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 OF SKIING
FUNDAMENTAL PRINCIPLES
SIDENOTES
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Deceleration is also a (negative) acceleration.
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Turning with a constant speed is also an acceleration.
As such, acceleration is created due to an external (unbalanced) force.
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Alpine skiing can also be seen as a kind of fall from the top of the slope to the bottom, while we also move forward.
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A turn on a nursery slope is like stepping down from a stool or chair.
The same on a black slope can be a jump from the first floor.
The dimensions of a whole run are like a tower block.
Whether we jump from the window or walk down the stairs is up to us.
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On the slope, gravity’s parallel component to the surface makes the object move into the fall line.
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In skiing, gravity provides speed but does not directly cause the skier to turn.
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Weight is a force created by gravity acting on mass.
It always acts vertically.
The skier cannot change either their weight or the direction of their weight.
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Acceleration is any change in velocity with time, in terms of both speed and direction.
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It follows that acceleration also occurs when the speed remains constant, but the direction of motion changes.
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Acceleration is due to an external (unbalanced) force.
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Force and acceleration are directly related; therefore, a larger change in velocity (speed or direction) in a shorter time comes with a greater force effect, and vice versa.
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Gravity is also an acceleration.
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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 ground contact, gravity causes the object’s mass to accelerate (to fall); on the ground, a force (weight).
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COM or COG (Center of Mass or Gravity) is a representation of an object’s mass, which can help to understand physical concepts, but not an existing body part.
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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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Momentum is the product of an object’s mass and velocity.
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Linear momentum’s direction is straight (never curved).
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In simple words, momentum is “mass in motion” or kinetic energy. As such, it is the primary physical factor 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 tendency of an object to resist any change in its state (position or motion). Momentum is a tendency to keep moving.
That’s why kinetic energy is related to momentum, not inertia.
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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.
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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.
SIDENOTES
The human mass distribution by body parts (approx.):
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Velocity is the displacement of an object per unit time.
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It consists of the speed and direction of movement.
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Velocity changes if either the speed or the direction changes.
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If velocity (and momentum with it) changes, we must assume an external (unbalanced) force causes the object to change its state.
SIDENOTES
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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Force is an interaction between objects.
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A force is an influence that causes an object to undergo a change.
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This can be a change in motion or a deformation of the object.
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If the sum of external forces and external torques exerted on an object is zero, the object is balanced:
- as it is stationary
- or moving with a constant velocity (uniform motion).
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Acceleration is due to an external (unbalanced) force.
isaacphysics.org/concepts/cp_force
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
FORCE & PRESSURE
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Pressure is the perpendicular force per unit area. (Encyclopædia Britannica)
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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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Force is an interaction between objects.
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A force is an influence that causes an object to undergo a change.
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This can be a change in motion or a deformation of the object.
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If the sum of external forces and external torques exerted on an object is zero, the object is balanced:
- as it is stationary
- or moving with a constant velocity (uniform motion).
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Acceleration is due to an external (unbalanced) force.
isaacphysics.org/concepts/cp_force
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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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Each can move under the influence of external forces if they aren’t stabilized.
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, 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.
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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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Kinetic chains are interconnected muscles, joints, 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 can move 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 activity, 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 will move 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.
A ski is like a vehicle; a skier is a driver or passenger.
Vehicle movement is determined by physics.
The passenger adapts their body to the vehicle’s movements.
The driver tries to influence the vehicle’s motion by their body movements.
In skiing, the vehicle moves primarily due to physical effects; it is not moved by the driver or the passenger. (Cycling, skating, and cross-country skiing differ from this point of view.)
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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.
They don’t need muscle activity to create their movement.
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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.
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The skier can not turn; only the ski can.
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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.
They don’t need muscle activity to create their movement.
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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.
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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.
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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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On the slope, the gravity’s parallel component to the surface causes the object to move into the fall line.
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An interaction between the ski and snow can cause the skier to divert from the fall line.
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(Theoretically, in some cases, e.g., the wind’s pressure on the skier’s body can also divert the skier from the fall line.)
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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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A strange paradox of skiing is that the parts that are close to the ski have the least kinetic energy.The more loosely connected further body parts (the upper body) have the most kinetic energy.
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Direction change = acceleration.
A greater change in direction requires greater force.
Smoother control results in a more balanced force distribution.