biomechanics

IS DYNAMIC SKIING A FORM OF WALKING?

The text below is from a sub page I put up on the home page in 2014 in which I posited that elite skiers use the same hard-wired processes as walking.

It was only recently after I connected pelvic alignment with the ball of the outside foot of a turn achieved by steering the foot into position with COM to create an alignment with the fall or gravity line did I finally put the last piece of the puzzle in place.


As bipeds, we propel our bodies forward by moving from one fascially tensioned base of support with foot to core sequencing on one foot to another fascially tensioned base of support with foot to core sequencing.

Dynamic skiing uses the same basic pattern. In skiing, we need to establish a fascially tensioned base of support with foot to core sequencing on one foot in order to be able to move with precision to another fascially tensioned base of support with foot to core sequencing on another foot. As far back as the 70’s, the famous French ski technician, Patrick Russell, said that the key to effective skiing is to ‘move from ski to ski’. What Russell was really alluding to is the process of alternating single limb support.

Ever since alpine skiing became formally established, it has been known that the best skiers move from the outside ski of one turn to the outside ski of the next turn. Although this may sound simple enough, the key to being able to effectively move from ski to ski (foot to foot) is the ability to establish a fascially tensioned base of support with foot to core sequencing one one foot and then use it to move the body or Centre of Mass to the new outside foot (current uphill ski) of the next turn. Good skiers do this so seamlessly that turns seem to have no beginning or end. The turns just flow together. When viewed in the context of stance and swing phases, the resemblance to walking becomes apparent

How to make skiing as intuitive as walking is what this blog is about. I devoted an entire series of patents to this subject commencing with US Patent No. 5,265,350 and associated international patents on the elements of a minimal ski boot necessary to accommodate the process of establishing a fascially tensioned base of support with foot to core sequencing on one foot and transitioning seamlessly back and forth between bipedal and monopedal stances.

The ability to balance multi-plane torques on the outside leg of a turn is, and continues to be, the secret of the worlds’ best skiers including Toni Sailor, Nancy Greene Raine, Pirmin Zubriggen and, today, Mikaela Shiffrin, Lindsey Vonn and Ted Ligety to name but a few.


A REVIEW OF GAIT CYCLE AND ITS PARAMETERS – Ashutosh Kharb1, Vipin Saini2 , Y.K Jain3, Surender Dhiman4 – https://ijcem.org/papers72011/72011_14.pdf

Dynamic loading of the plantar aponeurosis in walking – Erdemir A1, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA. – https://www.ncbi.nlm.nih.gov/pubmed/14996881

Active regulation of longitudinal arch compression and recoil during walking and running – Luke A. Kelly, Glen Lichtwark, and Andrew G. Cresswell – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4277100/

The Foots Arch and the Energetics of Human Locomotion – Sarah M. Stearne, Kirsty A. McDonald, Jacqueline A. Alderson, Ian North, Charles E. Oxnard & Jonas Rubenson – http://www.nature.com/articles/srep19403

Shoes alter the spring-like function of the human foot during running – Kelly LA1, Lichtwark GA2, Farris DJ2, Cresswell A2. – J R Soc Interface. 2016 Jun;13(119). pii: 20160174. doi: 10.1098/rsif.2016.0174. – https://www.ncbi.nlm.nih.gov/pubmed/27307512

The Science of the Human Lever: Internal Fascial Architecture of the Forefoot with Dr. Emily Splichal – https://www.youtube.com/watch?v=_35cQCoXp9U

 

 

 

 

TRANSITIONING TO A HIGHER LEVEL OF SKIER PERFORMANCE

The transition to a higher level of skier performance for my spouse and I started in the 2012-13 ski season. After a ten-year hiatus from skiing we were returning to the ski hills with renewed enthusiasm coupled with a desire to reach a higher level of performance. I purchased new narrow waisted skis for both of us. I intended to purchase new ski boots as well. But I quickly backed off from even considering this after assessing a number of new boots as too difficult to work with.

I started The Skier’s Manifesto in the spring of 2013 for a number of reasons. The primary reason was that the forum provided me with an opportunity to acquire new information and increase my knowledge so I could learn how to transition my spouse and I to a higher level of skier performance. The process of attempting to explain complex technical issues by writing articles and posts serves as the impetus for me to think deeply, thoroughly and analytically. As the process unfolded, I discovered issues I had overlooked in the past or not fully explored.

One issue I had not fully explored, let alone addressed, is a way of identifying the optimal ramp angle specific to each skier. Ramp angle is the angle of the ramp of the plantar plane under a skier’s foot with the base plane of the ski. Finding a method of identifying optimal ramp angle proved far more difficult than I had anticipated. But when I succeeded in identifying and then implementing the optimal ramp angles for my spouse and I last ski season this proved to be the gateway to a higher level of skier performance than I could ever have envisioned. After identifying and then confirming my optimal ramp angle as 1.2 degrees (bindings zero) I finally understood after almost 45 years how and why changing from the leather ski boots I learned to ski in to the new plastic boots had such a devastating impact on my skiing. It was the change in ramp angle. The ramp angle in my leather boots was much less than the ramp angle in my plastic boots.

NOTE: Since I published this post a little over a year ago I have since reduced the zeppa angle of ,my Head boots to close to zero)/

By 1978 I had subjectively found that a ramp angle greater than 3 degrees adversely affects skier performance with some skiers affected more than others. I knew there was no one size fits all, only that more than 3 degrees seemed to cause problems. From 1978 onward I was improving skier performance by ensuring the total ramp angle of the combined boot board/binding (zeppa + delta) was about 3 degrees. For females with small feet this required grinding the boot board in Lange boots flat or even negative (heel down) to compensate for binding ramp angle which increased as the toe and heel pieces moved closer together for small boots. I wasn’t always able to get the ramp angle set at 3 degrees. But getting it in the 3 degree range consistently resulted in significant improvement in skier performance.

It was becoming increasingly apparent to me that finding the optimal individual ramp was critical.

Critical Ramp Angle

In 2018 I identified the critical ramp angle as the angle of the plantar plane in relation to the base plane of the ski that enables a skier to apply maximum vertical force to the ball of the outside foot when the COM in the pelvis is stacked vertically over the head of the first metatarsal.

The vertical force is applied passively by force transfered to the plantar aponeurosis ligament (PA) by Achilles tendon (AT) tension.  As COM moves forward towards the head of the first metatarsal in the support phase where skier resists the force of gravity, AT-PA tension applies an increasingly greater down force to the head of the first metatarsal. Ramp angle is optimal when the vertical force peaks just prior to the end of the support phase in what is called Mid Stance in the Gait Cycle of walking.  I qualified this mechanism as enabling a skier to apply maximum vertical force to the head of the first metatarsal. Studies have shown in the skiing the position of the pelvis in relation to its vertical position with foot is the most reliable indicator of the position of COM. A skier is able to control the vertical force applied to the head of the first metatarsal by controlling the position of the pelvis.

The photos below show Marcel Hirscher and Tesa Worley applying maximum force to the head of the first metatarsal of their outside foot by stacking their pelvis over it.

The Problem with Adapting

The primary determinant of the critical ramp angle is the length of skier’s Achilles tendon (AT).

The length of the AT can and does vary significantly among the general and skier populations. The type of everyday footwear worn and especially what is called drop (heel elevated above the forefoot) can affect the length of Achilles tendon.

Drop affects the timing of the process that stiffens the foot transforming it into a rigid lever for propulsion. Over time, the predominate wearing of footwear with significant drop can cause the AT to shorten as a way for the body to adjust the timing of the stiffening process. In activities such as walking and standing, a shortened Achilles tendon may not have a noticeable affect on performance. But in skiing, the timing of the AT-PA tensioning process is critical. Those who learned to ski in boots with ramp angles close to optimal for the length of their Achilles tendon typically excel at skiing regardless of athletic prowess while gifted athletes who learned to ski in boots with sub optimal ramp angle can struggle in spite of innate athletic ability. For a racer whose equipment is close to their critical ramp angle a change in equipment that significantly changes ramp angle can be fatal to a promising career.

Most skiers would assume that they can just adapt to a sub optimal ramp angle. But adaptation is precisely the reason why skiers and racers with a sub optimal ramp angle reach a threshold from which they cannot advance. When their brain makes repeated attempts to apply force to the head of the first metatarsal without success it starts to make adjustments in what are called synaptic connections to create a new movement pattern to adapt to sub optimal ramp angle. The more the equipment with a sub optimal ramp angle is used the more the associated synaptic connections are strengthened and reinforced. Once the movement pattern associated with sub optimal ramp angle is hardened,  optimal ramp angle is likely to be perceived by the brain as wrong. Telling a racer with sub optimal ramp angle to get forward or get over it (what that means) will only make matters worse because a sub optimal ramp angle makes it impossible. Correcting the ramp angle and/or the length of the AT will not help because neither will change the hard-wired movement pattern in the brain. Deleting a bad movement program can be done. But it usually takes a structured program and a protracted effort.

Mid Stance Misinformation

A factor that I believe may have contributed to the critical ramp angle issue being overlooked is misinformation about mid stance. The story used to sell footbeds and even some orthotics is that skiing is a Mid Stance activity and in Mid Stance the foot is pronated and weak necessitating a foundation under the arch to support it. While it is true that the load phase of skiing occurs in Mid Stance the statement that the foot is weak is only partially true because it doesn’t encompass the whole picture.

The Stance or Support Phase of what is called the Gait Cycle of walking consists of four phases:

  1. Loading Response
  2. Mid Stance
  3. Terminal Stance
  4. Pre-Swing

All four phases happen in a ski turn sequence. The support phase, where one foot is flat on the ground and the leg is supporting the weight of COM, is called Mid Stance. The position of COM in relation to the head of the first metatarsal in Mid Stance and how fast COM can move forward over the head of the first metatarsal (center of the ski) of the outside foot in the load phase is a major factor in dynamic control and the ability of a skier to apply maximum force to head of the first metatarsal. But Mid Stance is a range and a sequential stiffening process, not a fixed point as has been misrepresented for decades by many in the ski industry.

The graphic below shows the relationship of 1. Achilles Tendon Force with 2. Plantar Aponeurosis Force with 3. Vertical GRF and how the tensioning process and transfer of force to the head of the first metatarsal occurs as COM progress forward in the Mid Stance cycle. The timing of the forward advance of COM/Pelvis to sync with peak AT-PA force transfer to the head of the first metatarsal is shown with a red circle and vertical arrow.

If I had only shown the segment of Mid Stance in the grey rectangle at the beginning of Mid Stance on the left I could have made a case that the arch is weak and in need of support since Achilles Tension is zero and Plantar Aponeurosis Force (called strain) is very low. But this would be misinformation because it does not show the whole picture. If the foot were weak as is alleged it would be impossible for it to act in the capacity of a lever in propelling the weight of the body forward in locomotion.

In my next post I will explain how I used NABOSO surface science technology to confirm my optimal ramp angle.

 

THE FUTURE OF THE SKI BOOT – PART 2

The introduction of the rigid shell ski boot served as a foundation for the evolution of what became a science of immobilization and splinting of the joints of the foot and a leg of a skier. By creating an encasement for the foot and the portion of the leg within the rigid shell, mediums such as foam could transfer force to the ankle and leg to substantially immobilize its joints. Supporting the foot in a neutral position with a rigid footbed or orthotic in conjunction with form fitting mediums ensures maximal immobilization that is described as the Perfect Fit. The science of immobilization has evolved over the years to include thermoformable liners and even thermoformable shells.

Even though the medical textbook, The Shoe in Sport, cautioned 30 years ago that “the total immobilization by foam injection or compression by tight buckles are unphysiologic (against physiologic function)” the proponents of immobilizing the joints of the ankle continue to claim that this puts the foot in it’s strongest position for skiing.

The paper, Recent Kinematic and Kinetic Advances in Olympic Alpine Skiing: Pyeongchang and Beyond,  published on February 20, 2019, cited better transfer of the skier’s action to the skis through improved boot-fittings with individual liners and insoles. If in fact, skier performance is improved due to improvements in the science of immobilization through boot-fitting then it should be evident in studies that look at skier performance.

One such study, Challenges of talent development in alpine ski racing: a narrative review, published in March of 2019 found:

Youth and adolescent ski racers report lower injury rates compared to World Cup athletes. The knee was the most affected body part in relation to traumatic injuries. The most frequently reported overuse injuries were knee pain (youth) and low back pain (adolescent level). Athlete-related modifiable risk factors were core strength, neuromuscular control, leg extension strength and limb asymmetries.

Neuromuscular Function (NMF) affects Neuromuscular control (NMC). NMC is an unconscious trained response of a muscle to a signal associated with dynamic joint stability. This system of sensory messages (sometimes referred to as “muscle memory”) is a complex interacting system connecting different aspects of muscle actions (static, dynamic, reactive), muscle contractions, coordination, stabilization, body posture and balance. The movements of the lower extremity, including the knee joint, are controlled through this system, which needs correct sensory information for accurate sequential coordination of controlled movement.

It has been known for decades that restricting the action of a joint or joint system, especially immobilizing the joint, will cause the associated muscles to atrophy. But a study, Effect of Immobilisation on Neuromuscular Function In Vivo in Humans: A Systematic Review, published in March 2019, suggests that the effects of immobilizing joints of the body are far greater than simply causing muscles to atrophy. This is the first systematic review to consider the contribution of both muscle atrophy and deterioration in neuromuscular function (NMF) to the loss of isometric muscle strength following immobilisation. The fact that the study, Challenges of talent development in alpine ski racing: a narrative review, cited core strength and neuromuscular control as issues in the development of talent is significant. The feet are part of the core in what is called foot to core sequencing. Immobilizing the joints of the foot can affect lower limb and core strength.

Immobilisation in the study the Effect of Immobilisation on Neuromuscular Function In Vivo in Humans: A Systematic Review, was achieved by using casts, braces, slings, unilateral suspension, strapping or splints with the following locations immobilised: knee, ankle, wrist and finger. All studies measured isometric muscle strength. No studies were cited that involved bilateral immobilisation of both ankles such as occurs in form-fitting ski boots. However studies did find that multiple joint immobilisation was likely to produce the largest change in the NMF of segments consisting of both mono and biarticular muscles. Other key findings were:

  • The greatest changes in all variables occur in the earliest stages of immobilisation.
  • The loss in muscle strength during immobilisation is typically greater and occurs faster compared to the loss of muscle volume.
  • The choice of joint angle for immobilisation using the brace or cast method appears likely to play a large role in the outcomes.

I started this blog six years ago for several reasons. A primary reason was to identify whether any influences existed in skiing that would serve to change the focus from immobilizing the joints of the foot and leg with the associated claims to a science-based focus. Since the future of the ski boot appears to be continued refinement of the science of immobilization this will be my final post.

I have learned a lot over the past six years that led to huge breakthroughs on skis for myself and those who I have worked with. Thank you to those who commented and contributed to The Skier’s Manifesto.

THE FUTURE OF THE SKI BOOT – PART 1

SHOEspiracy, a new feet-first documentary by barefoot/minimal shoe maker Vivobarefoot (1.) provided me with insights on the factors behind the unproven theory on which the design and modification of the rigid plastic ski boot is based that supporting and immobilizing the foot of a skier in neutral places it in the strongest position for skiing.

The intent of SHOEspiracy is to shed light on what amounts to a  ‘Shoe-shaped’ Public Health Scandal’.

There is a 20 billion pair a year, silent public health scandal a’foot and it’s shoe shaped!

It’s astonishing to us that the vast majority of shoes produced each year are actually bad for people’s feet—and the wearers are none the wiser.

VIVOBAREFOOT co-founder Galahad Clark

According to the documentary SHOEspiracy is intended to inspire viewers to reconnect with their feet and create a drive within the multi-billion-dollar footwear industry to establish a template for healthy shoes, healthy feet and natural movement. Most people are blissfully unaware of the problems footwear can and does cause and assume that what they put on their feet is benign.

I commend Galahad and Asher Clark and Vivobarefoot for taking the initiative to educate consumers on the problems shoes can and do cause and to establish a template for shoes that respect and accommodate the physiologic requirements of the user.

From Function to Fashion

At one time all humans were barefoot. This changed about 40,000 years ago when humans began to wrap animal skins around their feet to protect them against damage from the elements.  From crude beginnings as nondescript forms of protection, footwear evolved into a fashion entity; one that changed the shape and appearance of the foot, often radically, to render it more aesthetically pleasing. Heels first appeared in horsemen’s shoes as a device to help keep the rider’s feet in the stirrups.

As the evolution progressed shoes became corrective and lifestyle devices in addition to fashion accessories. In the footwear fashion era people have historically worn shoes that deformed their feet, the Chinese Lotus shoe  being an extreme example. But since a degree of deformation does not typically result in a noticeable impact on low-key locomotion the negative impact of restrictive footwear has generally flown under the radar unnoticed.  Adverse effects due to footwear such as joint and muscle pains and impaired balance are usually attributed to other factors.

Young feet are especially malleable. Their shape can be molded by footwear often resulting in permanent deformity as mine were when as a child my feet were put in orthopedically correct, supportive footwear to help them develop properly. The recent photos below show the state of my feet after more than 5 years of wearing exclusively minimal shoes, doing exercises like the short foot and using NABOSO insoles. Although they have become much stronger and healthier, it is doubtful whether the damage done when I was a child can ever be undone.

The left hand photo shows my feet with forefoot minimally weighted. The right hand photo shows my feet weighted. Note the difference in the robustness of the big toe of my left foot compared to the big toe of my right foot. I believe this at least partially explains why I am able to stand and balance with superior stability on my left foot compared to my right foot.

The photos below serve to graphically illustrate why I gave up road biking several years ago and now ride a touring bike with large flat platform pedals and minimal shoes fit with NABOSO insoles. As my feet became stronger and more functional I was no longer willing to abuse them with constrictive footwear.

The Jogging/Ski Boot Connection

About 50 years ago a new type of shoe appeared; one that would revolutionize the footwear market. The Sports Shoe was created in response to the running boom of the 1970’s. When I took up running on the cusp of the running boom, runners of the day ran in flats made for tennis or basketball. These were plain canvas shoes with no heel toe drop or special features.

Jogging, published in 1967 by Nike cofounder, William J. Bowerman, served as a catalyst for the running boom that emerged in the 1970s and with it the development of jogging and other sports footwear including plastic ski boots. At the time that he wrote Jogging Bowerman was working with elite runners looking for ways to improve their performance. His book was preceded by the introduction of the first Lange ski boot in 1962 followed by a racing model in 1965.

People who took up jogging who hadn’t run before started having problems with their Achilles tendon and calf muscles because their everyday shoes had heels. After consulting with doctors Bowerman made a decision to raise the heel of his jogging shoes by 1/2” (12 mm) to accommodate people who wore dress shoes. This feature was for the general public, not the athlete. Bowerman recognized that the sports footwear industry needed to create a consumer product that could be worn without causing discomfort. In an attempt to address problems caused by raising the heel the sports shoe industry responded by adding counters, arch supports and other features; in effect adding band aids in an effort to correct problems caused by raising the heel.

When the Nike Waffle Trainer was marketed as a shoe designed specifically for jogging the idea of sport specific shoes initially made sense to me. But even though I had been running with a heel strike technique in flats I experienced problems right away with ankle and knee shock at heel strike in my Nike Waffle Trainers. In comparing the Nike shoe to my canvas flats it became obvious to me that the flared heel was adversely altering the mechanics of heel strike. Trimming away the outer (lateral) and rear aspects of the flared heel reduced the shock of impact at heel strike.  I suspected that other aspects of the shoe were also adversely affecting my running mechanics. This incident caused me to question whether the design of sport specific shoes was supported by science.

When I started looking for answers I found out that it had been known for decades that footwear can negatively impact the physiologic function of the user. But the issue of the effect of footwear on athletic performance came into sharp focus in 1989 with the publication of the medical textbook, The Shoe in Sport (published German in 1987 as Der Schu im Sport). The Shoe in Sport brought together the collective expertise of 44 international authorities on orthopedics and biomechanics to focus their attention on the SHOE PROBLEM in the context of problems shoes can cause for athletes that compromise performance and contribute to injury. The Shoe in Sport focusses on the medical and orthopedic criteria of sports shoes in offering guidelines for the design of shoes for specific athletic activities including skiing and ice skating. The efforts of the Shoe in Sport was supported by the Orthopedic/Traumatologic Society.

In the Introduction to the Shoe in Sport, Dr. med. B. Segesser and Prof. Dr. med. W, Pforringer note that the buyers of athletic shoes are always looking for the ideal shoe. In their search for the ideal shoe they encounter a bewildering variety of options and are largely dependent for information on the more or less aggressive sales pitches directed at athletes from every angle.

Segesser and Pforrineger go on to state that the findings in the textbook should enable the interested reader to distinguish between hucksterism and humbug on the one side and scientifically sound improvements in the athletic shoe on the other. The Shoe in Sport makes it abundantly clear that it is not a question of if the structures of footwear will affect the physiologic function of the user but a question of how they will affect the physiologic function of the user and especially whether the footwear will compromise athletic performance and/or contribute to injury. The Shoe in Sport studies the biomechanical, medical and technical aspects of the shoe problem as it exists in various fields of athletic endeavour.

A number of leading footwear company executives have often said to me over the years that they know science and agree with the philosophy behind the benefits of barefoot shoes, but that consumers aren’t ready. – Galahad Clark

Clark’s statement seems to suggest that little has changed since the publication of The Shoe in Sport in 1987 and the subsequent publication of Nigg’s Biomechanics of Sports Shoes in 2010. One reason may be the difficultly in conducting objective studies that lead to definitive conclusions pertaining to effects on the user of specific features of footwear.

After I learned of the research done by Benno Nigg at the Human Performance Laboratory at the University of Calgary that found that anything appended to the human foot compromises physiologic function I set out to develop a minimal constraint device for rigid soled footwear such as hockey skates, ski boots, cycling shoes and the like that would create a functional environment equivalent to barefoot. Activities that employ rigid soled footwear are much easier to conduct in vitro and in vivo studies than other activities. The objective of the device I wanted to develop was to enable the study of the effects of interfering with the action of discrete joints or joint systems of test subjects by controlling variables against a standard reference. In 1991, I succeeded in developing such a device in a corroborative effort with a biomedical engineer. The device can be constructed at minimal cost and readily fit with instrumentation to capture performance data.

When I wrote my US patent 5,265,350 at the beginning of 1992 I described the research device in impeccable detail with the intent and hope that others would construct the device and conduct studies with it. Under the terms of a patent, research may be conducted using a technology for which patents are pending or granted without infringing. This meant that research vehicle could have been constructed and studies commenced as soon as my patent application was published.

The graphic below shows the Birdcage research device on the left and Figure 1 from US patent 5,265,350 published on  February 22, 1993 on the right.

Form follows Function

The designation of the research device as Figure 1 in the patent is symbolic of the priority I give to function and science over other considerations.

The design and development strategies used by David MacPhail are very holistic in nature, placing the human system as the central and most critical component in the biomechanical system. His intent is to maximize human performance and efficiency, while foremost preserving the well-being and safety of the users and minimizing biomechanical compromises.  Alex Sochaniwskyj, P. Eng.

In US 5,265,350 and subsequent patents granted to me I disclosed a series of accessories for use with the research device. I designed these  to enable the effect on the user of factors such as the position of key mechanical points of the foot in relation to the mechanical points of a snow ski appended to it to be studied. To the best of my knowledge the minimal constraint research device and accessories has yet to be constructed and employed by other parties.

…………. to be continued.


  1.  https://www.shoespiracy.tv

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 OF PLATFORM ANGLE: PART 4

In Part 3 of the mechanics of platform angle I suggested that some unidentified force or forces are at work that enable elite skiers to alter the angle of attack of the applied force R so that it is more aggressive in terms of cutting (carving) a step into the surface of the snow. I asked the reader what the components of the applied and reaction forces would look like.

One reader correctly identified two separate forces acting on the transverse plane of the platform of the outside ski; one oriented vertically at 90 degrees to the plane and a second force oriented parallel or 180 degrees to the transverse plane with the vector aligned into the snow.

The right hand graphic below shows the 90 and 180 degree components of the angular force acting on the platform in the left hand graphic.

The right hand graphic below is the same as the graphic above but with the angular force superimposed over the 90 and 180 degree components.

I am taking the discussion of platform mechanics in small steps in order to provide the reader with a chance to assimilate the issues and ask questions if my discussion is not clear.

THE SHOCKING TRUTH ABOUT POWER STRAPS AS A REFERENCE

Most of the views of the series on the Mechanics of Platform Angle are accompanied by views of The Shocking Truth About Power Straps which contains quotes from the medical textbook The Shoe in Sport (published in German in 1977 as Der Schu im Sport). This medical textbook has been invaluable to my efforts.

Here are some pertinent quotes by Dr. E. Stussi,  Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland

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). In this way, the skier can adapt to the requirements of the skiing surface and snow conditions.

These conditions can be met if the height, stiffness, angle and functions (rotational axes, ankle joint (AJ)/shaft) of the shaft are adapted, as well as possible to the individual skier.

The modern ski boot must be designed from a functional point of view, i.e., the design must take into consideration the realities of functional anatomy (axes etc.).

It (the design) should not make compromises at the expense of other joints (length of shaft, flexibility and positioning).It (the ski boot) must represent the ideal connecting link between man and ski (steering and feedback).

 Biomechanical Considerations of the Ski Boot (Alpine)

The question for this post is what is the source of the 180 degree force? Please consider Dr. E. Stussi’s comments above when contemplating an answer to this question.