An earlier post tried to work though the relationship between stance width, flexion, and ski tipping angles:
?Harald has always advocated relatively narrow stance as more functional. So how come he can carve and get deep angles? Is he defying physics or something?
I believe the trick is in relaxing the parallel shins restriction. In the upper C part of the turn aggressive free foot tipping will increase the gap between skis (inside ski will follow a shorter radius arc) and thus allowing impressive angles, however before transition into next turn skis have to come together again, so a little A frame is unavoidable.?
You are probably correct that there will sometimes be forces that will tend to separate the skis. Most of the time it is probably different edge angles or some knee steering on one of the legs. However, I?m not sure that there is a necessary increase in separation because the inside ski is following a shorter arc. I would guess the issue is more one of body movements (kinesiology) rather than one of the movements of bodies (physics). I?ll take a shot at working through the issue from what I know of PMTS and then anyone else can add or subtract what they will.
Let?s get back to an analysis of tipping and stance in terms of primary movements and see which movement(s) widen the stance during a turn. Just for a reminder: the three primary movements are 1) inversion / eversion of the feet 2) plantar and dorsiflexion of the ankle and pulling the feet back in general, and 3) flexion / extension of the legs. There are of course secondary movements of the hips, shoulders and arms, but they can be ignored here.
1) Rolling the skis onto a set of edges (inversion/eversion of the feet) will not necessarily widen the stance width. It moves the CM to the inside of the turn and if no other movements happen the body begins to tip into the turn. If the skis are put onto the same angle at roughly the same time they should follow parallel arcs of the same radius. Further, even if that is not the case, one can always adduct to pull the legs and skis together. .
2) Of course pulling the free foot back through plantar flexion of the ankle and the more general actions of pulling the leg back will enable one to invert the free foot to a higher angle, but if the angle is matched by the stance leg (as it does easily because internal rotation of the stance leg is so much easier), then this should not lead to a wider horizontal stance either.
3) Where we should begin to see an increase in the stance width is when the free foot leg is flexed. However, this is a widening of the stance vertically; not horizontally. The free foot glides up the stance leg in proportion to the angle of tipping and the level of pressure needed for the conditions. Flexing is done to allow a greater range of external rotation of the femur and open up a deeper range of angles that can be achieved by the movements in the foot and ankle. Flexion also allows the upper body to resist merely banking into the turn by allowing more separation in the movements of the lower and upper body.
Just as an exercise, place your hand on a wall and lean into it with fairly stiff legs. Now, without flexing your legs a) check to see how much inversion is available just through foot movements 2) see how far you can tip your upper body out over the stance leg. Then, let the imaginary free foot ride up the stance leg until it reaches knee height. Try seeing how much easier it is to a) get a greater tipping angle and b) get your upper body out over the stance leg. These two effects is what gives the ski ?bite.?
Vertical separation is functional and comes through flexion. Excessive horizontal separation is not functional and is either produced by leg steering and mismatched ski angles (the cause of this could be a either a movement deficiency or an alignment/equipment issue), or actively induced through abduction.
As Harald has posted elsewhere on the forum, there are some very good biomechanical studies looking at the negative effects of a wide stance (either parallel or wedged) on both ski performance, knee stress, and injuries rates. If anyone is interested, I can post a recent reference from a presentation at this year?s international congress on ski safety by North America?s leading researcher on these issues. From his results, he advocated abandoning the wedge and adopting a narrower stance with the emphasis on free foot tipping toward the little toe edge.
Two additional points. First, I am not advocating a ?tall in the transition? and flexed in the end of the turn rhythm. I only started my example with relatively stiff legs to first isolate the inversion / eversion movements from the other movements involved. The more flexion one can have at the beginning of the turn the greater range of movement and control one will have throughout the turn. Second, I am not advocating locked boots. The feet should be horizontally wide enough to work independently, but not so far apart to inhibit a functional release. Within the parts of a trurn the stance may tend to be the narrowest at the release because the narrower the stance and the more the flexion, the esaier it is to invert the old stance foot into a release. So, it would make sense to see a more definite narrowing at the release/transition.
By the way, I found Elvis on the mountain top last weekend:
http://web.pdx.edu/~petersj/HoodCamp/Elvis.htm
Enjoy
Jay
PS Hood still has great skiing left. Come on out!