Interne krachten.

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 zich moeten bewegen.

SKIING – A SPORT OF MOVEMENT

The PMTS has identified four movements to demonstrate how skiing works.
This section discusses the four movements and how they are applied when skiing. It is important for instructors and skiers to understand not only how the movements work individually but also how they work with, and affect one another, to create different ski/snow interactions. A thorough understanding of the four movements provides the instructor with the ability to teach skiing effectively and to accurately analyse a skier’s movements.

THE FOUR MOVEMENTS OF SKIING

The four movements of skiing are:
  1. FORE/AFT MOVEMENT
  2. ROTATIONAL MOVEMENT
  3. LATERAL MOVEMENT
  4. VERTICAL MOVEMENT
Each of the movements has a direct effect on the skis and how they interact with the snow. The movements and their effect are applied through the base of support, which is defined as the portion of the ski or skis under the foot or feet. Another important term referred to in this chapter is the centre of gravity
which is defined as the point in the body where all mass acts as if it is concentrated. For example the body rotates around the centre of gravity when it performs a spin in the air. The location of the centre of gravity
changes as we move and can even fall outside of the body. Standing still ina good athletic skiing position, the centre of gravity will be in our core area, approximately in line with our navel and a few centimetres in front of our spine.

FORE/AFT MOVEMENT

Fore/Aft movement occurs along the length of the skis and is controlled by the
ankle, knee and hip joints, movement in the spine and the position of the arms.
The goal of fore/aft balance is to allow the skier to stay balanced and maintain
an athletic position. The skier’s position along the length of the ski may vary
depending on terrain and the desired outcome to be achieved. Fore/aft balance
is always being challenged and is often referred to as dynamic balance and
skiers will be constantly moving to and from a balanced position.

The benefits of good fore/aft balance are:

• the skier stands in an efficient posture
• even pressure will be maintained along the length of the skis
• ski design is able to be utilised to its full potential
• it makes it easier to access the other movements

An athletic stance

An athletic stance is the reference point for describing how a skier is standing
on his/her skis. It is described here in a stationary position.
The centre of gravity will be directly above the centre of the base of support and directly between the two skis. The ankles, knees and hips will be bent relatively equally, with the hips aligned over the top of the feet and the upper body tilted forwards. The feet will be about hip width apart with the knees aligned approximately laterally over the second toe and the skis flat on the snow. Arms will be held in front of the body slightly wider than shoulder
width at approximately navel height. The skier’s weight will be spread evenly between the two skis.
Fore/aft balance is constantly challenged when skiing. The angle of the slope under the foot, the speed of travel and the phases of the turn are always challenging the relationship between the base of support and the centre
of gravity.
At a beginner level the environment is relatively simple and only small changes in pressure or forces are felt from the terrain, but at an advanced level this can become very complex. Fore and aft movements are required for adjusting the body parts as these changes under the feet occur, so that we can keep the centre of gravity in an ideal athletic skiing position relative to the base of support and the intent of the skier. One way of doing this is to move the body back and forth along the base of support by flexing or extending the joints in the ankles, knees, hips and spine. The muscles surrounding these joints control this movement. Another option is to move the base of support back and forth under the body. This can be achieved by flexing the ankles (dorsiflexion) to pull the feet back or by extending the ankles (plantar flexion) to slide the skis forward while the upper body remains stable. Either moving the body fore and aft above the base of support or moving the base of support underneath the body or a combination of both can be used.
At a beginner level it is generally easier to move the body above the base of
support. As skiers advance they can develop the ability to manipulate the base of support.

ROTATIONAL MOVEMENT

Rotational movement occurs when the body or parts of the body move in a circular path around a vertical axis. Parts of the body can move in the same or opposing directions.
In skiing the most common ways the body can rotate are:

Rotating the lower body.

In this situation the femurs rotate underneath the pelvis and each leg becomes the vertical axis. The two points we use to define this axis are the pivot point on the sole of the foot directly under the ankle joint and the head of the femur. Each leg is rotated independently around its axis. Most of the rotation in this axis occurs / originates in the hip socket and is controlled by using the muscles connecting the femur and the pelvis. As the leg rotates from this joint a rotational separation occurs between the femur and the pelvis.
Rotation of the femurs when skiing requires a combination of medial (internal) and lateral (external) rotation and adduction and abduction (the amount of each element required is dependent on the amount of bend in
the leg). Therefore during any given turn one femur is medially rotating and adducting while the other is laterally rotating and abducting. The muscles that are used to create adduction are able to create a stronger turning force than those used to create abduction so the leg on the outside of the turn (which is turning inwards) can create greater turning forces.

Rotating the upper body.

In this situation the spine becomes the vertical axis. The two points we use to define this axis are the cervical spine (neck) and the part of the spine that joins the pelvis (sacrum). The upper body rotates around this axis using the muscles surrounding the spine and mid-section of the body (core area). The rotation occurs along the length of the vertebral column, which results in the separation being incremental between the top and the bottom of the spine. All parts of the vertebral column can rotate to a certain extent but the most rotationally flexible part is the lumbar spine and throughout the thoracic spine.
The goal of rotational movement is to create a change in direction of the skis.
The most effective way to do this is with rotational movements of the lower body because this will have a more direct effect on the base of support.
The benefits of initiating rotational movement with the legs are:
• it has a more direct influence on the ski
• the skier is able to maintain good alignment
• it aids in controlling edge angle
• the rate and intensity can be altered with minimal effort
Good alignment is maintained due to the fact that when the legs are used to initiate rotational movement rotational separation occurs. This means that the legs will turn further than the pelvis and the pelvis will turn further than the upper body. Rotational separation occurs in the hip joint and the lower spine. The pelvis and upper body will follow the direction of the turn but will in effect lag behind the legs and the skis.
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.

 LATERAL MOVEMENT

Lateral movement occurs across the base of support, or from side to side, and
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 goal of lateral movement is to edge the skis and/or to allow the skier to balance on the outside ski.

The two main components to lateral balance are inclination and angulation:

Inclination.

Inclination generally occurs in the initiation of a turn and is created by movement of the centre of gravity away from the base of support or by moving the base of support away from the centre of gravity. This movement begins at the feet and continues up through the body until the whole body is tipped inside the line of the skis. Inclination will move the centre of gravity further away from the base of support than angulation. At higher speeds this allows the skier to maintain a strong aligned position to balance out the forces being created.

Angulation.

Angulation is the result of lateral movements of the parts of the body relative to one another and is achieved by abduction/adduction movements of the femurs and lateral flexion. Angles are created as the lower body moves towards the centre of the turn and the upper body moves towards the outside of the turn or remains stable. Angulation allows the skier to adjust the edge angle without necessarily affecting the degree of inclination. Angulation through the end of the control phase and completion phase also allows the skier to move the centre of gravity towards the base of support (laterally) while maintaining sufficient edge angle to hold the skis in the turn. This aids the transition of the centre of gravity across the base of support as the new turn starts. At lower speeds angulation allows the skis to be tipped on to their edges without the need for the centre of gravity to move too far inside the turn.
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.

VERTICAL MOVEMENT

Vertical movement moves the centre of gravity closer to (flexion) or further away (extension) from the base of support and is controlled by movement at the ankles, knees, hips and spine.
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

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

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 four movements, when performed accurately, work with one another to help skiers achieve the desired result, whether it is a wedge turn or skiing a zipper line through the bumps. This section outlines how the movements interact with each other to create positive results.

FORE/AFT MOVEMENT

When the skier is centred on his/her skis it will have the following positive relationship with the other 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)

ROTATIONAL MOVEMENT

Rotating the legs to turn the skis will have the following positive relationship with the other movements:
• 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)

LATERAL MOVEMENT

Balancing accurately laterally and effective use of inclination and angulation will have the following positive relationship with the other movements:
• 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 MOVEMENT

Moving vertically will have the following positive relationships with the other movements:
• 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

As well as having a thorough understanding of the four movements, it is also important to be able to use consistent terminology when describing how the movements are applied. The movements do not work independently but are always blended together. From beginner level the co-ordination of movements needs to be developed to progressively advance a student. As soon as the skier can turn, each of the four movements is evident in some form. Co-ordinating these movements accurately will allow skiers to advance their skill level as they encounter the forces created by terrain, speed, turn shape, snow conditions and equipment.
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.

1. DURATION

Duration is the length of time a movement is sustained. The skier must choose how long to maintain a movement in any given situation. A simple example of the application of duration within rotational movement is when a shallow wedge turn skier is progressing to a round wedge turn skier. For the turn to become rounder the duration of the rotating of the legs needs to last
longer.

2. INTENSITY

Intensity refers to the amount of power or strength used when a movement is applied. High intensity implies a high degree of application. The intensity of movement used will help control the shape and characteristics of the turn. An example of intensity within vertical movement is if a skier flexes with intensity (muscular tension) through the completion phase of a dynamic medium radius turn he/she will be able to maintain and control pressure on the skis and therefore create more ski performance. A skier flexing passively will not however maintain pressure on the ski and the ski will flatten on the snow and lose performance

3. RATE

Rate is described as the speed with which a movement is applied. The skier must choose the rate at which movement is applied in any given situation.
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.

4. RANGE

Range is the amount of movement which is applied. The skier must choose how much movement to apply in any given situation. Choosing the correct amount will result in precision and good balance as well as controlling the shape and characteristics of the turn. An example of range within lateral movement is that as a skier becomes more dynamic in a medium radius turn, the range of movement of the centre of gravity to the inside of the turn will increase.

5. TIMING

Timing is when a movement is applied at a particular point in time or in a particular phase of the turn. From beginner level through to advanced skiing, the timing of movements will alter. This is because of changes in terrain, snow conditions, speed and turn radius. Correctly timed movements will give the skier rhythm and flow enhancing the ability to stay balanced and in control. An example of timing within vertical movement is when skiers change the timing of the vertical movement while skiing bumps, so they are flexing during the transition of the turn (compression turn) rather than extending, providing better pressure control.