SKIER LEG SHANK ANGLE VS. BOOT SHAFT ANGLE


The lack of proper technique seem so often is not due to a lack of ability, but to an unsatisfactory functional configuration of the shaft in so many ski boots.

–  Sports Medical Criteria of the Alpine Ski Boot – W Hauser P. Schaff, Technical Surveillance Association, Munich, West Germany – The Shoe in Sport (1989) – published in German in 1987 as Der Schuh Im Sport – ISNB 0-8151-7814-X

In a conventional ski boot, the rear aspect and sides of the shaft are fixed in relation to the shell lower with the result that the angle of the rear spine of the shaft is fixed. The leading edges on either side of the shaft, overlap at the forward aspect where they are drawn together by closure means. In this configuration, the angle of the shank of a user is dependent on the degree with which the closures draw the uppermost forward aspect of the leading edges of the shaft towards each other, and towards the rear spine, in proportion to the amount of overlap created by the operation of the closure mechanism.

The graphic below shows two photos of a right ski boot shell. In the left photo, the shaft buckles are operated to the minimal closure position. In the right photo, the shaft buckles are operated to the maximal closure position. A red reference line at the rear spine indicates the fixed shaft angle. A red reference line at the front aspect of the shaft overlap indicates the variable aspect of the shaft angle is it pertains to the shank angle of the user. An arbitrary reference center with which to gauge the variance in the shank angle is shown in black. The reference shank angle in the maximal closure position (right boot) is 8 degrees less than the reference shank angle in the minimal closure position (left boot). In terms of angles of ankle flexion, the ankle of the same foot and leg in the boot shell in the right photo would be 8 degrees plantarflexed when compared to the ankle of the same foot and leg in boot shell in the left photo.

Shank difference

The implications of this arrangement are that the shank angle of the user will change in response to changes in the operation of the closures of the shaft, especially changes in the top shaft closure and/or the amount of tension in the power strap, if equipped with one. For the most part, racers are unaware of the critical nature of the correct shank angle. They have erroneously assumed, or have been taught, that a securely tightened shaft is essential for good control of the ski. The tighter the shaft is secured to the leg, the better the control. As shown in the photo below, boot makers provide shaft buckles with high leverage features that facilitate a secure closure of the shaft with the leg. Closing the boot shaft beyond the shank reference angle can have serious implications for balance and control of the ski.

Leverage boost

The angle of the shank of the user is dependent on the degree of overlap of the leading edges of the shaft, including the tension of any power strap. The cross-sectional area of the leg of the user at the boot top, in particular, the front to back dimension of the cross-sectional area, is also a factor that affects the correct angle of the shank.

The two photos below compare the shaft angle of a stock boot shell to the shaft angle of a boot shell that has been modified to reduce the shaft angle by 8 degrees in order to correct for excessive shank angle.

8 degree difference

Operating the closure mechanism of a shaft beyond a certain point creates another problem, as shown in the photo below, deformation of the interfaces of the overlap elements of the shaft.

Overlap deformation

The angle of the shank of a racer is critical. It must be maintained within a narrow range for optimal performance. Anything that has the potential to alter an optimal shank ankle should be carefully evaluated.

Related post: GETTING SHAFTED BY THE (SKI BOOT) SHAFT -https://skimoves.me/2014/04/09/getting-shafte…ski-boot-shaft/

 

 

3 comments

  1. David, please comment on the effect of power strap since the 3º of foot/ankle freedom seem compromised by any compromise in dorsiflexion. I note when I loosen my buckles and tighten my power strap the effect of buckle loosening is gone and dorsiflexion is still restricted.

  2. Dave, what determines the optimal shank angle? I would think this is dependent on the individual mechanics and physiology of each racer. Thanks.

Comments are closed.