What I call the Resistive Shank Angle is the angle of the shank or shin bone of the leg in relation to the plane of the surface the sole of the foot is supported on. When the foot is in a ski boot, ramp angle puts the plane of the boot board or zeppa in a significantly different plane from the sole of the boot. In my last post, I described how to construct a simple Stance Ramp that allows the effect of ramp angle on stance to be sensed free of interference from the ski boot. Unless a skier has a kinaesthetic sense of a strong stance, the odds of finding it by chance are about as good as winning the lottery. Once a kinaesthetic sense of a strong stance has been acquired it can be used to set up ski boots.
Ankle Range of Motion vs Resistive Shank Angle
In order to understand Resistive Shank Angle, it is important to know the difference between ankle flexion/extension in weighted and unweighted states.
Anatomical references typically list the normal range of motion of the ankle in plantarflexion (toes pushed away) at 0 to 50 degrees and the normal range of ankle dorsiflexion (toes pulled back) at 0 to 20 degrees. These ranges of motion pertain to changes in the angle of the ankle resulting from extensor/flexor concentric muscle contraction. The ranges of motion in weight-bearing are much greater.
The graphic below shows the approximate angles of the shank or shin bone in relation to the plane of the surface under the sole of the foot. The vertical (90 degree) reference is shown in blue. The angle of the shank relative to vertical is shown in red. The center image shows the Resistive Shank Angle. The right image shows the approximate maximum angle of plantarflexion typically associated with an athletic or power stance. It is sometimes referenced to a good ski stance. However, the biomechanics of the two shank positions are very different.
The Resistive Shank Angle is the angle of the shank associated with the late phase of stance in walking and running. To activate the Resistive Shank Angle, stand in the Anatomic stance shown in the left image and relax the ankles. Shiffrin refers to this as settling on the foot. When the ankle is allowed to relax, the ankle and knee joints will flex. Keep the torso erect.
Now do the same thing on the 2.5 degree Stance Ramp. Pay close attention to the sensations in the muscles in the legs, buttocks and back. The ankle relaxation exercise ramps up what is called eccentric gastrocnemius-soleus complex muscle contraction.
Next, starting from the Resistive Shank Angle position, activate the abdominal core muscles. Bend forward at the waist and round the back and shoulders by pulling the shoulders outward and forward into a bow shape. As you do this, the knees and ankles will straighten slightly. Next, move forward in the hips until you feel the pressure move to the balls of the feet.
Try the same exercise on the Stance Ramp. Repeat several times, alternating between the flat floor and the Stance Ramp. Pay careful attention to the sensations of the muscles in the legs, buttocks and back. You should feel as if your shoulders are pulling on the balls of your feet.
In my next post, I will explain how the Stance Ramp ramps up eccentric muscle power of the posterior chain that connects through the Achilles tendon to the balls of the feet and how to assess the effect of increasing and decreasing ramp angle on the Stance Ramp.