In terms of stance at ski flat, and especially shank angle, the position of the aspect of the tibia that forms the joint with the knee is more important than the actual angle of the shank as reflected in the angle of dorsiflexion of the ankle joint. The reference shank angle is really the position of the proximate center of the aspect of the tibia that forms the knee joint in relation to the proximate centre of the head of the first metatarsal in terms alignment with the vector of W emanating from COM. The best way to determine reference shank angle in a ski boot is by rehearsing the late phase of stance out of the ski boot and then trying to assume the reference shank angle while in the boot.
The reference shank angle is the angle of the shank in the late phase of stance associated with maximum transfer of load to the forefoot. In the reference shank angle, a vertical line (force vector) emanating from COM passing through the proximate centre of the head of the first metatarsal will pass through the proximate center of the tibia where it forms the knee joint with the femur. When the kneecap and the thickness of skiwear are taken into consideration, the reference shank angle line will appear to be in the toe area of the boot shell. When the reference shank angle is correct, strong pressure will be felt under the heads of metatarsals 1-2-3 and to some degree, the toes, with the majority of pressure felt under the head of the first metatarsal. The shin should be against the tongue of the boot. But the load or pressure felt under the under the head of the first metatarsal should not be diminished by force applied to the tongue which serves as a reference position for the shank.
The best way to establish and confirm the correct reference shank angle is through kinaesthetic training done in bare feet. Pay close attention to the feelings and tensions in the legs and especially the buttocks in the various exercises.
OUT OF SKI BOOT EXERCISES – done on a hard, flat, level surface
TWO FOOT EXERCISES
- Stand erect with both feet equally weighted and with your hands by your sides. The weight should be felt more on your heels than on the balls of your feet.
- The toes of both feet should be resting on the floor. However, it is important that they are not being consciously pressed down on the floor.
- Using ankle flexion only, slowly move forward in the hips until you feel the weight (pressure) move to the balls of each foot. It should not take much forward movement of the hips to transfer the weight from the heels to the balls of the feet.
- Rehearse this exercise until weight transfer to the forefoot is predictable.
Do not actively press down on the balls of the feet or especially the toes. Allow the weight of the body to create the pressure.
- Assume the same erect stance.
- The toes of both feet should be resting on the floor. However, they should not be consciously pressed down on the floor.
- Allow the ankles to ‘relax’ and the knees to bend (flex).
- The weight should move from the heels to the balls of the feet. As this happens, the torso (COM) will drop towards the floor.
- The ankle joints will reach a point where they stop flexing on their own. When this happens, it should not be possible to to move forward in the hips very much without feeling as if you will fall forward. The angle of the shanks in this configuration is the reference shank angle.
- While maintaining the reference shank angle, bend forward at the waist. As you bend forward, the hips will move back and the angle of the knee will increase. The angle of the ankle joint should change very little, if at all.
- Round the shoulders and back until the shoulders are felt to be pulling on the buttocks. As the tension on the buttocks increases, you should feel your buttocks move rearward and the angle of the ankle (shank) decrease slightly.
- While maintaining a rounded back and shoulders, move as far forward in the hips as you can without feeling as if you will fall. You should feel very strong pressure under the balls of your feet.
- Keeping strong pressure on the balls of the feet, bend the knees to get as low as possible with the upper body. You should be able to press your torso onto your thighs. This is the lowest possible position at ski flat where you can transfer the load to the balls of the feet at ski flat.
- The reference angle of the shank should change very little when bending at the knees and hips.
ONE FOOT EXERCISES – alternate feet
Exercise 3 – Hard, flat, level surface
- Repeat Exercise 2 but on one foot. Use the hands and arms to steady the body by placing them on chairs or rails on either side of the body. Find the side-to-side position of COM that gives the highest pressure on the head of the first metatarsal.
- Repeat on the other foot.
Exercise 4 – Hard, flat, surface at same net ramp angle as the boot-binding-plate system
- Tilt a large piece of plywood stiff enough to not bend to the same angle as the net ramp angle. Repeat exercise 1 above while standing on the ramp at the net ramp angle.
Exercise 5 – Insoles/Footbeds
- Repeat exercises 1 and 2 above on a hard flat surface but with the insole from the boot liner placed under the foot. If a custom footbed is used, place the insole under the foot.
- Alternate back and forth between flat surface only (no insole/footbed) and with the insole/footbed under each foot. Make sure that there is no difference in the load under the head of the first metatarsal and especially that it is not necessary to rock the arch of the foot to load the head of the first metatarsal or that the load under the heel needs to be reduced in order to transfer the load to the ball of the foot. Keep in mind that typical GS turns involve loads of 3 to 4 Gs or more. These loads will significantly increase compression and lowering of the arch.
The next post will describe a series of exercises done in the ski boot designed ascertain whether the reference shank angle is correct and what factors can cause it to change.