There are 9 specific events associated with the extension/pendulum effect technique.
- 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).
- 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.
- Ski Flat – Ski flat provides a brief source of contiguous Ground Reaction Force that enables load transfer induced pronation.
- 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.
- Load Transfer Induced Pronation
- Edge Change
- 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.
- 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.
- 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.
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