# GS AND KNEE INJURIES – CONNECTING THE DOTS

When the FIS reduced side cut on GS skis, many were confused as to why GS was singled out over other disciplines. A new injury study (1)  sheds some light on this issue. And while the study falls short of actually identifying the injury mechanism, it provides enough clues to connect the dots. The study, Here are some key statements.

Competitive alpine skiing is considered to be a sport with a high injury risk. Injury rates per competition season and per 100 World Cup (WC) athletes were reported to be 36.7, with the knee being the most frequently affected body part.

“The injury rate was highest for giant slalom,

Associating the number of injuries per hour in WC skiing with skiers’ mechanical characteristics, injuries in super-G and downhill seem to be related to increased speed and jumps, while injuries in giant slalom may be related to high loads in turning.

It has recently been found that many injuries occur while turning, without falling or being the result of a crash.

“Recently been found?” Seriously? Skiers have been experiencing knee injuries for years without falling and while apparently skiing in control. Without seeing any data, I can predict with confidence that, with rare exceptions, knee injuries are always associated with the outside ski of a turn. The other issue I can state with confidence is that if the moments of force (torques or twisting forces) acting on the outside ski are not tending to rotate the ski into the turn, they will be tending to rotate the ski out of the turn. In the mechanics of an outside ski on its inside edge, there can be no neutral. There are implications to moments of force that tend to rotate the outside ski out of the turn that are associated with speed and the length of any moment arm that exists between the reaction force at the inside edge and the center of the force applied at the sole of the foot of the skier. The force of gravity causes skier to accelerate in the fall line and decelerate as the skier crosses the fall line. (Force = Mass x Acceleration). The shorter the radius of the side cut of a ski, the longer the potential moment arm.

Skiers are turning for approximately 55% of the time in downhill, 80% in super-G and 93% in giant slalom. Moreover, it was shown that small turn radii might be related to an increased injury risk in giant slalom since they provoke the skiers to use their full backward and inward leaning capacities, and thus skiers have less buffer if an additional factor causes an out-of-balance situation.

Note the reference to additional factor.

Downhill had the largest mean turn radius, while giant slalom had the smallest mean turn radius.

Out-of-balance situations themselves are known to be a critical part of typical injury mechanisms, such as the ‘slipcatch’ and ‘dynamic snowplow’.”

What constitutes balance?

This is the central issue. Postural responses on a single limb involve two coordinated and interdependent balance strategies or synergies; 1) a plantarflexion strategy that resists the tendency of a disturbing force (typically gravity) that tends to topple the vertical column supporting CoM forward by causing the ankle to dorsiflex and, 2) an inversion strategy that resists the tendency of a disturbing force to topple the vertical column supporting CoM sideways (ergo, cause the foot to evert or turn away from the centre of the body). In terms of the latter strategy, if one is standing on the right foot, the disturbing force will tend to cause the foot to rotate about it’s inner or medial aspect into the ground. This is called eversion. In skiing, the external forces would tend cause the ski to rotate into the turn. The postural responses of the skier’s balance system act to control the degree of eversion. Eversion is mechanically coupled through a joint in ankle complex called the subtalar joint. When the angles at the knee are relatively small, the leg as a whole will rotate about its vertical axis on a 1:1 ratio with rotation of the foot about its long axis in eversion. Elite skiers don’t consciously cause these rotations to occur. The external forces cause them to occur. The balance system of the elite skier controls the rotations.

The ‘balance problem’ is that there is only a very short window when the outside ski of the new turn is flat on the snow between edge changes in which to set up the biomechanics that engage the external forces that drive the moments of force into the turn. Among other things, this requires that the skier be able to rapidly dorsiflex their ankle so they can move CoM forward; ergo so the foot can pronate. It is relatively easy to prevent the foot from pronating, but extremely difficult to stop the foot from supinating especially under the influence of a high instantaneous peak force. The configuration that engages the external forces must be established before the outside ski acquires a significant edge angle and especially before the external forces start to increase.

The ‘slipcatch’ technique is the worst possible way to engage the inside edge of the outside ski. The outside ski is slipping sideways as the angle of inclination of the skier and associated edge angle are increasing until a point is reached where the inside edge catches or locks up. Should the slipping ski encounter a frozen ice formation the ski could suddenly decelerate. Should this happen, the offset or moment arm between the inside edge and the centre of the force applied at the sole of the foot will tend to rapidly rotate the foot about it’s long axis out of the turn. This will also rotate the tibia on its vertical axis against a well-stabilized femur.

Comparing the mean and minimal turn radii between discipline, it is evident that giant slalom has substantially smaller turn radii than super-G and downhill. Additional analysis of the data showed that the radial component is the main contributor to the increased FGRF in giant slalom. Thus, the combination of small turn radii and speed leads to larger mean and maximum FGRF in giant slalom compared with super-G and downhill. Furthermore, in giant slalom, skiers’ balance might be challenged simultaneously by small turn radii and high forces.

Injuries in giant slalom were linked to high loads in turning;

It gets worse.

First, the model for the computation of FGRF does not capture the high frequency force components and, therefore, might underestimate the work load (impulse), in particular for giant slalom.

In other words, the actual impulse forces in a GS turn could be much higher than the model predicted.

Furthermore, giant slalom includes a larger number of turns (52.0±3.5) compared with super-G (40.0±3.5) and downhill. Hence, skiers have to find balance in turning more frequently in a run and thus might be more often susceptible to balance-related mistakes in turn initiations.

The implications are that the skier needs to be able to set up the processes responsible for dynamic equilibrium in the outside leg at the initiation of every turn.

dot-dot-dot-dot

sidecut radius + length of moment arm + instantaneous peak moment of unbalanced force out of the turn on the outside foot + high GRF

1. Mechanics of Turning and Jumping and Skier Speed Are Associated With Injury Risk in Men’s World Cup Alpine Skiing – A Comparison Between the Competition Disciplines: Matthias Gilgien, Jörg Spörri, Josef Kröll, Philip Crivelli, Erich Müller: Br J Sports Med. 2014;48(9):742-747′, is available for free at Medscape (www.medscape.com).