THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: BALANCE PLATFORM MECHANICS


Turntable rotation generated by the powerful internal rotators of the pelvis (the gluteus medius and minimus) in combination with second rocker mechanics can create a platform under the body of the outside ski and foot that a skier can stand and balance on using the same processes to balance on solid ground. The associated mechanics creates a platform under the body of the outside ski by extending  ground reaction force acting along the portion of the inside edge in contact with the snow, out under the body of the ski.

In order to understand the mechanics, we need to start with a profile through the section of the body of the ski, binding and boot sole under the ball of the foot. The graphic below is a schematic representation of a ski with a 70 mm waist and 100 mm shovel and tail with an arbitrary length of 165 mm. The total stack or stand height from the base of the ski to the surface of the boot that supports the foot is 80 mm. The uppermost portion of the schematic shows the shell sidewalls of a 335 boot in relation to the 70 mm width of the stack. A ski with a 70 mm waist will place the center ball of the foot of skiers with US Men’s 10 to 12 feet close to over the inside edge. The heavy black line at the bottom of the stack shows the projection of the sidecut width beyond the waist.The schematic serves as a base on which to overlay a free body diagram showing the forces acting across the interface of the inside edge with the snow. This is where the rubber meets the road.

There are two possible scenarios in terms of the axis on which the center of pressure W of the skier will act. Unless the foot can sufficiently pronate and especially generate impulse second rocker loading, W will lie on the proximate anatomic center of the foot and transverse center of the body of the ski as shown in the graphic below. In this location, W will create a moment arm due to the offset with the GRF Pivot under the inside edge at the waist. The resulting moment of force will externally rotate the ski and foot under load out of the turn while simultaneously rotating the leg externally.The graphic below shows the second scenario where the center of pressure W lies directly over the GRF Pivot under the inside edge. In this position, W will load the inside edge under the ball of the foot and assist edge grip. But in this configuration, rotating the ski onto its inside edge necessitates overcoming the moment of force created by the moment arm resulting from the offset between the GRF Pivot and GRF acting at the limits of the sidecut. This requires a source of torque that acts to rotate the ski into the turn about the pivot acting at the inside edge at the waist of the ski.An obvious source of torque is to use the leg to apply force to the inner aspect of the shaft of the foot; aka knee angulation. But this will not create a platform under the body of the outside ski. Applying a load to the vertical wall of the shell opposite the ball of the foot will apply torque load to center at the GRF pivot as shown in the graphic below. The moment arm is formed by the point at which the Turntable Torque is applied to the boot sidewall (green arrow) to the center of rotation at the GRF Pivot.

 

The torque applied to the vertical sidewall of the boot shell is the Effort. The sidecut of the ski is the resistance. What effect will this have on the body of the ski under the foot? There is a lot more to this subject that I will begin to expand on in my next post.

2 comments

  1. I believe i commented last winter on dealing with a short leg issue by raising the short leg to level out my pelvis and shoulders. With all your posts of Dr. Emily and foot heath I’ve been playing with different things such as the toe spread, especially on my left foot. So walking hasn’t been perfect so i asked my wife to watch me walk in different configurations and give me feed back.
    Started with a ‘baseline’ of walking barefoot on the floor. First feed back; my left knee kicks to the outside every step I take:( Then to make it easier I, walked in my sandals so i could integrate various thickness of ‘lifter’ for my shorter left leg. That improved things and determined on the thickness that gave the best results. Next; toe spreader (one I’ve been using daily with a smaller lifter) without lifter and that looked better also. Toe spreader with lifter, worse. Went back to barefoot (her request) with only toe spreader. Then increased the toe spread width and she said that was the best. I let her go and tried the couple of ‘best’ configurations and had to agree; the one my wife said looked the best to her, felt the best to me; more toe spread and no lift.
    Integrated that into my ski boots and on the easy little ski area here it felt…. like the best ever 🙂 🙂 Time will tell.
    My conclusions thus far (and no farther). One can ‘support’ a collapse arch by manipulating the big toe laterally. Fortunately, while my left big toe turns to the middle of my foot a bit it isn’t completely deformed with a bunion. Which it will be if I don’t stop it RIGHT NOW. While the lifter under foot served me well it wasn’t the correct place to fix it, the big toe is because the re positioning levels out my body side to side and simply feels better in all my footwear and ski boots.
    I tried engaging the big toe by pressing it down. It created tension in my body but actually more than I needed for the situation. Pretty well aligned for at least that slope because it would instinctively engage big toe press as necessary. I’ll see what happens when i get on ‘real’ terrain, may be helpful when the going gets real tough:)
    Was pleasantly surprised assuming I’d have to blow out plastic for my left big toe but never seemed to touch (went up one boot size last year; 25 years too late!)
    To do: Felt like my left navicular joint was hitting the side of the boot too much so need too move some plastic there

    1. The one thing I do know for certain is there is a whole lot in skiing that is not known but needs to be known and should have been known long ago. One area that needs study is boot board ramp angle (zeppa) and binding ramp angle (delta) and how the 2 interface with each other.

      My first day on the ski hill this year, I tested a NABOSO insole in my left boot. It was nothing like I expected and nothing like I have been experiencing in my zero drop Xero and Lems shoes. Instead of a strong feeling under the ball of my big toe and toe, I felt like I had way too much boot board ramp angle (my bindings are zero). When I pulled the boot board from my left Head ski boot, I discovered that I had marked the boot board ramp angle as 3.3 degrees. I can’t recall where it was originally. But I had one helluva time in 2012 getting it down to 3.3. I had to grind the heel way down and add a 2 mm shim to the forefoot of the zeppa.

      I am going to add a 4 mm gasket cork shim to the forefoot. No wonder I felt like I had far to great a ramp angle. I did! I suspect that wearing zerio drop shoes has lengthened my Achilles. Plus, the NABOSO made me more aware of the lack of sensitivity in the small nerve balance zone of the foot. A dynamic ramp angle assessment indicated my optimal ramp angle at around 2.0 degrees.

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