This is an excerpt from Ultimate Skiing by Ron LeMaster.
The Skier's Frame of Reference
You might think that the proper description and analysis of the forces of skiing are universal—that they should be the same for someone skiing down the hill as they are for someone on the side of the hill watching the first one go by. It isn't so. Take the concept of centrifugal force. To the stationary observer, there's no such thing as centrifugal force acting on the moving skier. A complete and accurate analysis of the moving skier's motion doesn't include such a force. But for a skier making a turn, centrifugal force is entirely real, meaningful, and measurable. The difference is that the two people have different frames of reference: One is moving along a curved path, and the other is not. We will consistently choose to view the world through the eyes of the skier in motion because that reflects our personal understanding of the sport. We understand it from how it feels while we're making turns down the hill, not from how it looks when we see someone else doing it.
Biomechanical descriptions of human motion are generally based on a formally defined frame of reference, and ours will be too. For that definition we'll use the planes of the body defined by conventional biomechanics and physiology, as shown in figure 1.17, and anchor them on an axis specially chosen for describing skiing.
The axis that anchors our framework is a line through the skier's center of gravity that's aligned with the resultant of gravity and centrifugal force acting on the skier. You might notice that when the skier is in balance, this line is the same as the line of action of the force from the snow. We'll call it simply the balance axis, and we'll refer to it throughout the book. As previously mentioned, this axis goes through the skier's center of gravity, somewhere in front of the navel, and it's constantly tilting one way or another as the skier goes in and out of the fall line and through turns (figure 1.18). About the only time the balance axis is aligned with gravity is when the skier is in a straight run on a perfectly flat surface or in a perfectly flat traverse.
The planes we use are the frontal, transverse, and sagittal planes. Again, see figure 1.17. The sagittal plane divides the skier into left and right parts and goes through the balance axis. The frontal plane divides the body into front and back halves and also goes through the balance axis. Finally, the transverse plane divides the skier's body into upper and lower parts and is perpendicular to the balance axis.
You can easily feel the balance axis when you're skiing. It's the line along which all the pressure on your body and skis is focused when you're arcing a turn, setting your edges on a steep slope, or skiing into a mogul. Even when you're not skiing, you can feel it. Stand and rock from one foot to the other, and from your heel to your toe. The spot where you feel the maximum pressure on the soles of your feet tells you where the balance axis is fore and aft in the sagittal plane. If the pressure is equal on both your feet, the balance axis is right between them in the frontal plane.
This is an excerpt from Ultimate Skiing.