SKI BIOMECHANICS

SKI EDGE MECHANICS It is important to also consider ski edge mechanics and the effect of side-cut of the ski. The side cut of the ski is described as the difference in width of the ski from the tip to the waist to the tail. The side-cut offsets the tip and tail from the waist of the ski where force is applied. This results in a moment arm that oppos- es the rotation of the ski onto its edge – if you think of this anatomically as a tendency to rotate in inversion. Eversion of the foot in pronation resists this inversion moment. If the geometry of the ski is suitable, the forces of pronation will balance the forces of the ski that cause it to rotate in inversion.

COMMON SKI INJURIES There are a wealth of injuries associated with skiing, however, most injuries are traumatic with aetiological factors mainly being skier error! Nevertheless it is the knee that is often the most affect- ed in non-traumatic lower limb injuries with 25% of injuries occurring in the knee (2). This is not surprising when considering the opposing rotational forces that are applied across the knee joint during skiing. During the ski turn, if the tibia is forced into lateral rotation by either the forces from the ski edge or from poorly fitting ski boots, and the femur in medial rotation, a strong rota- tional force is created across the knee joint (see figure 1).

Figure 1: Diagram

showing the opposing rotational

forces which may be

exerted on the knee

Medial rotation of tibia

Lateral rotation of tibia

In addition, the large angle of the knee joint required for knee angulation during edging opens the condyle of the knee therefore reducing knee stability. Injuries commonly found in the knee are soft tissue injuries involving the MCL, ACL, patellar tendon and joint capsule tears. Other injuries presented in the lower limb during skiing are medial tibial border pain, medial malleolus pain, Achilles tendon/calf muscle pain, plantar fasciitis, iliotibial band syndrome and calcaneo- cuboid joint pain. It is important to consider that these may be injuries that manifest in the patient on an everyday basis how- ever they may become aggravated during skiing.

BIOMECHANICAL ASSESSMENT OF THE SKIER When trying to identify intrinsic factors that may be causing or predisposing to lower limb injuries in the skier, it is important to assess the skier’s biomechanical structure and alignment.

Common lower limb deformities affecting function during skiing are: ■ Sub-talar joint hypermobility ■ Mid-tarsal joint hypermobility ■ Pes planus foot type ■ Tibial varum ■ Genu-valgum ■ Functional hallux limitus ■ Reduced ankle joint dorsiflexion ■ Sacroiliac joint dysfunction ■ VMO, gluteus medius or peroneal weakness ■ Functional and anatomical leg length difference.

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Figure 2: Carver ski showing the side cut. The side cut is the width of the tip/tail minus the width of the waist of the ski.

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