An essential mechanism to the ability to create a platform under the outside ski to stand and balance on using the same processes used to stand and balance on stable ground, is the Heel to Forefoot Rocker. A slide presentation called Clinical Biomechanics of Gait (1.) by Stephen Robinovitch, Ph.D. (Simon Fraser University – Kin 201) is a good reference for the various aspects of gait.
Slide 19 of the Gait presentation describes the ankle Inversion-Eversion-Inversion sequence of the ankle. The sequence begins with heel strike (HS), followed by forefoot loading (FF), followed by heel off (HO) followed by toe off (TO).
The normal foot is slightly inverted in the swing phase (unloaded) and at heel strike. It is everted through most of the stance phase. The ankle begins to invert in late stance. The kinetic flow of pressure is from the heel to the ball of the foot and big toe. This is what should happen in the transition phase of a turn sequence when a skier begins to transfer more weight to the inside foot and ski from the outside foot and ski. Up until the start of the transition, the skier’s center of mass is behind the inside foot with the majority of pressure under the heel on the transverse center of the foot and ski where is exerts an inversion torque that is tending to rotate the ski into contact with the surface of the snow. The skier maintains the edge angle by applying a countering eversion torque with a combination of external rotation-abduction of the inside leg.
When the skier begins to transfer more weight from the outside ski to the inside ski, the leg releases the countering eversion torque and the ski begins to invert in relation to the surface of the snow.
The presentation on the Clinical Biomechanics of Gait did not include important aspects of the stance phase that occurs in late stance. Nor, did it mention Achilles forefoot load transfer.
The Three Rockers
Slide 23 shows the Three Rockers associated with the gait cycle.
First Rocker – occurs at heel strike. It causes the ankle to plantarflex and rock the forefoot downward about the heel into contact with the ground. The rocker movement is controlled by eccentric dorsiflexor torque.
Second Rocker – shifts the center of pressure from the heel to the forefoot. Eccentric plantarflexor torque controls dorsiflexion of the ankle.
Third Rocker – occurs at heel separation from the ground that occurs in terminal phase of stance.
Slide 13 shows how the knee shifts gears and transitions from flexion in early stance to extension in late stance. In late stance, the Achilles goes into isometric traction. At this point, further dorsiflexion of the ankle passively tensions the plantar ligaments to intiate forefoot load transfer. Load transfer is accentuated when the knee shifts gears and goes into extension moving COM closer to the ball of the foot increasing the length of the lever arm.
Two Phase Second Rocker
Classic descriptions of stance and the associated rockers do not include a lateral-medial forefoot rocker component that occurs across the balls of the feet from the little toe side to the big toe side in conjunction with the heel to forefoot rocker creating what amounts to a Two Phase Second Rocker.
In his comment to my post, OUTSIDE SKI BALANCE BASICS: STEP-BY-STEP (2.), Robert Colborne said:
….… regardless of where the centre of mass is located relative to the centre of pressure in the above-described mechanism, when you go into a stable monopedal stance, as you would when you are in a turn, the ankle is dorsiflexed forward and as this occurs the tibia rotates internally several degrees.
COMMENT: The tibia rotates internally (i.e. into the turn) as a consequence of ankle dorsiflexion. It does not require conscious action by the skier.
This means that the main muscle forces acting across the ankle (the plantarflexors) are no longer acting along the long axis of the foot, but rather partly across it, medially toward the big toe.
So, the beneficial effect of that muscle force is to force the base of the big toe into the ground, and that becomes the centre of the turn (centre of pressure).
In the absence of this internal rotation movement, the center of pressure remains somewhere in the middle of the forefoot, which is some distance from the medial edge of the ski, where it is needed.
The photo below shows a skier in bipedal stance with weight distributed equally between the two feet standing on a plush carpet with foam underlay. Black hash marks show the positions in space of key aspects of the right foot and leg.
The photo below shows the same skier in monopedal stance with all the weight on the right foot. Forefoot loading from the Two Phase Second Rocker has pushed the toes down into the carpet by compressing the underlay.
The video below shows the dynamic action of the Two Phase Second Rocker.
The Two Phase Second Rocker results in a heel to ball of foot diagonal rocker action acting towards the centerline of the body; i.e. diagonally across the long axis of the ski with the load acting inside the shovel.
A primary objective of the Birdcage studies was to validate my hypothetical model of the Two Stage Diagonal (heel – forefoot) Second Rocker in creating a balance platform under the outside ski for a skier to stand and balance on.
The graphic below shows the alignment of the Two Stage Diagonal (heel – forefoot) Second Rocker.
In my next post, I will discuss the Two Stage Diagonal (heel – forefoot) Second Rocker Turntable Effect.