BIOMECHANICS
The mechanics of biological and especially muscular activity
(as in locomotion or exercise) (Merriam-Webster)
The physical forces that affect human […] movement or the study of these forces (Cambridge Dictionary)
BIOMECHANICS IN SKIING
FUNDAMENTAL PRINCIPLES
SIDENOTES
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This is especially relevant to skiing.
A skier carving a turn may keep a steady speed, but because their direction is constantly changing, they are continuously accelerating.
According to Newton’s Second Law (F_net = m × a), the greater the unbalanced force, the greater the acceleration.
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Gravity is also a form of acceleration. Near Earth’s surface, it pulls objects downward at roughly 9.8 m/s².
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In skiing, gravity provides speed but does not directly cause the skier to turn.
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Deceleration is also a (negative) acceleration.
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Acceleration is the rate of change of an object's velocity with respect to time.
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A key consequence is that an object moving at a constant speed but in a changing direction—such as a skier carving a turn—is continuously accelerating.
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Acceleration is always caused by an external (unbalanced) force,
as described by Newton's Second Law (F_net = m × a).
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This law establishes a direct relationship: a larger net force results in a larger acceleration, meaning a more rapid change in velocity.
F = M · a
No acceleration, no force:
F = m · 0
F = 0
a = acceleration
m = mass
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Linear momentum’s direction is straight (never curved).
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Momentum describes “mass in motion”.
It is the product of mass and velocity, and because velocity has direction, momentum does too.
As such, it is an essential physical factor in skiing.
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A skier’s momentum depends on both size and speed.
The heavier or faster they are, the more momentum they carry.
This makes them harder to stop or redirect—a crucial reality in skiing.
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A bigger mass with the same velocity has more kinetic energy.
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The same mass moving with a higher velocity has more kinetic energy.
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Inertia is the resistance to changing motion, tied only to mass.
It is the tendency of an object to resist any change in its state.
Momentum is the actual motion that must be managed to stop or turn.
It is a tendency of an object to keep moving.
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Kinetic Energy: the energy of motion, which also increases with mass and speed but is a different measure than momentum.
SIDENOTES
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In practice, a larger skier—or one skiing faster—has more momentum and more kinetic energy. That makes control, turning, and stopping more demanding.
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We don’t perceive kinetic energy when we are moving in a straight line with a constant speed. However, as soon as the direction or speed changes, this energy can be transformed into enormous force effects:
- A small bullet can cause huge damage.
- Biking at 20-25 km/h is a calm and comfortable ride.
Hitting a car at the same speed can cause serious injuries and ruin the bike.
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In reality, every separate (loosely or indirectly connected) part of an object has its own velocity, momentum, and actions.
F1 racers have strong necks to keep their head when cornering under 4-5G’s load.
Ordinary people in an F1 car or fighter jet wouldn’t even be able to keep their hands on the steering wheel or yoke, let alone operate them, because the forces move their limbs in turn differently than their strapped-in bodies. Skiing can be a bit similar to this.
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In skiing, we experience momentum mostly in turns, which is why we may have the impression that momentum also acts on a curved line, but it is just an illusion.
Momentum always acts in a current straight direction.
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70-80% of the body mass is far from the skis, therefore, this part of the body have greater kinetic energy
The vast majority of the masses are connected to the ground through many joints, giving them varying degrees of freedom of movement.
That’s why the upper body, like every other object on the slope, tends to travel close to the fall line, even when the ski begins to turn.
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While skiing, the majority of the force effects come from momentum, not from the direct effect of gravity.
No matter how we move a body part, it will not have a direct effect on the forces on the ski unless it influences the effects of momentum later.
Posture does not directly cause significant force effects only if it modifies momentum.
The role of posture is to manage the forces caused by momentum appropriately.
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The skier’s mass is accelerated by gravity near the fall line.
If the ski starts to turn due to its contact with the snow, the body will not automatically follow it; instead, it will continue to move downhill, which can easily lead to a fall, especially in deep snow (this is the most typical type of fall in powder or slush).
Therefore, many skiers prefer to reduce the ski’s turn radius and let it skid in the direction of the body’s movement (this is the most common reason for losing control).
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Ski technique and posture are the expedient connection between ground contact and large masses.
The human mass distribution by body parts (approx.):
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Velocity is the rate of motion in a given direction.
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Unlike speed, which only tells us “how fast,” velocity also specifies “which way.”
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Velocity changes if either the speed or the direction changes.
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If velocity changes, we must assume an external (unbalanced) force causes the object to change its state.
SIDENOTES
Any change in velocity—whether in speed, direction, or both—is called acceleration. Newton’s First Law tells us that without an external, unbalanced force, an object will keep moving in the same way indefinitely. So whenever velocity changes, we know a force is at work, altering momentum (mass × velocity) and shifting the object’s state of motion.
Athletes in the gym may perform their exercises without displacement, so velocity is not a significant factor in such sports activities.
In alpine skiing, velocity is a fundamental physical factor.
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A force is simply a push or pull—an interaction that changes motion or shape.
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A force is an influence that causes an object to undergo a change.
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Forces can:
Change motion (start, stop, speed up, slow down, or turn).
Cause deformation (bend, compress, or stretch an object).
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If all forces are balanced, an object is in equilibrium:
- It stays still, or
- It moves in a straight line at a constant speed.
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Acceleration—any change in velocity—only happens when forces are unbalanced.
SIDENOTES
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If the motion changes, an external force must be assumed to be acting.
If an external force acts, the motion will change.
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A force effect can also deform an object.
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“Whenever there is an interaction between two objects, there is a force upon each of the objects.
When the interaction ceases, the two objects no longer experience the force.
Forces only exist as a result of an interaction.”
www.physicsclassroom.com/class/newtlaws/lesson-2/the-meaning-of-force
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If somebody "puts weight to the outside ski," then they move mass vertically over the ski, but it is not about changing the force effects or the load on the ski in a turn. In this case, it is an incorrect use of words.
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A turning skier cannot be in balance (in equilibrium) in a physical sense because the turn is caused by an unbalanced external force.
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„Balance” in movement suggests that the effects acting on a skier are balanced.
A turn can only happen because there is an unbalanced force.
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Eliminating the unbalanced force, balancing the external force effects prevent the turn itself.
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„Balancing” is more about equalizing the circumstances themselves than adapting to dynamic, unbalanced conditions.
This is what I see with ordinary skiers: they try to be balanced because they feel safe in it, so they drift instead of turning.
They want to balance the circumstances.
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In contrast, skiing is unbalanced. This is a physical fact.
It is better to accept the imbalance and develop methods for stable behavior under unbalanced conditions.
FORCE & PRESSURE
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Pressure is the perpendicular force per unit area. (Encyclopædia Britannica)
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Pressure can be interpreted together with the surface area
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The pressure is present due to the forces acting on the ski and the skier.
A skier does not create pressure.
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Pressure is an existing circumstance, not a goal for a skier.
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Too little pressure is not typical for skiing.
Too much pressure can be more of an issue.
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The skier cannot essentially influence the amount of pressure, only its distribution over time during the turn and on the surface of the ski (e.g., front-back).
SIDENOTES
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The ski bends to its native radius even under a small force (a fraction of the body weight is enough for this).
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The ski cannot bend below a certain level.
- In general, only a small fraction of the forces acting on the skier are needed to bend the ski.
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The primary function of the forces on the skier is to direct the skier into the turn.
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An efficient skier strives to use the minimum force required for the turn, nothing more.
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Pressure is more of a feedback or signal than an intention, something the skier pays attention to but does not create.
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How much pressure do you need to bend a ski?
Practically none. At least none in the middle.
The pressure occurs on a surface.
As long as the ski is not bending, the middle of the ski is barely in contact with the snow, so there is no real pressure either. There is pressure at the two ends.
When it bends, the middle is also supported by the snow, the pressure increases and equalizes with the two edges, and you can’t really bend any further.
So the pressure in the middle of the ski prevents bending. The middle of the ski bends as long as there is not too much pressure on it.
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Mass is the amount of matter of an object.
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Weight is a force exerted on an object due to gravity.
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In general, flexion-extension movements are considered simpler in terms of movement coordination.
In skiing, the primary function of these movements is adaptation to the conditions (inclination, terrain) and adjusting the joint position for more complex movements (rotation, abduction-adduction).
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Rotation is more complex and important in skiing.
The ski and the turn generate and require many rotational effects, which can be controlled by rotational movements.
In simple terms, there are four primary rotational levels:
- ankle (foot-shin);
- knee (shin-femur);
- hip (femur-pelvis);
- dorsal spine (pelvis-shoulder).
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Abduction-adduction are the movements when a limb gets further away from the midline or gets closer to the midline.
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In skiing, there are many external forces that impact the foot from the side.
If the skier cannot manage these, they compromise the connection between the ski and the body, making the ski’s behavior or the skier’s posture unstable.
SIDENOTES
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Extended limbs:
- Less effective levers for rotation, or completely unable to rotate. There is only one way to move: flexion. -
Moderately bent limbs
- Effective leverage, better rotational conditions, better freedom of movement. -
Extreme flexed limbs
- The joints are exposed to injury. Less effective muscle power (contraction insufficiency). There is only one way to move: extension -
Active flexion: Active use of flexor muscles (e.g., hamstrings, hip flexors).
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Passive flexion: Flexion due to external forces (e.g., gravity, weight).
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Each joint can move under the influence of external forces if they aren’t stabilized.
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Stabilizing only one level is not enough; the adjacent levels must also be fixed, sometimes in the opposite direction, resulting in transverse stabilization.
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The difficulty of rotational movements lies in the fact that the main joints lack dedicated rotators.
Most muscles can also rotate in addition to their primary function.
For example, the rotators of the knee joints are the hamstrings and the calf muscles, which bend the knee or perform plantarflexion in the ankle as a main task.
Therefore, rotation typically occurs in conjunction with other movements.
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In skiing, many external forces impact the foot from the side. If the skier cannot manage these, they compromise the connection between the ski and the body, making the ski’s behavior or the skier’s posture unstable.
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On the outside ski, the adductors are important. The difficulty here is that these muscles also have a rotating effect on the femur, which must be appropriately neutralized with the glutes.
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On the inside ski, the abductor muscle groups play an important role.
The problem here is that the leg does not really have abductor muscles.
The hip muscles also have a relatively weak abductor effect in addition to their primary functions.
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Another difficulty with both the rotator and abductor-adductor muscles is that their function changes with different joint flexion angles.
They don’t work the same way for higher or lower postures, which makes it difficult to learn the movement in different conditions.
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There are four primary rotational plains:
- ankle (foot-shin);
- knee (shin-femur);
- hip (femur-pelvis);
- dorsal spine (pelvis-shoulder).
(Each can move under the influence of external forces if they aren’t stabilized.)
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Examples:
Rotators of the knee joint == knee flexors (hamstrings, sartorius, calf muscles).
Their main functions include knee flexion and plantarflexion of the ankle, among others.
That's why rotation mostly happens in conjunction with other movements.
In the case of the outside ski, the adductors play the main role.
Adductors are also rotators. Neutralizing the unwanted rotation may be necessary.
Abductors can stabilize the inside ski. Unfortunately, there are no really effective abductor muscles.
Some hip muscles also have a relatively weak abductor effect (in addition to their primary functions).
Neutralizing the unwanted rotation here may be also necessary.
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Kinetic chains are interconnected muscles and connective tissues that work together.
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Initially, the concept of kinetic chains was used in physiotherapy, but it works in any functional movement.
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The parts of the kinetic chain do not function separately, but always in a larger context, acting together.
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In real-life scenarios, we must always consider not only the function of one body part but also its relationship with the other parts of the kinetic chain.
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A muscle action affects both sides of the overlapped joint. The actual displacement is determined by which side was more firmly fixed.
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The firmly attached side is the closed end of the chain; the loosely attached side is the open one.
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In sports, generally, the outcome of the action is at the open end of the chain, and the closed chain provides support. (e.g., throwing or hitting a ball while standing on the ground; running or jumping away from the ground)
Skiing is exceptional because skiers expect an outcome from the closed side.
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Many skiers think there is no point in moving the foot in the ski boot, because it practically cannot move.
However, in this case, the foot is the closed end of the chain, but the muscles are longer than the ski boot, so they affect the upper part of the connected body parts.
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Multijoint muscles are, for example:
- The calf muscles overlap not only the ankle but the knee, too. If the foot can’t move, they can flex the knee as the other part of the chain.
- Hamstrings are knee flexors and hip extensors at the same time.
- Quads are knee extensors, but one part is a hip flexor as well.
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So, ankle flexion can result in knee flexion as a secondary effect through the posterior chain.
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Ankle plantar flexion can push down the big toe ball, but this is the closed part of the chain (the ground won’t move down), so the biggest movement is expected on the other side of the chain; that’s why there is a chance that the shin canmove back.
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The ankle’s freedom of movement is greater in the case of an open ankle (plantar flexion) and restricted in the case of a closed ankle (dorsal flexion). Foot movements with an open ankle have less effect on the shin and the upper body parts, while they have a more direct influence on the rest of the body when the ankle is closed.
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Looking for an outcome at the closed end of the chain is like a diver trying to loosen a rusty bolt underwater with a big wrench. The bolt doesn’t move; the diver will spin around it.
A skier can use the bigger mass at the open end as a counteraction, but only if the different body parts are connected appropriately.
SIDENOTES
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Muscle actions create internal forces.
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Depending on which is greater (external or internal force), the outcome will be different:
- The muscle can move the limb if the external forces are weaker (concentric action);
- There is no displacement if the internal and external forces are equal (isometric action).
- The limb moves under the influence of an external force, and the muscle is lengthening if the external force is greater than the muscle’s force (eccentric action).
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In all cases, the same muscle tries to act the same way, but the outcome will be different.
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Some muscles cross more than one joint. They can exert complex action by moving multiple body parts simultaneously.
The Skier and the Ski: A Unique Vehicle.
Vehicle movement is determined by physics.
The passenger adapts their body to the vehicle’s movements.
The driver tries to influence (control) the vehicle’s motion by their body movements.
In alpine skiing, the vehicle moves primarily due to physical effects.
In road cycling, skating, and cross-country skiing, the vehicle is moved by the driver.
Think of a ski as a vehicle, and the skier as its driver. Gravity is the engine, but the skier is far from just a passenger. Unlike cycling or skating, skis don’t move forward through direct muscular effort alone.
Instead, skilled skiing is about guidance and energy management. The skier uses strength and balance to pressure the skis, shape turns, and generate speed—similar to an ice skater pushing off the ice. At the same time, they harness and redirect the forces created as the skis interact with the snow. True mastery lies in influencing the ski’s path with intention, not simply riding along.
SIDENOTES
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Gravity is force that attracts a body towards the Earth.
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Gravity is constant, including its magnitude and direction.
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Gravity acts vertically toward the center of the Earth.
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Without a supporting surface, gravity causes every object—regardless of mass—to accelerate downward (to fall).
With a supporting surface, gravity shows up as weight (mass × gravity).
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To make motion easier to understand, we often talk about the Center of Mass (COM) or Center of Gravity (COG).
This is the balance point where the mass of a body can be thought to act.
It’s a useful model for analyzing movement, even though it isn’t a physical “part” of the body.
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On the slope, gravity accelerates objects in the fall line.
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For skiers, gravity provides the speed in the first place.
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The vast majority of the kinetic energy in alpine skiing comes from the vertical drop (difference in altitude).
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Alpine skiers have enough external energy to ski or make a turn.
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A turn has approximately as much kinetic energy as the vertical drop of the turn (if the speed is the same at the beginning and end).
Skiers cannot really influence the amount, but they can manage the distribution.
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Two-thirds of the skier’s body mass is only indirectly connected to the ski.
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If the skier is poorly (loosely) connected to the ski, the ski diverts from its original direction, but the skier’s body still moves in the original direction.
The primary purpose of ski technique and posture is to connect the ski as the support to the body as the mass.
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The skier can not turn; only the ski can.
SIDENOTES
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In skiing, the only direct interaction between the skier and the environment is between the ski and the snow.
After that, the lower leg interacts with the femur, the femur with the torso, the torso with the arms, etc.
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In skiing, the force effects resulting from the snow-ski interaction cause a change in the motion of the ski (and the stably connected body parts).
This change can be a direction change (turn) or a change in speed (braking).
In other words, there is no turning or braking without snow-ski interaction.
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This force, resulting from the snow-ski interaction, directs only the ski into the turn.
Skis and the closely connected body parts are only about 10-10% of the total body mass.
The further away the other, more massive body parts are, the more they will continue to move in a straight line according to their momentum.
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When the ski-snow interaction ceases (lost edge grip, sliding), the skier’s masses will move in a straight line from the last moment.
Sometimes, it is said that someone flies out of the corner.
This is not possible because there is no force that pulls the skier out of the turn.
They will not slide “out” (radially); they don’t turn anymore, but they slide or fly tangentially further without turning.
On the slope, gravity’s parallel component to the surface makes the object move into the fall line.
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Alpine skiing is partly a continuous fall from the top of the slope to the bottom. A skier can lose every turn from a few tens of centimeters to a few meters of height.
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Alpine skiing is partly a continuous fall from the top of the slope to the bottom, while also moving forward.
A skier can lose in every turn from a few tens of centimeters to a few meters of height.
A turn on a nursery slope is like stepping down the stairs.
The same on a black slope can be a jump from the first floor.
Going down the stairs one by one is easy, but taking two at a time is a big challenge, if not impossible, for many.
Passive initiation, late braking is like jumping.
Active, early initiation, distributed forces are like walking down the stairs.
Whether we jump from the window or walk down the stairs is up to us.
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On the slope, the gravity’s parallel component to the surface causes the object to move in the fall line.
An interaction between the ski and snow can cause the skier to divert from the fall line.
Less edge grip and more sliding result in less efficient force use, so the skier deviates less from the fall line.
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The ski bends to its native radius even under a small force (a fraction of the body weight is enough for this).
The ski cannot bend below a certain level.
In general, only a small fraction of the forces acting on the ski are needed to bend it.
The primary function of the forces acting on the ski is to direct the skier into the turn.
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An efficient skier strives to use the minimum force required for the turn, nothing more.
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Pressure is more of a feedback or signal than an intention, something the skier pays attention to but does not create.
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Direction change = acceleration.
A greater change in direction requires greater force.
Smoother control results in a more balanced force distribution.