In this article we will analyze how we generate and resist torque through skeletal levers and muscular activation. By understanding torque, we not only optimize energy transmission to the edges while turning but also identify the critical limits for preventing ligament injuries.
The Femurs as Third-Class Levers
In physics and biomechanics, levers are classified according to the relative position of the fulcrum, the effort (force), and the resistance. The femur being a third-class lever means that the muscular force is applied between the fulcrum and the resistance.
The three components in the femur are:
- The pivot (fulcrum): the hip joints (the heads of the femurs within the acetabulum or hip sockets). It is the axis on which the legs rotate.
- The effort (power): the muscles (adductors/abductors). These muscles insert into the femur very close to the hips (greater trochanter and linea aspera).
- The load (resistance): the weight of the skis plus the force the snow exerts against the skis at the end of the limbs (at the feet).
Mechanically, third-class levers have a mechanical disadvantage in terms of force but a great advantage in terms of range and speed of movement:
- Force disadvantage: because the muscle is “attached” very close to the axis (hip), it must exert an enormous amount of force to move the foot, which is far away. Therefore, to generate 10 Nm of torque at the ski, the hip muscles must generate much higher internal force.
- Speed/Arc advantage: a small contraction movement at the base of the femur translates into a very large and rapid displacement of the foot. This allows us to move from edge to edge explosively; a small action at the hip “moves” the ski a considerable distance.
Conclusion #1
The femur is “third-class” lever because the effort is applied between the pivot point (hip) and the load (foot). This has an advantage: speed. Even though the muscles have to exert a lot of force because they are “close” to the axis, a small muscle movement translates into a fast movement of the foot. The femurs sacrifice “brute force” (requiring very powerful muscles like the adductors) to gain edging speed and range of motion, which is essential for turn dynamics.
The Law of the Lever
To calculate the actual force the muscles must exert, we use the Law of the Lever. We mentioned that, as a third-class lever, the muscles have a “mechanical disadvantage”: they are very close to the axis, while the skis are far away.
We also mentioned that the femurs act as the main lever arms to transmit muscular force from the hips to the skis. However, to be technically precise, we must distinguish between two lever arms acting simultaneously:
- The Longitudinal Moment Arm (femur): its function is to transmit adduction/abduction torque from the hip joint to the boot. The longer a skier’s femur, the greater the natural torque generated on the ski with the same muscular contraction, but the load on the ligaments is also greater.
- The Transverse Moment Arm (ski and boot): this is the lateral distance from the edge (fulcrum) to the vertical axis of the leg. Here, the lever arm is composed of the ski’s waist width and the height of the boot/plate.
Stabilizing Torque
In the context of the knee, this term refers to the strength muscles have to brake and control movement, rather than just generating momentum. It is, essentially, the “handbrake” and “suspension” of the legs.
As mentioned, torque is the rotational force acting on a joint (in this case, the knee). Stabilizing capacity is the ability of the muscles (quadriceps, hamstrings, and glutes) to keep the knee aligned and firm, preventing it from “giving way” inward or outward under pressure from snow or a bump.
- Isometric capacity (static resistance): the force exerted when the muscle is under tension but does not change length. It allows us to maintain the correct posture during long runs without the legs trembling from fatigue.
- Eccentric capacity (muscular “braking”): the force exerted by the muscle while it is lengthening under load. This is the most powerful type of contraction and the one that best protects the ligaments. It is crucial because it is the primary defense against ACL (Anterior Cruciate Ligament) tears. If eccentric capacity is low, the muscle fails to dampen the impact against a bump, for example, and all that energy is transferred directly to the bones and ligaments, potentially causing injury.
Conclusion #2
Having a good “stabilizing torque capacity” means the muscles are capable of holding the knee firmly (isometric) and absorbing the shock of a jump or bump without the joint collapsing (eccentric).
Critical Maintenance Torque
The Critical Maintenance Torque (CMT) is the magnitude of the minimum moment of force that the musculoskeletal system (prime mover) must generate to exactly counteract the snow’s resistance torque and the gravitational fall torque, thereby maintaining a constant edge angle. It is the precise rotational force value that the muscles must sustain so the ski does not deviate from its trajectory or lose its inclination angle.
Biomechanically, the CMT is the magnitude of the moment of force generated by the isometric contraction of the lower kinetic chain, necessary to neutralize external torques and maintain the integrity of the edge angle during the steering phase of the turn. In practical terms, it is the “holding force” felt in the hip and adductor while on the arc of the turn. It is not a force for movement, but a force to resist deformation under snow pressure.
Model Variables
- Effort Arm (power arm): distance from the hip joint to the insertion of the adductor/gluteus on the femur. Average: 0.05 m (5 cm).
- Resistance Arm: distance from the hip to the foot (length of femur + tibia). Average: 0.90 m (90 cm).
- Critical Maintenance Torque (CMT): using the torque for a competition ski with a lifter plate as a reference: Average: 16.52 Nm.
In academic terms, torque in skiing is the rotational response of the leg-ski system to external forces. When the moment arm increases (due to a wider ski or a lifter plate), the resulting torque varies proportionally, altering the skier’s mechanical advantage. It is measured in the International System in Newton-meters (N·m).
In the above numerical example, we calculated (16.52 Nm), that was the CMT needed to keep a 65mm ski from moving at a 30° tilt angle.
Load Comparison between Narrow vs. Wide ski
| Ski Type | Torque at the Foot | Actual Force at the Hip | Weight Equivalent |
| Competition (65mm + Plate) | 16.52 Nm | 330.4 N | ~33.7 kg |
| Freeride (110mm) | 38.10 Nm | 762.0 N | ~77.7 kg |
Load Analysis Conclusion
- Effort Multiplier: because the femur is a third-class lever with such a short muscular insertion (5 cm), the adductor must perform a force 18 times greater than the force that ultimately reaches the ski.
- Wide Ski Impact: switching to a freeride ski causes the hip muscles to go from holding 33 kg to sustaining nearly 78 kg of constant tension per leg during the turn.
- Fatigue: this explains why, even for strong skiers, the adductors are often the first muscles to “burn out” or cramp in heavy snow or with wide skis on groomers.
Final Conclusions
The biomechanical system for generating torque possesses the following characteristics:
- There is a distinction between Rotational Torque (skidding) and Maintenance Torque (carving).
- The femurs constitute the primary moment arm in the frontal plane and they are levers of speed, not of force.
- The length of the femurs and the angular orientation relative to the pelvis determine the magnitude of the torque that the adductor and abductor muscles can project onto the skis’ edges to overcome surface resistance.
- While torque is “felt” in the feet, it is generated at the level of the femurs rotating within the hip sockets.
- Torque is not the product of a single muscle, but of a rotational chain working in a coordinated manner.
- We sacrifice a significant amount of muscular energy to move the feet with speed and precision.
