Once the initial Birdcage experiments had validated my hypothetical model of the mechanics, biomechanics and physics of skiing, we moved on to experiments that looked at the effects of things like second stage cuff end point resistance and total stage one/stage two cuff cuff range of motion effects. Unlike conventional ski boots that have a fixed cuff, the Birdcage had a cuff that actually rotated about an axis that was close to the proximate centre of the true axis of the ankle joint. The hinge points of a conventional ski boot cuff amount to nothing more than a means to join the cuff to the lower shell. Any forward flexion of the shin is achieved by distortion of the boot shell as a whole in what amounts to a L shaped tube bending through deformation. What appears to be a hinge is an illusion.
The Birdcage experiments compared the Birdcage to a conventional Salomon boot. The tester for these trials was an intermediate skier who had been skiing in the same Salomon boot for several years. We started the experiments with several runs in the Birdcage. Keep in mind that the tester had been trained by the Salomon boot to ski a specific way. It takes time to replace learned motor patterns with new ones. Here is a data sheet from the second test trial done in the Birdcage with my notes added. The hand written notes are those of the scientist who oversaw the experiments.
Compare this testers’ skiing to that of a World Cup champion and Olympic medalist. Note the marked difference in the pressure under the ball of the foot, ankle flexion and absence of cuff pressures.
Here is the first trial done by the tester in his own Salomon boots.
The problem with the bent tube configuration of the conventional ski boot shell is that the glide path of the shin bone is obstructed by the throat of the shell. The throat is the point where the instep of the lower shell transitions into the cuff.
Here is a collation that compares the data fields for the 1st metatarsal (ball of the foot), front cuff and the instep-shin force plate that is pulled down and back towards the heel by a cable-buckle system. When pressure builds in the shin glide path, the contraction of the soleus muscle is interrupted resulting in a clipping of pressure under the ball of the foot.
The 2-piece tongue system with a flexible link disclosed in my US Patent No. 4,534,122 provided a partial solution to this issue (see my post OUTING THE ARCH COLLAPSE THEORY). But the configuration of the Salomon rear entry boot actually made the situation worse with an instep-shin plate that pulled the obstruction of the glide path of the shin diagonally down and back towards the heel.
Once a baseline optimal performance configuration for a skier has been captured by a Birdcage or Birdcage-like device, it is possible to conduct experiments that can accurately assess the effect of obstructing specific joint actions or comparing the effect on the user of another ski boot or a device like an insole or custom formed insole. If for example someone claims, as many are, that the foot functions best in skiing when its joints are completely immobilized we can compare this state to that of the baseline optimal performance configuration the same skier. The technology used in the Birdcage is over 20 years old. Today’s technology has the capability quantify the effect on the user of virtually any piece of ski equipment that resides between the sole of the user’s foot and the snow surface.