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.


In LESS REALLY IS MORE I talked about how I gone in a direction opposite from that of the industry after my perfect fit experience with Mur. I was now removing material from ski boots instead of adding material and expanding shells where necessary to make room for the structures of the foot. While this seemed to generally have a positive effect on skier balance and the ability to control skis, especially edging, removing material from the sides of the boot liner  exacerbated the fact that in the majority of cases I was encountering the shell wasn’t loading the instep of the foot. The reason for this turned out to be  that there was a void between the top of the tongue of the liner and the inner surface of the shell over the forefoot. This was allowing the foot to move upward into the void space or unload from contact with the sole plate (aka boot board) in response to changes or perturbations in ground reaction force. I coined the effect Separation Anxiety because of the alarm bells it was setting off in the skier’s balance system.

After I became aware of this effect, I started doing experiments to try and understand how it was affecting a skier’s balance and ability to control their skis. While riding ski lifts with foot rests (the old slow chairlifts) I would let one of my feet drop off the foot rest and try and feel what was happening with my foot and leg inside the boot when the foot unloaded from the boot board. At that time, I wasn’t thinking in terms of trying find a solution for knee injuries. I saw this as an issue that would be addressed by refinements in bindings which at that time were rapidly evolving. Through my experiments I had come to the realization that the unloading and reloading of the sole of the foot with the boot board, such as occurs when a skier is moving over irregular terrain, was setting off a chain-reaction of physiological events that were creating balance issues. Although I didn’t know exactly how, this unload/load cycle  seemed to be placing stress on the knee. But my focus was trying to find a way to reduce the effect on skier balance. In effect, I was trying to achieve a net improvement in skier balance by reducing negative balance artifacts.

The standard solution in those days was to attempt wedge the heel with heel or L-pads inserted in the liner. The objective was to keep the foot from lifting. I tried this approach. But I  found it didn’t work as advertised. The pads invariably caused problems with the Achilles tendon or they prevented the heel from seating in the back of the shell, or both. The latter had the effect of making the liner shorter and the boot hell to put on. I was looking for a better solution. But it wasn’t until 1980, while working on Podborski’s boots, that I came up with a device that eventually led to my being granted US Patent Number 4,534,122.