In the physical system of skiing, skis, lifters, and boots act as a critical coupling mechanism between the lower legs (the levers) and the skis-boots system (the load).
Skis
To relate torque transmission in skis, we will compare the resistance torque a skier must overcome to keep the ski on its edge. We will use two types of skis with market-standard dimensions:
- Ski A (Competition/Slalom): Narrow waist width of 65 mm.
- Ski B (Freeride): Wide waist width of 110 mm.
1. Comparison of Results
| Feature | Competition Ski (65mm) | Freeride Ski (110mm) | Difference |
| Lever Arm | 32.5 mm | 55 mm | +69% |
| Resistance Torque | 22.52 Nm | 38.10 Nm | +15.58 Nm |
2. Biomechanical Conclusion
When switching from a standard ski to a wide ski, the torque trying to “flatten” the ski increases by nearly 70%. This means that:
- Muscle Fatigue: The abductor muscles (gluteus medius) and ankle invertors must exert 69.2% more effort to maintain the same edge angle.
- Snow Leverage: On hardpack or ice, a wide ski feels much “heavier” and harder to edge because this type of snow has a much longer lever arm against the ski boot.
- Use of Lifter Plates: This is why wide skis rarely use high lifter plates; the total torque combined with the waist width would place excessive strain on the knee ligaments.
Lifters
The use of lifter plates under the binding drastically alters turn biomechanics by modifying the vertical lever arm. Here is a detailed breakdown of how this affects the ski behavior:
1. Increased Lever Arm
In the formula for torque applied to the joint, the variable represents the distance from the edge (pivot point) to the axis of rotation (ankle/knee). By adding a plate, we add its height to the system: the higher the plate, the more torque the skier can generate on the edge using the same muscle force. This creates a force multiplier effect (mechanical advantage).
2. Relationship with Edge Angle
The plate allows for more aggressive edge angles without boot-outing (dragging on the snow). By increasing the height with a plate, the critical angle increases. This allows the skier to tilt the ski further and, therefore, carve shorter-radius turns with greater grip on ice.
3. Modification of Reaction Torque
While the plate facilitates edging, it also increases fatigue torque. When the ski hits a bump or vibrates, the impact force is transmitted through a longer lever arm. Consequently, the skier feels that the ski is more “aggressive” and harder to tame. The adductor muscles and the gluteus medius must work harder to stabilize the leg against this increased torque.
4. Summary of Variations
| Variable | Without Plate | With Lifter Plate |
| Lever Arm | Smaller (boot only) | Larger (boot + plate) |
| Ease of Edging | Requires more physical effort | Requires less initial effort |
| Maximum Angle | Limited (boot hits the snow) | Much larger |
| Lateral Stability | More stable / Less sensitive | More unstable / Requires more stabilizing force |
| Precision | Lower | Maximum (typical for racing) |
5. Comparative Analysis
The resistance torque for the 65 mm normal ski is 22.52 Nm, where “Nm” stands for Newton-meters. It is a standard scientific unit used to measure torque, which is rotational or twisting force.
When using a plate, the skier gets a mechanical “boost”:
| 65mm Ski | Resistance Torque | Leverage Torque | Net Muscle Effort |
| Without Plate | 22.52 Nm | 0.00 Nm | 22.52 Nm |
| With Plate (15mm) | 22.52 Nm | 6.00 Nm | 16.52 Nm |
6. Conclusions on the Plate’s Effect
- Effort Reduction: The 15 mm plate reduces the muscular demand required to hold the edge by 26.6%. This reduction in net muscle effort is calculated by dividing the 6.00 Nm decrease in required torque (resulting from the plate) by the initial 22.52 Nm of effort required without a plate (6.00 / 22.52 = 0.2664). This figure represents the percentage decrease in muscular demand when moving from the base ski setup to the setup with a 15mm plate.
- Mechanical Advantage: The skier can now drive the edge into the snow much more easily. This is why high lifter plates are used in speed disciplines (Downhill/Super-G): to handle immense pressure with less fatigue.
- The Biomechanical “Cost”: Although the plate “helps” initiate the turn, it increases the twisting torque on the knee during a lateral impact. The lever arm works both ways.
Conclusion
The Net Torque felt by the skier is the snow resistance minus the assistance of the vertical lever arm. As the angle increases, the sine function increases, making the plate more effective the further we lean the ski. In the next article, we will analyze the muscle fatigue paradox when using plates.
