Downhill skiing mechanics

- Physical requirements during downhill skiing

Let us take a look at some physical basics of downhill skiingSkiløb handler naturligvis om at kunne styre sine ski, og en forudsætning herfor er, at man kan holde balancen..   Downhill skiing is about keeping your balance by resisting the external forces that act on your body. 

In the vast majority of skiing situations, it is gravity, which is the moving force, and our muscles have to create the opposite effect so we don’t get “pushed around.” It is only in special cases that we allow the muscles to create a moving force when skiing. If for example we want to jump or otherwise defy gravity. We can also save a crash by creating a moving force in the opposite direction of where we are about to fall. But when downhill skiing, we mainly use our muscles to help us stay stable and not get pushed around.

The safest way to create stable situations is to ensure that the gravity force line is kept within the supporting surface. When this cannot be done, we need to rely on the stregth and reactivity of our muscles.

How much strength is needed depends on the terrain, and how aggressively we are skiing.  Steep terrain, high speed and actively putting pressure on the ski and driving it are all factors that will increase the reaction forces at play. Other factors such as heavy snow, skiing in the backseat and deep snow also requires more of our muscles.  Of course you decide yourself how much intensity you will put into your skiing and where you want to go – provided that you are the one in harge, and not your skis!

 Downhill skiing – lateral plane:

In the picture to the right you see the external forces that act on the skier in the lateral plane:

• På billedet til højre ses de eksterne kræfter, der virker på skiløberen:Reaction Force from the ground (represented by the yellow arrow)

•  Tyngdekraften (repræsenteret med den orange pil).The effect of gravity (represented by the orange arrow).

As you can see, the gravity force line (the vertical red line) falls outside support surface (blue surface). If we remove the green arrow representing combined muscle force, we will fall back into the mountain and the center of the turn.  

It is thus muscle strength (the green arrow), that has to counteract the external forces to keep the skier in balance.

More precisely we need muscle strength in the legs to put pressure on the skis (especially the external ski), while we seek to edge and carve.

We also need strength in the hip muscles (especially gluteus medius and minimus) to keep the angle in the hip that helps getting your gravity line closer to the supporting surface and thus create better stability. Without angling the hip, our skis will tend to slide away beneath us and we will fall into the hill.  

Billedet ovenfor begrænser sig til at vise, hvad der sker i det transversale plan (sidevejrs), men skiløberen påvirkes også af kræfter i det sagitale plan (profil), som det fremgår af nedenstående billede.The picture above is limited to show what happens in the transverse plane, but the skier is also affected by the forces in the sagital plane (antero/posterior), as you can see on the following picture.

Skiskolen på langs:Downhill skiing – antero/posterior plane:

In this plane, the supporting surface is much longer because of the length og the skis. This helps us from falling forward or backward. It does however give you a bit more length to control.  It can for instance be hard to control the front of the ski if your weight is far behind and vice versa.Forvægt / bagvæg

I  dette plan har vi en forstørret understøttelsesflade takket være skienes længde. Det hjælper naturligvis mod at falde forover eller bag 

This is why you want to keep you center of gravity somewhat over your feet (blue surface) and not to far back, nor too far in front. 

For at opnå en god balance og god styring af skiene skal tyngdelinien gå igennem føddernes understøttelsesflade (den blå flade). Når den går bag fødderne taler man i skiskole-sprog om bagvægt og når den går foran, taler vi om forvægt.When it goes behind your feet, it is called “being in the backseat” and when it goes in front, we are talking about being forward.  

På billedet holder skiløberen tyngdepunktet over fødderne, men det ses til gengæld at underlagets reaktionskraft (den gule pil) virker foran tyngdepunktet og vil dermed skabe et rotationsmoment mod uret omkring tyngdepunktet.  Det medfører at overkroppen vil blive skubbet bagover og fødderne fremefter og dermed forårsage bagvægt, hvis ikke musklerne står imod.  De skal også i dette plan arbejde sammen for at modvirke resultanten af de eksterne kræfter.  Den resulterende muskelkraft skal således virke som den grønne pil for at holde en stabil position.  Det kan gøres effektivt ved at arbejde excentrisk med de muskler, som strækker hofterne og knæene.

On the picture the skier keeps The upper body will be pushed backward and feet forward and the skier would end up in the backseat, if she didn’t use her muscles to resist this movement. The combined muscle force needs to act as the green arrow to keep a stable position. This can be done effectively by working hip and knee extensors eccentrically. That means, contracting these extensor muscles so as to resist the hips and knees being flexed by external forces.

You can make downhill skiing significantly easier for yourself by preparing your body with the right physical training from home. Once you are at the ski resort, you have a lot to win by hiring a ski instructor and take few ski lessons once in a while to get some proper feedback on your downhill skiing technique.