Thursday, December 15, 2011

Why did my ski instructor tell me to turn with my legs?

Solid skiing involves rotary movements, some of which occur with the legs.
  • "Steering with your feet"
  • "Steering your skis with your legs"
  • "Turning your feet to steer your skis"
These catchphrases all imply turning using your legs. We can loosely describe this as leg rotation.



Ted Ligety Go Pro Slalom from Ted Ligety on Vimeo.


Why is leg rotation important? Why would an instructor or coach emphasize turning with our legs instead of turning with our whole body?


By maintaining a stable upper body while using our legs to steer our skis, we can use our largest muscle groups to manipulate our skis. We can create functional torque and be in the best
position to maintain dynamic balance. We can increase and decrease edge angles appropriately.


Movements that occur at the hip joint (more accurately called the acetabulofemoral joint and less accurately the iliofemoral joint) combine so that our knees and toes change their orientation about our vertical axis and direction of travel. The mobility of the hip joint allows lateral / medial rotation, flexion / extension, abduction / adducation, and combination movements such as circumduction. To complicate things further, the femoral neck is angled roughly 126° to the femoral shaft. Another anatomical consideration is that the acetabulum (the "cuplike" articular surface in the pelvis that holds the femoral head) is uniquely different in orientation to the transverse and sagittal planes for each person. Finally, hip stability and mobility varies widely between skiers with different levels of conditioning.


Whoa... Complicated...


Should we even use the term "leg rotation"? Biomechanically, it's not accurate. But it captures the general idea in powerful, effective skiing.


Here's a basic movement cue:
"When you are in a turn, the legs should turn [or twist] more than the upper body turns"


This is hard for many skiers to grasp. There are few other sports where both legs have a similar rotary move. It's hard to simulate without both feet being on a slippery surface. With an experienced instructor, students can perform some drills and exercises to develop preliminary kinesthetic awareness of these sensations before stepping into their bindings.


But for the visual learners, see Ted Ligety's backpack cam in the embedded video above. This video clearly shows how the legs turn more than the upper body does. It's rare to find footage like this where you can see the legs turning below a stable upper body, and where you can get a feel for what it's like to move the base of support (skis) so far out from below the center of gravity at the apex of a turn. Ted has captured a really good perspective to demonstrate this important aspect of skiing. It seems easier to imagine yourself skiing with these movements after watching this video. High level skiing is a great way to show effective movement. Even skiing with half of this skill would put you in the top percentile of skiers.


The GoPro provides a great perspective to see leg rotation. But the wide-angle lens doesn't do Ted's skiing justice. The GoPro makes it look easy. See the next embedded video to see Ted's skiing in action. He is one of the best technical event skiers in the world:





Most skiers tend to underestimate the mobility and stability in the hip to make dynamic short radius turns like Ted's. A skier's hips need to be very stable and highly mobile to make accurate dynamic short radius turns, where the center of gravity moves laterally over a meter and the turns are at a tempo well over one turn per second. A very rough estimate of the bare minimum necessary strength is being able to do a bodyweight one-legged squat. For a combination of stability and mobility, a skier should be able to perform a hurdle step (see Gray Cook's FMS test) with the hurdle at least as high as the tibial tuberosity.


Ted Ligety is considered by many to be the world's best GS skier. To be expected, his slalom turn is exceptional and mostly carved. Once you've got the strength, mobility, stability, and experience, it's possible to start turning it up a notch and work towards some short radius turns like Ted's. Understanding and being able to "turn with the legs" is a critical component.

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