LETTERS
on their 27 m sidecut skis vs. 35 m sidecut skis. More knee angu-
lation will cause an unstable leg geometry leading to uncontrol-
lable, non-linear, generation of loads.
A typical World Cup GS skier on current skis will angulate in such
a way that a line from the inside edge of their outside ski through
the center of their knee will fall slightly outside their COM. This
leg geometry is stable as a sudden increase in load will cause the
edge angle to be reduced and the load to be reduced during the
abrupt transition, giving the athlete time to react appropriately.
The line from the ski edge through the knee of a skier using 7
degrees more knee angulation will fall well inside of the COM.
This leg geometry is unstable as a sudden increase in load will
cause an increase in the edge angle as the knee collapses in-
ward leading to additional loading, additional knee angulation,
more loading, and so on until the skier can react. By the time the
skier reacts this load generation can cause serious injury and/or
loss of control. This is a non-linear reaction to natural control in-
put and is to be avoided at all costs. Aggravating this problem is
the likelihood that more knee angulation will make it more difficult
for the inside ski to track parallel with the outside ski since add-
ing more knee angulation on the inside leg is very difficult. This
may encourage the skier to transfer weight from the inside ski to
outside ski thus increasing the load on the now more vulnerable
outside knee.
A supporting fact is that many of the best World Cup skiers
choose skis with larger sidecut radii than the allowed minimum
27 m and 23 m for men and women respectively. For example
Ted Ligety and Lindsey Vonn use 29 m and 27 m sidecut radii, re-
spectively. It is reasonable to assume that Ted and Lindsey pre-
fer these skis because they encourage postures that are stable,
strong and safe. A less accomplished or weaker skier would need
more sidecut to achieve the same postures. Too much sidecut for
the skier’s ability and strength causes the line of force to fall too
far outside the COM. Too little sidecut for the skier’s ability and
strength cause the line of force to fall too far inside the COM.
Both are undesirable, especially the latter.
The well-known geometric relationship between sidecut radi-
us, edge angle and turning radius shows that a 35 m ski will fit
the FIS description of a ski that is “too aggressive”. Of course it
would take some training time on these 35 m skis for the athletes
to learn that they can ski faster using more knee angulation, so it
is unlikely to show up in short-term tests.
Skiers should be allowed and encouraged to choose skis with
sidecut radii that promote the most stable, strongest and safest
postures. Coaching guides should be developed to help athletes
achieve better postures through a better understanding of the
relationship between ski design variables and skier postures.
The geometric relationships described above are well known
and understood. The implications cannot be responsibly dis-
missed or ignored.
Higher probability of phantom foot ACL injury
The slip-catch injury mechanism is identified in the Oslo3 study
as the predominant cause of knee injuries on World Cup athletes. It is, in my opinion, a combination of the well-known phantom foot and BIAD mechanisms. A review of the video and pictures included with the Oslo studies shows a loss of edge grip on
the downhill (outside) ski followed by a transfer of weight to the
inside (uphill) ski. Prior to the loss of grip the skier’s COM was
balanced between the skier’s feet. The sudden loss of grip on
the outside ski caused an out-of-balance situation with the center of pressure suddenly moving uphill and forward causing the
skier to start falling downhill and backwards. This backward rotation combined with the downhill rotation produces a precessional
rotation on the third vertical axis that rotates the skier’s mass
away from the hill. Of course, all these rotations are undesirable,
and the skier responds by retracting his uphill ski to reduce the
forces throwing him out of balance. This allows the downhill ski
to reengage. In order to arrest the unwanted rotations, the skier
naturally pressures the tail of the ski to correct the backwards
and downhill rotation, but this creates an uphill rotation acting
against the precessional downhill rotation of the skier’s COM,
causing the skier’s upper body to twist downhill, producing an
internal-valgus rotation of the knee joint.
The skier is now in a position universally recognized as the final
stage before a phantom foot ACL rupture, except that his leg is
relatively straight. Please note, the ski sidecut had nothing to do
with this scenario. All that is needed is a specific set of out-of-bal-
ance rotations and a lever extending backwards from the foot.
If the ski has less grip in the tail, the skier will continue to fall
backwards; the ski will skid and continue to rotate uphill due to
the slope of the snow surface relative to the skier’s COM. If the
skier fights these rotations with sufficient vigor he will most prob-
ably rupture the ACL on the downhill knee in a classic phantom
foot posture. If he gives up the recovery attempt, pulls his body
into a safer posture and lets the fall progress naturally he will al-
most certainly avoid an ACL injury. No World Cup racer is likely
to do this without extensive training on how to avoid the phantom
foot trap. Ettlinger et al5 have shown that such training can dra-
matically reduce the likelihood of phantom foot ACL injuries.
If the ski has aggressive grip in the tail, the skier may be able to
reverse the backward and downhill fall and re-center his fore-aft
balance — quickly enough that he is able to reengage the uphill
ski in a way that arrests the downhill rotation without resorting to
twisting his upper body in a way that applies the injury producing
internal-valgus rotation of the downhill knee.
Supporting the above analysis is a large (34-year, 6,780,940
skier-day) epidemiological injury study by Ettlinger et al4 show-
ing that the knee injury rate increased steadily from 1972 until
approximately 1992-1993 unabated by all the equipment devel-
opments during that time period. Since 1992-1993 the trend has
reversed. Shorter, shaped skis became popular in the 1992-1993
timeframe. Superior edge grip at the tail, stronger self-steering
effects and the shorter length are the differentiating features of
these skis compared to the older skis they replaced. It is there-
fore reasonable to conclude that one or more of these features
led to the reduced knee injury rate.
It is clear that a less aggressive ski will reduce the forces that
appear to contribute directly to the injury in the slip-catch sce-
nario. However a less aggressive ski limits the skier’s ability to
arrest the rotations that lead to phantom foot type injuries.
