In this post, I am going to discuss some of the Birdcage data from the experiments done in 1991 on Whistler Mountain’s summer glacier. After we did the experiments, we decided not publish the data. The intent was to share the information with the ski industry. But there was no interest. So this is the first time this data has ever been shown in public forum. In order to correlate the data it needs to be referenced to the legend below.
Here is one data sheet. The pressure under the ball of the foot is in the second column, third row down. Every peak is moving the Centre of Pressure to the ball of the; ergo the foot is pronating. You can see the corresponding ‘coupling’ of the horizontal torque into the turn with the lateral heel or outer corner of the heel. The medial or inside corner of the heel is loading because the foot is pronating. The reference to ‘LIGHT SPRING BEGINNER’ means that the end point forward travel resistance of the cuff was provided by a light spring. Note that there is zero pressure on the front of the cuff and very little pressure on the rear of the cuff.
Most telling is the almost complete absence of force on any aspect of the cuff of the Birdcage. According to conventional wisdom. the best skiers use the inside or medial aspect of the boot cuff to hold the outside ski on edge. The data in the pink highlighted zone clearly shows that the best skiers do not use the cuff in this manner. Instead, they use the cuff as an alignment device to direct the application of force to the ski through the ball of the foot in combination with force under the heel. Also note the consistent forward and back flexion (dorsi-plantarflexion) of the ankle joint (2nd column, last row). In future posts I will present more data from the Birdcage experiments.