The Birdcage was more than just a research vehicle. It was a device that allowed us to fine tune the interface that connects the user to a ski and capture what amounts to a Performance DNA or Performance Signature that represents the optimal performance prescription for a specific skier. As in any properly done science experiment, I had formulated a hypothetical model in advance that predicated specific outcomes. This is very different from the process of subjective observation where gross movement patterns are observed and noted without any attempt to understand the underlying process responsible for the movement pattern. In the Birdcage experiments, specific constraints were placed on a skier while all other factors were kept constant. This ensured that any change in a specific constraint was actually responsible for any change in performance where a change was seen in the data. Hard data from the instrumentation package was correlated with tester comments (the testers were mic’d and recorded) and video analysis.
One of the outcomes that I did not predict, and did not believe possible when several testers suggested it, was doing experiments with zero resistance in the cuff and no forward stop. In other words a free hinging cuff. Early in the experiments we had determined, based on theoretical and practical considerations, that a rear cuff stop with the cuff set at a functional angle was essential as a rearward reference. And contrary to conventional wisdom, my model had predicted that a specific amount of zero resistance cuff movement was essential to maximize the contraction of the soleus muscle. The soleus is part of a chain of muscles in the back of the leg that is responsible for postural responses in skiing and especially the loading of the ball of the foot when the foot is pronated. The graphic below shows the soleus muscle.
In order to understand the pressure readings on the four aspects of the cuff of the Birdcage (front, back, inside, outside) it is necessary to know how the inside ankle bone moves when the foot is pronated. The graphic below shows a neutral foot (left) and a pronated foot (right). There is a mechanical line from the ball or head of the femur that runs to the base of the tibia where it forms the ankle joint with the talus. No matter how curved the leg is the force from the pelvis is transferred to the foot on the mechanical line. In order for the foot to pronate the inside or medial ankle bone must move inward, towards the shell wall of the boot. The top of the boot cuff and the sides of the heel of the boot act as axes or pivots for the inward movement of the tibia. The graphic below shows this movement. The Figures are from US Patent No. 5,265,350 – MacPhail with notes added.
The Birdcage was intentionally configured with sufficient clearance to ensure that no testers’ inside ankle bone could contact the arm of the cuff. The photo below shows the Birdcage in the process of being fit to the foot and leg of a tester.
Here is another sheet of Birdcage experiment data. The data was originally in digital form which made it much easier to expand and overlay. This process was used to arrive at the notes on the hard copies. At this point I only have the hard data sheets. So the data is not as easy to expand and overlay to compare different data fields. In the first data set there was almost no pressure on any aspect of the cuff. The reason for this is that the cross-sectional area of the testers’ shin was smaller than the cuff enclosure. In the data field below, the cross-sectional area of the testers shin is a little larger than the cuff enclosure. Hence we see a constant pressure line. This is not force being applied to the cuff for the purpose of putting the outside ski of a turn on its inside edge. In fact, when the feet of the elite skiers were allowed to pronate, there was little or no pressure applied to the sides of the cuff at any time.
Here is the data sheet for an experiment done with a free hinging cuff. Much to my amazement testers skiing with a free hinging cuff were able to ski through mogul fields like they were skiing on groomed runs while the muscles in their feet and legs absorbed the energy that would normally have disrupted balance and thrown them all over the place. Click on the data sheet to enlarge it.