While Anatomical terms of motion describes all possible movements of the body, let's narrow down on those that are useful for skiing and relate them to actual skiing movements and techniques. These are the basis for biomechanics, afterall.
Also, note that while the motion is usually analyzed in the context of the plane of the respective joint, we will look at the planes of movement in relation to the skier, aka the entire body, as it goes through the turn. For instance, flexion/extension which are mostly in the vertical plane, will continue to be in the vertical plane even as the skier is in the middle of a turn with the skis edged 80 degrees (hip to snow). The planes of movement are relative to the ski plane. This is often a point of misunderstanding...
Here's a tip: when analyzing or learning skiing, always start from the snow up, as we will do below.
These are at the movements at the base of the "kinetic chain" and are then complemented by moving the knees and femurs (active or passive). We use these to put the skis on and off the edge and fine balance adjustments.
Continuing with the movements of the lower body, we have Dorsiflexion and plantarflexion:
These, as you can guess, are very useful for fore-aft and pressure adjustments. Dorsiflexion, i.e. "closing" the ankle will tend to put the hips more forward, while plantar flexion, i.e. "opening" the ankle will put the hips further back, depending on how flexed the leg is. These are a big part of why Keep your feet underneath you is important.
When in closed chain mode, i.e. the skis are weighted, these movements are not very strong, as we try to leverage the skis and boots to bring the body forward or back. In open chain mode, when the skis are unweighted, these can have a large impact on fore/aft adjustment, together with with moving the boots fore/aft.
These two movements are also important for pressure application and fine adjustments. One could apply more pressure to the tips with plantar flexion (i.e. "pedal") but the reaction to that will be to move the hips even further back, so it should only be a temporary adjustment.
Here's a detailed animation and explanation of the ankle movements:
Flexing the knee is not a movement that happens in isolation - we need to flex/extend the ankles and hips as well, to stay in balance and keep our feet underneath the hips. We call this entire set of movements flexing, as in flexing the leg and we call its opposite: extending the leg. In normal every day chat, you'll hear these referred to as short leg and long leg, i.e. flexed and extended.
At the femur, we have:
Also at the femur, we have:
The Femur rotation can be passive or active and it also has multiple effects, depending on the other movements. For instance, rotating the femur with a long leg will result in pivoting the skis while rotating the femur with a flexed knee results in more edging (presumably while carving at big angles, too):
It is important to note that the flexing and extending of the legs are independent movements of each leg... you can see both in the photo above. They're not just moving the COM (Center Of Mass) in relation to the BOS (Base Of Support), but sometimes simply the feet in relation to the hips (as the skier is on the outside ski exclusively, so flexing the inside ski has nothing to do with BOS) or just allow the hips to drop/move inside and increase edge angles, as above (again, flexing the inside leg).
Here's a trick question: above... did the femurs rotate and the tibias abduct and adduct or did the upper body move laterally? It does look like the legs moved under a stable upper body, doesn't it?
Is the femur rotation there passive or active? Is there any inversion of the foot? Do you see any dorsiflexion? You may be surprised what it takes to ski with good ski performance! (The answer is yes to all those questions!
With the huge range of motion at the hip joint, the hips and femurs play a central role in skiing, with flexing and extending (folding or hunching at the hip) as well as sideways.
Out of the many things the components of the upper body can do, the movements important to skiing as far as the upper body is concerned are:
Separation of the lower body from the upper body is a critical principle for effective skiing and we minimize stress and bending of the spine, so the upper to lower body decoupling should occur mostly at the pelvis, between the femurs and the hips.
What do you think? That stress you feel in your core and upper body to keep it upright at the apex - is that passive?
The frame of reference is not just space. It's also about time and leverage - they make a big difference.
The upper body is connected to the lower body, so the most natural thing for it to do is to follow the lower body: if the skis turn, the shoulders tend to turn as well. If the legs incline into a turn, the body tends to incline.
To create or maintain separation, the upper body must actively counter the lower body.
Here are some of the important muscles and their uses in movement.
The functions of the glutes includes extension, abduction, lateral(external) rotation and medial (internal) rotation of the hip joint. The gluteus maximus also supports the extended knee through the iliotibial tract. (source: Wikipedia).
The calf muscle is involved in stabilizing the ankle and plantar flexion.
The tibialis anteriori muscle is very important, as it is responsible for both dorsiflexion and inversion, in open chain mode. In closed chain mode it helps balance the leg.
The kinetic chain is the interconnection of all the body's elements and how they move and affect each-other.
One important thing to note is that movements at different joints have different effects (and take different muscle activations) when performed in open chain mode (i.e. the joint is not under weight) versus closed chain mode, where the joint is weighted - especially for the feet and ankles.
For instance, am I doing a low-energy turn, where flexing is more like a squat, i.e. relaxing the feet allows the body to drop? Or am I in the transition between two high-performance turns, where I am unweighted and flexing is more like retracting the feet up?
Understanding and focusing on the movements that create expert skiing is very important for any skier. Decomposing great movement into components and working on each component, to learn it and refine it, is how we evolve into expert skiers.