by increasing or decreasing the pressure made to bear on the medial aspect of the foot through the main contact points at the heel and ball of the foot via the mechanism of pronation. As medial pressure increases, horizontal torque (relative to the ski) increases through an obligatory increase in the intensity of internal rotation of the tibia. Thus, increasing medial pressure on the plantar aspect of the foot tends to render the edge-set more stable.

There are many figures that illustrate the concepts expressed in the above text which I will include in future posts.

The photo below shows the strain gauges (black disks) fit to the 1991 research vehicle. These gauges recorded first metatarsal forces under and to its inner or medial aspect and the outer and rearmost aspects of the heel bone.

I’ve learned a lot since the above information was made public after the patent was issued on November 30, 1993.

In Part 10, I will discuss the mechanism by which forces applied by the ball of the foot to what I call the Control Center of the platform provide quasi ground under the outside foot and leg in the load phase of a turn for a skier to stand and balance on.

THE MECHANICS + BIOMECHANICS OF PLATFORM ANGLE: PART 9

In my previous post I the described a mechanism by which whole leg rotational or steering force can be applied to a vertical extension of the platform by inner (medial) aspect of the head of the first metatarsal. The associated user biomechanics have a number of requirements the structures of a ski boot must meet in order to apply this force. These include, but are not limited to the following:

  • in the load phase in what is called ‘the bottom of the turn’ the foot must be able to rapidly pronate with minimal interference from the structures of the ski boot.
  • force applied to the vertical extension of the platform must be localized on (medial) aspect of the head of the first metatarsal and not from other structures of the foot, including the inner (i.e. medial) aspect of the big toe and the medial boney structures of the ankle and midfoot.
  • the big toe must be able to be aligned straight ahead on the anatomical center of the long axis of the foot without significant interference from the structures of the ski boot including structures of the liner.

In addition to the above, there must also be minimal interference with the ability of the Achilles tendon to transfer high loads to the head of the first metatarsal (i.e. ball of the foot) to the platform underneath as the 90 degree component of edge cutting force. The magnitude of force, especially peak impulse force, that a skier can apply to the head of the first metatarsal has a direct effect on the degree of force that can be applied to the medial aspect the head of the first metatarsal.

Data from the 1998 University of Ottawa study of pressures under the feet of elite skiers (1.) found that maximal forces ranged from a low of 522 N to a high of 1454 N; a difference of 279%. The data also found significant differences in the maximal forces recorded between the left and right feet of all elite skier test subjects for all turn types except dynamic parallel.

Table 1 below from the shows the forces generated from the pressure data acquired in University of Ottawa study.

The large differences seen between a range of elite skiers and especially between left and right feet of the same skier has significant implications for the ability to apply force to a vertical structure with the head of the first metatarsal, a force not considered in the University of Ottawa pressure study or any study I am aware of.

To the best of my knowledge my 1992 skier force study that used a research vehicle called The Birdcage is the only study even today that examined force applied by the medial aspect of the head of the first metatarsal to a vertical structure of the platform of a ski boot/ski. The Birdcage studies also examined the interaction and effect of vertical plantar forces applied to the platform in conjunction with horizontal force applied to a vertical extension of the platform.

Center of Force

Sometimes call Center of Pressure in gait/balance studies, Center of Force (COF) or Center of Pressure (COP) do not represent a point application of a force vector. COF and COP are point centers of force applied to an area of a surface or body. (2.)(3.)(4.)

In platform mechanics, the sole of the foot applies force to a large area of the platform. The closest point to the inside edge of the outside ski where the Center of Force can act is under the head of the first metatarsal. Force applied to the platform of the ski will always apply a force to the running surface of the inside edge. Even if CoF is aligned over one aspect of the GRF acting on the inside edge of the outside ski it is impossible for COF of the outside foot to be aligned over the entire sidecut arc of the inside edge in contact with the snow. Since the foot cannot access GRF (i.e. ground) under the entire length of the inside edge of the outside ski, ground needs to be brought out under the platform.

In order to successfully solve a problem all aspects of a problem must be identified and their implications understood. The solution to the platform/ground problem is finding a way to extend the ground under the entire running surface of the inside edge of the ski out under the platform. In my next post I will begin to explain how this is tied to the ability to apply robust force with the head of the first metatarsal to a vertical extension of the inner aspect of the platform.


  1. ANALYSIS OF THE DISTRIBUTION OF PRESSURE UNDER THE FEET OF ELITE ALPINE SKI INSTRUCTORS – Dany Lafontaine, Mario Lamontagne, Daniel Dupuis & Binta Diallo,
    Laboratory for Research on the Biomechanics of Hockey, University of Ottawa, Canada: Proceedings of the XVI International Symposium on Biomechanics in Sports (1998), Konstanz, Germany, p.485.
  2. WHAT THE TWO HIGH PRESSURE COPS IN THE UNIVERSITY OF OTTAWA STUDIES MEAN – https://wp.me/p3vZhu-1fV
  3. IMPLICATIONS OF THE UNIVERSITY OF OTTAWA PRESSURE STUDIES – https://wp.me/p3vZhu-1e2
  4. AN INDEPENDENT STUDY IN SUPPORT OF THE UNIVERSITY OF OTTAWA FINDINGS – https://wp.me/p3vZhu-1gR