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.


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.


  1. Hi David ,
    Thank you for your reply. In your first sentence you said “As…,I do like guessing.” Unless you were being sarcastic, I believe you may have meant to say “…I do not like guessing”

    Indeed, this is what attracted me and I am sure many other followers of your work to this blog. You don’t guess or spew out eminent opinions or theories as if they are facts like so many other sources of information out there. And, you point the way to information that is lacking and research that needs to be done. The most desperate need, in my opinion, is to connect the fit and function aspect to the safety aspect with comparative studies. Of course, this is the most fearful subject for the industry. Hopefully, an independent research group with recognized authority will take this on and not have the results squashed by corporate thugs.
    Your last paragraph and the letter from Salomon to their sponsored racers comes to mind.

    The Birdcage data was and is ahead of the technology curve. Today, it’s not hard to envision a sensor packed liner which could be used to direct shaping and fitting boots in the near future. This has enormous implications for teaching skiing and coaching racers as well.

    I await the pictures of your boots eagerly and with an open mind. I also follow Lou Dawson’s “Wildsnow” blog where he and others are constantly modifying backcountry oriented equipment. As you say, There is no box in my world. For me the limitation is skill and equipment, sometimes materials, to make the mods.

    Best regards,

    1. Hi Herb,

      Good catch. I meant “I don’t like guessing”.

      I believe the issues affecting skier and especially racer performance, requires the collective efforts of an integrated, coordinated, multi-disciplinary team of experts in their respective fields similar to a formula one team.

      After I read The Shoe in Sport one of my greatest fears was that the ski industry would stay their course in spite serious red flags raised by international authorities on biomechanics and especially safety and injuries and that “an independent research group with recognized authority will take this on and not have the results squashed by corporate thugs”. If this happens they could decimate the ski industry and take down skiing with it. The last thing skiing needs is a Ralph Nader, Chevrolet Corvair ‘unsafe on any skier’s foot’ moment after which lawyers can and usually do outgun those who make and sell defective products. But the technology has existed to do this to skiing for a while now. I hope my greatest fears do not become reality.

      1. Hi David,

        My initial response to your comment regarding fear of killing skiing by exposing it to a Ralph Nader type of attack was that I was being perhaps, a bit naive not to have considered that possibility. I agree, I really did not think of that when I made that comment. However, upon further consideration I realised that while Nader and others have awakened the auto industry to the idea that they need to be responsible for more than just the appearance of their products, the overall effect was to raise safety awareness in the industry and the public. Not a bad thing in the long run, in my opinion. Also, the Corvair was one model in a plethora of car models and manufacturers. Its demise did not kill the General Motors or the entire auto industry. And, how did the Volkswagen “Bug” and other models similar to the Corvair, not get caught up in that process?

        I believe that we cannot let the fear of such an attack paralyse our behavior and stifle our curiosity, imagination, desires, and abilities to invent, design, develop, and produce better products based on the knowledge derived through well designed research. It’s possible that a poorly designed, improperly tested model may be killed but, I believe that the ski industry as a whole would be resistant to devastation on a large scale. The defense of the ski hardware manufacturers would likely be ignorance in such an attack, despite studies such as “The Shoe In Sport”, etc., which have pointed to a better direction for some time, yet remain in obscurity. I have not been able to find a copy of that book so far. Please let me know if you have a source.

        Yes, absolutely, your “synergy sports performance consultants” highlighted in your blog “Ski Boot Assessment Protokol” was a brilliant concept for a “coordinated multi-disciplinary team”, which needs to be integrated into ski teaching and coaching systems everywhere, and should be mandatory for instructors/coaches at least at higher levels.

      2. Hi Herb,

        Thank you for continued insightful comments. I will be addressing some of the issues you raise in future posts. For now I will speak to the liability issue.

        Up until I retired I managed over $100 million in liability each year. My job involved the formuation of procedures and policies to defend against liability. I am qualified as an expert witness in court.

        The defense of a lawsuit I was involved with that was filed by a lawyer representing a high earning consultant serves to illustrate what can happen.

        The plaintiff fell over a retaining wall, dropped 6 feet and landed on their head causing serious brain damage that rendered the plaintiff, who was in their 40s, a parapalegic and unable to work. At the time of the incident the plaintiff was severely intoxicated. The retaining wall was about ten feet from a sidewalk leading from the bar. The plaintiff’s lawyers filed suit against a number of parties incuding nearby bars.

        When the plaintiff was wheeled into court in a wheelchair slobbering and drooling it was obvious to everyone in the court room that this person’s life had been ruined.

        The defense for the owners of the bars argued that the plaintiff was injured because they chose to overconsume alcohol. No one forced them to. Most people would agree. But that’s not how liability in the face of the law works.

        The plaintiff’s lawyer said to the Court: “Of course my plaintiff was drunk your honor. That’s how the bars make their money. The more alcohol their clients drink the more money they make.

        Then the lawyer asked the judge to add an estimate to the evidence. The estimate was for the cost of erecting a guard along the top of the retaining wall to prevent anyone from falling over. The cost? Six hundred dollars. The plaintiff’s lawyer then said to the Court, “If the bars had spent $600 my client wouldn’t be here today. The reason the bars didn’t spend $600 is because they don’t care about protecting their clients.”

        Do I have to tell you how the jury ruled?

        More later.

      3. I’m not, nor have I ever been interested in holding those accountable who caused the problems that all but destroyed my enjoyment of skiing 45 years ago. My sole interest is in finding solutions and especially understanding how to reach my maximum performance in skiing that gives the most enjoyment and safety for the least energy so I can enjoy a good ride.

        But a major concern I have is that the ski industry did not heed warnings by those such as Dr. E. Stussi, Member of GOTS – Chief of Biomechanical Laboratory ETH, Zurich, Switzerland when he said in The Shoe in Sport; “…..improvements in the load acting on the ankle make it biomechanically very likely that the problems arising in the rather delicate knee joint will increase.”
        My read is that Stussi was saying to the ski industry, You loaded the ankle and sent the forces of skiing up to the knee. Bad idea. It caused knee injuries. If you keep improving the loading of the ankle joint (by improving fit) it is biomechanically very likely that knee injuries will increase. Guess what. The industry kept on improving the fit and knee injuries increased just as Stussi predicated. This warning came from one of the most qualified experts in the world. Ignoring an expert of Dr. E. Stussi’s caliber is really bad idea especially if one ever ends up sitting across from him in Court. I wasn’t going to go forward with a new boot without taking steps to address the concerns raised in The Shoe in Sport.

  2. Hi David,

    As a backcountry skier, I use boots with a walk mode. Current technology and materials (Grilamid, carbon fiber,etc) has allowed production of lighter boots with good support and walk modes which can be relatively easily modified to allow free flex in the cuff when in walk mode. In your current thinking, would this be a better way to achieve flex than modifying the cuff padding of the liner to allow adequate free flex? This would allow one to keep the cuff fit snug while still allowing flex as you did with the Birdcage. The flex resistance or distortion index is not near race boot stiffness especially in the boot sole. (A carbon fiber sole joined to a Grilamid nylon upper would help if it was possible). The Tecnia Zero G Tour Pro and Atomic Hawx XTD 130 models are the lightest/stiffest of this type.

    Back in the 70’s Rosemount made a boot with acuff that flexed freely controlled with an elastic tensioner and an adjustable forward lean angle. It was made of cast epoxy composite with upper and lower connected by a stainless steel hinge. The design was similar to the Birdcage without the adjustable instep support feature which probably could have been added to the design fairly easily. It skied well but, had fit issues which today’s technology would likely solve nicely.

    The Birdcage data is an incredible resource of applicable information. You can see what the foot and ankle are doing so clearly on the readouts. This is much more information than the Novel Pedar, Techscan and Carv technologies provide, from what I have seen in your posts. It is delinquent and irresponsible of the industry to ignore this wellspring of knowledge which you have so generously made freely available, criminal, even, considering the injury rate on the World Cup circuit!

    Best regards,
    Herb Jones

    1. Hi Herb,

      As you are well aware, I do like guessing. There are far too many factors that can affect skier performance that to shoot from hip. The objective and intended eventual application of the Birdcage was to use it to acquire what amounts to an optimal individual setup for the intended application of the human system as confirmed by data based on personalized functional baseline data similar to what is emerging for assessing whether an impact has breached the concussion threshold.

      I fully agree that the Birdcage goes far beyond technologies such as Novel Pedar (which I have used), Tekscan, CARV and others. In a technologic age of microprocessors and micro data wireless sensors it is inexcusable technology is not being applied in skiing as is they are in recent studies which I will discuss soon.

      You are correct in that the Rosemount boot would have worked well with the instep loading device which I just discussed in my last post.

      I wish I could speak to your questions about you backcountry boots. But due to the lack of standards in the ski boot industry there are two many unknowns. I will post photos soon of my own boots. Be sitting down when you see them. In my world function comes first and I do what is necessary to get it.

      I think the future of skiing may be best served by third party providers of components that can be fit to existing products. I suspect this is already happening out the background in some World Cup race teams. I did this for many years and everyone involved keep quiet about it especially me.

      Best regards,

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