EXTENSION PENDULUM/EFFECTS EVENTS


There are 9 specific events associated with the extension/pendulum effect technique.

  1. Transition – (aka Get Over It or Up and Over). As the turn phase ends, the racer shifts the load from the outside ski to the inside ski while it is still on its inside (current) edge by extending on the inside leg and moving the hips up and over the inside ski to bring COM over the inside foot and in front of the ankle joint (hence Up and Over).
  2. Pendulum Effect Initiation – As the racer begins to transfer the load to the inside ski, the action releases the load on the outside ski. This initiates load induced rotation of the inside ski into the new turn. The inside (uphill) edge of the inside ski acts as a pivot for the rotation. The column of the skier rotates about the pivot like an inverted (upside down) pendulum imparting rotational momentum that will take the ski and skier past ski flat and into the new turn.  The pendulum effect begins at the transition phase and ends when the transition phase for the next turn begins.
  3. Ski Flat – Ski flat provides a brief source of contiguous Ground Reaction Force that enables load transfer induced pronation.
  4. Coordinated Ankle Dorsiflexion/Knee Flexion/Hip Flexion – As the ski flattens on the snow, the racer assumes a position of balance on one foot (monopedal stance) made possible by coordinated ankle dorsiflexion/knee flexion/hip flexion. As ankle dorsiflexion progresses, it drives the intrinsic mechanism that causes the foot to pronate. As the foot pronates, the load shifts from the proximate anatomical centre of the foot to the proximate head of the first metatarsal and over the inside edge of the outside ski.
  5. Load Transfer Induced Pronation
  6. Edge Change
  7. Whole Leg Top Down Internal Rotation – As the ski edge changes and the ski rotates into the new turn, the racer rotates the leg internally (into the turn) with the powerful hip rotators. This rotates the ski horizontally about a centre between the outer aspect of the heel and the inner aspect of the head of the 1st metatarsal.
  8. Edge Set/Closed Kinetic Chain – As the ski rotates across the trajectory of the racer, the ski progressively acquires edge angle until a point is reached where it engages with the snow and creates a closed kinetic chain. When this happens, the edge ski presents a source of resistance to the skiers momentum as the skier aligns the resultant force R emanating from COM with the load W and both loads align through the proximate centre of the head of the 1st metatarsal. When the kinetic chain closes, internal rotation of the whole leg is converted through the torque converter in the subtalar joint into rotation of the foot and ski into the turn. Using this mechanism, the skier is able to wind the ski about its long axis into the turn like a corkscrew while controlling the amount of torque applied to the ski.
  9. Inside Leg Pelvis Position/Torque Control – The inside leg is used to restrain the pelvis against the torsional load arising from internal rotation of the outside leg and bottom up torsional loads from perturbations if Ground Reaction Forces.

A key aspect of the extension/pendulum effect technique is that when it is correctly executed at ski flat, it sets up a vertical force acting at right angles to the transverse aspect of the ski in opposition to the Ground Reaction Force at the portion of the inside edge 0f the ski under the centre of the head of the first metatarsal. The alignment is maintained from initiation of the new turn until it is released at the end of the turn when the transition phase commencement.

In my next post I will illustrate these events using still shots of racers from the recent women’s slalom and men’s GS at Are, Sweden.

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Biomedical perspective by Dr. Kim Hewson

This is a description of a fluid blending of biomechanical events in a ski turn.   A specific emphasis should be on responses to coupled dorsiflexion and foot pronation (no.4).  Simultaneously, a mandatory Internal rotation of the lower leg occurs.

After load transfer and edge change, further  internal leg rotation occurs  through the internal rotators of the pelvis and hip (no.8) rotating the leg into the turn.   The initial edge set also triggers a kinetic closed chain leg extension through the knee and hip from a solid platform at the foot.

Turn mechanics utilize muscle co-contractions in flexion, extension, internal and external rotation in a blending of stabilizing agonists and antagonists.

These fluid transfers of energy in a stacked(balanced) skeletal system result in minimal muscular effort while providing highly efficient turn mechanics.

Dr. Kim Hewson is a Telluride Ski School Alpine Instructor and Staff Trainer in the Biomechanics of Alpine Skiing

8 comments

  1. Thx Dave for reply! like ‘door frame’ task….particularly ‘looking down’ on joint rotation and using the reader as ‘their own best model’ As you develope #9 could you develope this furthers to what you see from flat/ski stable base to start of new transition?
    ‘have thoughts on ‘hip’ but will leave those for when you expand on 9… Like you said “write a book”! huge subject and believe,largely’poorly understood.
    Thx also for plantar facia response. Will ‘remind’ later.
    included 2links Shifferin. #1 free skiing Hood 2011. 2.. Shifferin GS up close+personal 🙂
    http://m.youtube.com/?#/watch?list=FLFKwbcXMGYZvJ_q_ikD082Q&v=5nkwr9Dglf0

    http://m.youtube.com/#/watch?v=9eB9bWMZsz0

    1. Thank you for the video links especially Shiffrin up close + personal. What country is the YouTube site this from? In future posts I will expand on #9 and the implications of tension in the plantar fascia. The events around ski flat are what I referred to in an older post as Ligety’s Moment of Truth. In fact, this is any skier’s moment of truth. By moment, I mean moment of force or torque. The key is to arrive at ski flat in balance on the new outside limb of the turn with load transfer induced pronation then rotate the leg internally (into the pelvis) as the ski rotates onto its new inside edge. The timing must be perfect. It is usually late because of interference with the essential articulations of the foot and leg caused by the ski boot. It was disconcerting watching Mikaela Shiffrin struggle horribly in the early part of the season. I was sure that she had changed boots and possible some aspect of her ski equipment which turned out to be correct. She was starting to come around in the GS at Kuthai. But she was still not right. But her second run in the slalom was breathtaking, possibly the best I have ever seen from this enormously talented female racer.

      In a a few days I will start to post regularly once we get past Christmas break.

      1. Will check out Shifferin sl. Don’t know country of origin Shifferin GS…title looks like cyrillic? Found video on Shifferin fan site…lot of video here. link:
        http://www.mikaelashiffrin-fanclub.com/
        Agree with Shifferin equip. set up …as you’ve said ‘on again off again’ trying to compensate with gross large muscle movements to get where she needs to be. also agree with ‘timing must be perfect’ like setting dominoes perfectly spaced around an arc. tip them… fluid motion. fine muscle accrutement to large..any pause is a compensation+refers to gross muscle movements in an attempt to get timing back. looking forward to future posts.

      1. Greetings Dave—-You are ‘spot-on’ with 1-9! Can’t wait for discussion of #9….the wonderful world of ‘countering movements’ of the hip/upper torso with the inside legs’ roll in maintaining your ‘closed kinetic chain’ in the stance leg. ‘like your hint #9 ‘the inside leg is used to retain the pelvis’…as you allude to ‘that’s not all’

        I noted in on of your ‘bio posts’ you mentioned ‘tension in plantar facia when COP has moved to the head of the 1st met. would you comment on this in reference to 1-9?
        Personal observation on snow….it seems that when COP is on 1st met leg internally rotated the ‘evertors’ in lateral aspect of the lower stance leg are very much involved. The arch has firmed up and lays against the medical side of the boot. It seems this heaps stabilize the ankle joint+the whole structure now provides a base of support for Dr. Kiss’ “stacked skeletal system”
        Looking forward to more posts .. just received Stenmark.

      2. I could write a whole book on Event 9. I may end up doing this at some point. The key point to understand is that internal rotation of the leg in a close kinetic chain event is relative. Eversion of the foot will rotate the leg internally or internal rotation of the leg will evert the foot.

        Internal rotation of the stance leg in monopedal stance is initiated from the foot up by load transfer induced pronation. Part of the pronation event involves rotation of the foot about its long axis in eversion (i.e. the sole turns away from the centreline of the body). Eversion is translated through the STJ torque converter into whole leg internal rotation. Once load transfer induced pronation engages the torque converter, torque conversion can go either way so long as the kinetic chain is closed. What closes the chain? A ski with its edge engaged in the snow. At ski flat, the torques about the long axis of the ski and horizontal plane are minimal. So it doesn’t take a lot of constraint to close the kinetic chain. The key event is top down rotation of the leg as a whole with the hip rotators that happens as the ski changes edges. This is what I look for when analyzing World Cup video. Very few racers get this event right. When whole leg rotation creates sufficient edge angle of the ski, the edge locks in the snow thus engaging the ski lever mechanism. When this happens, external forces drive internal rotation.

        Transient peaks resulting from perturbations in GRF cause associated transient torque peaks. The transient torque peaks create a tendency to unwind internal rotation of the outside leg. How? By causing the pelvis to rotate in the opposite direction as a reaction. This has the net effect of externally rotating the leg. Try this. Stand in a doorway with your body aligned with the door frame. Grasp one side of the door frame with your hand. Stand on the same foot. Make sure pressure is on the ball of the foot. Now rotate your hips towards the side of the frame you are holding onto. This will have the effect of rotating your leg internally.

        In summary, the best racers use the inside leg for pelvic stability and torque control. This becomes readily apparent when the transition phase begins. The prelude to extension is a release of pelvic torque control. Extension of the inside leg does not commence until significant unwinding of pelvic torque has occurred similar to that of a spring uncoiling. Scrubbing video of this event forward and backward reveals the torque control mechanism.

      3. I recently found a very good paper on the plantar fascia aka the plantar aponeurosis. I am reading it now. I can tell by Stenmark’s skeletal joint angles that he is maximizing tension of the PF. This is critical component of the extension/pendulum effects technique. Please remind me in a few days to comment on this if I don’t get back to you. In essence, the conclusion of the paper is that PF tension peaks in late stance which is why coordinated ankle dorsiflexion, knee and hip extension are critical at ski flat.

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