This is an excerpt from Ultimate Skiing by Ron LeMaster.
General Alignment and Stance Principles
A few biomechanical principles will help us understand what makes for good alignment and stance. Taken on their own, they are basic principles of good skiing.
The Balance Axis is the Reference for Alignment. Balancing against the forces of skiing means balancing along the balance axis. While gravity plays a part in where the balance axis is, it's only one of several factors, so a true vertical line is not of much use for us here.
Each Segment of the Body Balances on the One Below It. Your torso balances on the heads of your femurs. Your torso, hips, and upper legs balance on top of your knees. Your torso, hips, and legs balance on your ankles and feet. When you're in a turn on firm snow, most of your body's mass is balanced on one leg, knee, and ankle. Depending on your alignment, different muscles are called on at each of the major joints to do the work of supporting and balancing the body segments above it. Good alignment enlists the strongest and best muscles.
The Upper and Lower Body Must Work Independently. The lower body—the legs and feet—plays a distinctly different role from the upper body (that portion from the hips up). You ski with your legs. You balance with your upper body. On average, about 65 percent of a person's mass is contained in the upper body and about 35 percent in the legs and feet. Looking at a skier from the front, the center of gravity is located somewhere just above the hips. As a result, small movements of the upper body have a disproportionately large effect on a skier's balance and the distribution of force and pressure on the skis. That's why the best skiers keep their upper bodies quiet. They put their torso over the part of the ski they want to apply force to, and they don't move it much. If they want more force on the outside ski, the upper body goes over that ski. If they want even pressure between their skis, the upper body is in the middle.
The best skiers use their legs, whose joints have many degrees of freedom, to manipulate the skis underneath the massive, quiet upper body. The legs turn the skis, control their edges, and adjust their fore-aft position relative to the skier's center of gravity.
The Legs Must Work Independently. Racers have always known that the functionally best technique in skiing is based on using their legs independently. Like the independent suspension on each of the wheels of a high-performance sports car, each leg must respond in its own way to variations in the shape of the snow surface. When you're in a turn, you apply force to the inside edge of one foot and the outside edge of the other, and your inclination requires your inside leg to be bent more than your outside leg. Any element of stance that prevents them from being able to do their own thing hinders you.
The Hips Must Be Stable and Well Controlled. Some of the best coaching methods focus on the hips: “Move your hips forward into the turn.” “Stabilize your hips.” “Keep your hips over your feet.” The hips play a central role in skiing because they are, literally, central to your body. The hips are where the upper and lower body meet, and if the upper and lower body are to work independently, the muscles around the hips must constantly contract and relax in a coordinated fashion to adjust your balance and manipulate your skis while adapting to changes in external forces. These muscles support and balance the mass of the upper body over the heads of the femurs, and good alignment in the midbody is largely a matter of arranging the hips in the sagittal, frontal, and transverse planes so that the strongest, most effective muscles are used.
The hips are also near your center of gravity. So it's more than poetic license to say that your body revolves around them. If they move erratically, your entire body moves that way, and your center of gravity with it. Be careful, though, not to confuse your hips with your center of gravity, because it's the latter that determines your state of balance, not the former.
A lot of skiing happens in the ball-and-socket joints where the femurs meet the pelvis. Some of their movements are controlled by the biggest muscles in the body, and their three degrees of freedom and range of articulation give them great versatility. Having so many options for movement provides you with many ways to accomplish any particular task, but only a few of them are optimal. Consequently, the alignment of the hips in relation to the legs and upper body, which is one of the main subjects in this chapter, plays a central role in ski technique.
You Must Be Able to Act and React From a Neutral Stance. Skiers can learn a lot about stance by watching athletes in other sports. A football linebacker defending against an oncoming opponent provides a good model for a skier's neutral, home-base stance. The linebacker needs to be in a posture from which he can move in any direction as quickly and powerfully as possible, and take a hit without getting knocked out of balance. This stance is flexed at all the major joints. The hands are at the same height, about halfway between the hips and shoulder. The athlete's balance is on the balls of his feet. With a couple of tweaks—the stance is generally narrower and the balance maybe a hair farther back—this is the basic neutral stance of an athletic skier.
Each of the movements of skiing has its own range. A skier's vertical movement, for example, can range from standing almost bolt upright to a low tuck or lower. There's a point within each of these ranges that we can think of as its neutral point: a place the skier will go to to be ready to move in one direction or another within that range. The collection of all those neutral points defines a skier's overall neutral stance or home base.
Good skiers continually pass through the neutral points of various movements when they're skiing, but they don't linger there, and there are few moments in dynamic skiing when all movement patterns are in their various neutral points at the same time.
This is an excerpt from Ultimate Skiing.