Skien is een sport van bewegingen en daar zijn interne krachten bij nodig. Om te kunnen bewegen heeft het lichaam krachten nodig. Het lichaam zal moeten bewegen.
biomechanics Hier wordt de biomechanica beschreven van het menselijk lichaam.
SKIING – A SPORT OF MOVEMENT
THE FOUR MOVEMENTS OF SKIING
- FORE/AFT MOVEMENT
- ROTATIONAL MOVEMENT
- LATERAL MOVEMENT
- VERTICAL MOVEMENT
The benefits of good fore/aft balance are:
An athletic stance
Rotating the lower body.
Rotating the upper body.
Rotational separation results in stability of the upper body and is controlled through use of the core muscles. This stability allows the skier to maintain a strong biomechanically aligned position as the skis are turned.
Rotation of the lower body can create an increase in edge angle. To achieve this the legs must be flexed or the centre of gravity must move to the inside of the turn. As the femurs rotate and adduct and abduct, the knees move towards the inside of the turn, creating angulation and increasing the edge angle.
Rotational movements are adjusted depending on the amount of friction on the base of support, the shape of the turn and the forces acting on the body.
As well as leg (lower body) rotation and upper body rotation there are two other types of rotation that are commonly referred to:
• Counter rotation is created by turning the upper body and lower body in opposing directions and is an example of Newton’s third law of equal and opposite reactions Counter rotation has a high rate of turning and a short duration and is most effective when the skis have very little or no resistance, such as on the top of
a mogul or in the air.
• External fulcrum is often referred to as a blocking pole plant and can be useful when skiing steep terrain and in moguls. If the ski pole is placed in the snow with the tip of the pole ahead of the handle, resistance
is created that pushes back on the skier’s hand creating torque and a fulcrum to turn around. This works best during the initiation phase when the skis are at their flattest in relation to the snow.
is defined in two parts:
• movements of the body or parts of the body travelling in a sideways relationship to the base of support
• the base of support moving sideways from the body.
The two main components to lateral balance are inclination and angulation:
At higher speeds angulation is used to allow the skier to increase the edge angle without having to move the centre of gravity further inside the turn. It stabilises and aligns the body so that inclination can be maintained as the forces pull the centre of gravity to the outside of the turn. The timing of the angulation movement is important and will depend on such variables as turn radius, snow conditions and speed.
Angulation is created at the ankles, knees, hip joint and spine.
Angulation at the ankles is created by inverting and everting the feet.
Inversion is created by tilting the sole of the foot inside (supination) i.e.
lifting the big toe and eversion is created by tilting the sole of the foot to the outside (pronation) i.e. lifting the little toe.
The degree of angulation required from the ankles, knees, hip joint and spine is determined by such variables as turnshape, speed, friction and snow conditions.
The goal of vertical movement is to allow the skier to move with the skis,
maintain an athletic stance and to continually reposition the body to achieve the desired outcome (balancing, rotating, edging, pressuring) throughout the phases of any given turn. Vertical movements need to be continuous, consistent and controlled to be effective.
As a stand alone movement vertical movement simply moves the centre of gravity towards and away from the base of support, but when combined with the other three movements – fore/aft, rotational and lateral – its real benefits are obvious. Accurate and well timed vertical movements allow the other three movements to blend together and have the optimum effect on ski/snow interaction.
The two components to vertical movement are flexion and extension:
The benefits of accurate lateral movement are:
• the centre of gravity is able to move inside the turn, creating edge angle and a platform to balance on
• edge angle is able to be maintained, increased or decreased as needed
• pressure is created/controlled on the skis (primarily the outside ski) causing it to penetrate the snow and make the ski bend, therefore utilising ski design.
• balancing on the outside ski allows use of the stronger adductor leg muscles to make rotation easier
• the skier is able to balance against the forces created.
Flexion moves the centre of gravity towards the base of support. This is achieved through bending at the ankles, knees, hips and spine. The ankle is limited by the stiffness of the ski boot which means that the knees, hips and spine need to compensate to ensure that the centre of gravity remains over the base of support. Flexion can be active which is achieved through eccentric muscle contraction to shorten the legs, or it can be passive, which is achieved by relaxing the leg muscles. Passive flexion is commonly used to
maintain ski/snow contact during changes in terrain.
Extension moves the centre of gravity away from the base of support. This is achieved through an opening of the ankle, knee, hip and spine. The centre of gravity will still maintain a perpendicular relationship to the base of support in this taller position.
The benefits of accurate vertical movement are:
• the skier is able to maintain/increase/decrease pressure on the skis
• the skier is able to maintain a desired relationship with the snow surface
• vertical movements provide rhythm and flow
• the skier is able to blend the other three movements.
Extension and Flexion
THE INTERACTION OF THE FOUR MOVEMENTS
• the balance point of the skier will be over the centre of the ski, allowing ski design to be utilised (rotational and lateral)
• the skier will be able to rotate the skis effectively due to good alignment allowing access to the appropriate muscles (rotational)
• joints will be able to be turned appropriately and incrementally, creating rotational separation (rotational and lateral)
• the entire length of the edge of the ski will contact the snow when the skis are edged and the ski will bend and be pressured from the middle (lateral)
• good alignment will allow all joints and muscles to flex and extend appropriately (vertical)
• the rate at which the legs are turned affects the skier’s ability to stay appropriately balanced fore and aft. As the skis turn down the hill the slope angle increases and the skis speed up. The faster the skis are turned down the hill the quicker the skier needs to make fore/aft balanceadjustments (fore/aft)
• the skier will be able to maintain alignment and balance on the outside ski (lateral)
• rotating the femurs can increase edge angle (lateral)
• turning the joints incrementally creates rotational separation. This angles the upper body more toward the outside ski creating lateral balance (lateral)
• appropriate rotational movements will allow the skier to both incline and balance on the outside ski, enabling the skier to manage both centripetal and centrifugal forces which are created by turning (lateral)
• good alignment will be maintained allowing the skier to flex and extendas required (vertical)
• engaging the edges of the skis will create a solid platform to balance on and to flex and extend from (fore/aft and vertical)
• inclination and angulation allow the skier to balance with the forces created by turning and to maintain balance along the length of the ski (fore/aft)
• balancing on the outside ski allows skiers to utilise stronger turning mechanics (rotational)
• the amount of edge angle affects the amount of intensity required to rotate the femurs. The higher the edge angle the more intensity required (rotational)
• vertical movements allow the skier to balance as required along the length of the ski (fore/aft)
• the muscles used for leg rotation vary with the posture of the leg (rotational)
• flexion of the leg will increase the amount it can be rotated (rotational)
• a flexed and rotated leg will increase edge angle (rotational and lateral)
• the centre of gravity is able to be directed inside the line of the base of support (lateral)
• pressure on the skis is able to be adjusted (lateral)
THE MOVEMENT DESCRIPTORS
To help describe the movements of the skier and the ski/snow interaction, whether it is a wedge turn or skiing a direct line through the bumps, we have identified five movement descriptors to provide an accurate description of how a particular movement or movements should be or are being performed.
The descriptors are also used to describe the performance of the skis.
An example of rate within rotational movement is when a medium radius turn is changed to a short radius turn; the rate at which the legs are turned is increased.