Gravitational Torque occurs when gravity pulls on an unsupported body part, creating a rotational force around the joint that acts as a tipping force against our skiing stability.
Gravitational Torque vs Gravitational “Fall” Torque
In physics, gravitational torque is the general term, while “gravitational fall torque” is a way to describe torque in the specific context of falling or toppling. Here is the breakdown of the nuance:
1. Gravitational Torque (The General Concept)
This is the rotational force exerted by gravity on any object or body part that is not supported from directly above or below its center of mass.
- It’s always happening: even if we aren’t falling, we are experiencing gravitational torque. For example, when we hold our arm out to the side, gravity creates torque on our shoulder, but our muscles “cancel it out” so we don’t move.
2. “Fall” Torque (The Result)
When people use the phrase “fall torque,” they are usually describing the moment where gravitational torque wins.
- The tipping point occurs when our center of gravity moves outside our base of support (our feet). At this point, the torque is no longer being balanced by our muscles or our stance and it causes a rotation that results in a fall.
Gravitational Torque vs. Inclination
To calculate Gravitational Torque, we must understand how our weight projects outside their base of support as inclination increases. Torque is the product of the force (weight) and the horizontal moment arm (the distance from the ski edge to the line of action of the weight).
As we lean, the moment arm increases proportionally to the sine of the angle. This means that at greater inclinations, gravity has “more leverage” to pull us toward the snow. For an 80 kg skier, the torque attempting to “flip” the skier into the turn grows non-linearly:
- 15° (Initiation): the torque is approximately 203 Nm. It is relatively easy to compensate with minimal support.
- 45° (Carving): the torque rises to 555 Nm. At this point, the moment arm is 71cm, requiring powerful centrifugal force and adductor torque to avoid collapse.
- 60° (Competition/Extreme): the torque reaches 680 Nm. Here, gravity pulls with nearly triple the force than at 15°, explaining why GS racers require exceptional physical conditioning of their kinetic chain.
Gravitational Torque and Forward/Inward Inclination
Moving our body forward or sideways changes the “wrench handle” (moment arm) in different ways to control our skis:
- Leaning Sideways (The “Tipping” Move)
When we move our weight to the side, we create a gap between our center of mass and the ski edge.
- The Goal: to engage the edges and turn.
- The Physics: this creates the torque that pulls us into a “lean.” The further we lean, the more gravity helps us carve.
- The Risk: if the gap is too wide and we don’t have enough speed (centrifugal force) to balance it, we simply fall over inward.
2. Leaning Forward (The “Pressure” Move)
When we move our weight forward or backward, we create a gap between our center of pressure and the middle of the foot.
- The Goal: to control the length of the ski.
- The Physics: by leaning forward we move our weight toward the tips of the skis. This makes the front of the ski bite into the snow, helping us steer or carve the turn.
- The Risk: if we lean too far back (creating a gap toward the heels), the tips of our skis get “light” and we lose control of our steering/carving.
The Secret: Combining Both
The best skiers do both inclinations at the same time:
- They move sideways to get the ski on its edge (using the moment arm to tip).
- They move forward to make sure the front of that edge stays locked into the snow.
- Think of it like this: sideways controls the angle of the turn; forward controls the accuracy of the turn.
Conclusion
The Gravitational Torque is the moment of force resulting from the action of gravity on our CoM when in an angulated position while turning. It acts upon the axis of the ski edge, tending to increase the medial inclination of the system, and must be counteracted by the combination of centrifugal force and the stabilizing torque of the lower kinetic chain.
In other words, it is the force we feel “sucking” us toward the snow on the inside of the curve. If our muscular torque is less than Gravitational Torque (plus snow pressure), our kinetic chain collapses, resulting in an inside fall. Our job is to constantly adjust our body position so that this “inward pull” exactly matches the “outward push” of the turn.
The reason why we specify the “lower kinetic chain” rather than the whole kinetic chain is because the physical battle against Gravitational Torque happens primarily between the skis and the hips.
In skiing biomechanics, the “lower kinetic chain” (feet, ankles, knees, and hips) performs a different mechanical role than the “upper kinetic chain” (torso and arms) during a high-speed turn.
1. The Point of Contact
The torque mentioned acts upon the axis of the ski edge. Because our feet are the only part of our body connected to that axis, the forces are immediately transmitted upward through our lower joints. The lower kinetic chain must remain rigid and powerful to hold the edges against the snow while gravity tries to pull us inward.
2. Separation of Roles (Angulation)
In turning at moderate to high speed, we use angulation. This means:
- Lower Chain: acts as the “active” engine and support structure. It manages the lean and resists the twisting forces.
- Upper Chain: acts as a stabilizer and “counter-weight.” Its job is to stay relatively quiet and balanced to maintain the center of mass (CoM), rather than directly fighting the rotational torque at the skis’ edges.
3. Energy Dissipation
If the “whole” chain were used to counteract the torque, our entire body would be under maximum tension. By focusing the stabilizing torque in the lower chain, we can keep our upper body flexible enough to adjust to terrain changes and maintain a balanced line.
Summary: the lower kinetic chain is the “front line” of defense. While the whole body is involved in skiing, it is specifically our legs and hips that must generate the internal force to stop gravity from “folding” our legs toward the inside of the turn.
