Kinetics Notes

Kinetics vs. Kinematics

Kinetics focuses on forces that cause movement, while kinematics studies movement without considering the forces behind it. Kinematics is movement, Kinetics is forces.

Key Terms

Mass

Mass is the amount of matter in a body, referring to the quantity of substance present. Mass differs from weight.

Inertia

Inertia is the resistance of a body to changes in its state of motion. The amount of force required to initiate movement depends on the object's mass. For example, it's easier to push or slow down a Shetland pony than a Shire horse due to its smaller mass.

Weight

Weight is the effect of gravity on mass and it is calculated as: Weight=Mass×GravityWeight = Mass \times Gravity Gravity acts downwards, and weight varies depending on gravitational force (less on the moon than on Earth), while mass remains constant.

Linear vs. Angular Motion

Linear motion involves moving forward, with inertia directly proportional to mass. Angular motion involves rotation, considering forces that cause the rotation.

Forces

A body's state of motion changes when an external force acts upon it. In jumping, once a horse is airborne, its trajectory is determined, and the only force acting on it is gravity. Force is measured in newtons.

Forces can be:

  • Push/Pull
  • Compression: Force from either end (e.g., on bone during locomotion).
  • Tension: Stretching force (e.g., on tendons and ligaments).
  • Shear: Forces acting against each other.
  • Torsion: Twisting motion (e.g., in a hot joint).
  • Combined Loading: Multiple forces acting simultaneously (e.g., torsion and compression during stance).

Ground Reaction Force (GRF)

The force exerted by the ground on a body in contact with it. According to Newton's third law, every action has an equal and opposite reaction.

During standing, ground reaction force equals the weight of the horse. During movement, GRF changes with acceleration (F=maF = ma), where FF is force, mm is mass, and aa is acceleration. Because mass remains constant during a session, changes in acceleration (and therefore speed) alter the GRF.

GRF has three directions:

  • Vertical: sends us upwards, opposing gravity
  • Longitudinal: sends us forwards (accelerating) or backwards (decelerating)
  • Transverse: sends us side to side

GRF is a vector with magnitude and direction. The summation of these directions is called the ground reaction force vector.

GRF in Static Conditions

In a static horse, there is more GRF in the forehand (58%) compared to the hind end (42%) due to the weight of the head.

GRF During Locomotion

GRF varies with speed (acceleration) and gait. It differs between limbs and depends on factors like mass and gait.

Postural Sway

Postural sway is measured using a force platform to observe the center of pressure shifts. Better postural control benefits the horse, and can be improved with rehabilitation programs like water treadmill training (Mother King, 2013).

Phases of Stance

  • Impact: Foot hits the ground (deceleration).
  • Mid-Stance: All the horse's weight is over limb.
  • Push-Off: Hoof breaks over and leaves the ground.

GRF only acts on the limb when it's in contact with the ground. The stance phase includes:

  • First Impact: Vertical deceleration; hoof decelerates while the body hasn't.
  • Second Impact: Longitudinal Deceleration; sliders stopped
  • Mid-Stance: All the horse's weight is the limb is vertical over it.
  • Breakover: Propulsion causing acceleration.

Ground reaction force vector: Magnitude of force. The direction of acceleration and amount of force

Locomotion and Center of Mass

Locomotion manages the trajectory of the center of mass (around T13). Horses aim for efficiency, using different models:

  • Inverted Pendulum (Walk): Center of mass moves over the limb from one link to the other. The the trajectory becomes exaggerated up and over/not efficient.
  • Spring Mass (Gallop). The faster gait smooths trajectory, and passes the center of mass from one limb to the other smoothly.

Force Plates - Hillary Clayton

A horse traveling over force plates shows the center of mass moving up and down. The ground force vector gets bigger and then goes forward for every single limb.

Ground Reaction Force Curves

GRF is measured using force plates in three directions during stance. They can change depending on the football pattern, the gait, how we're moving our center of mass, etcetera.

  • Transverse: Minimal during straight-line locomotion.
  • Longitudinal: Deceleration at the start of stance and acceleration past mid-stance.
  • Vertical: Varies between forelimb and hindlimb due to weight distribution.
  • Forelimb: Braking deceleration
  • Hindlimb: Propulsion, smaller peak. The difference is from propulsion and braking

Vertical Ground Reaction Force

The forelimb looks anatomically for braking and turning, and hindlimb is for propulsion

Gait-Specific GRF

  • Walk: Two peaks due to inverted pendulum model, with two to three limbs on the ground at any time (First peak is slowing down, the second peak is to push it off).
  • Canter: Three-beat gait with the highest GRF on the trailing forelimb.

Impact

With increased speed also comes increased force. How quick the short sharp peak vertical ground reaction force. The less amount of time means more force. How many stride cycles
Frequency.

Vertical Impulse

Calculated as the area under the GRF curve, representing the change in momentum. A horse can elongate stance phase to reduce peak GRF while maintaining the same vertical impulse.

Stress and Strain

Applied force results in stress (resisting force) and strain (deformation). Tendons can be trained to become stronger and more elastic. A stress-strain curve illustrates tissue deformation and potential fracture points.

Torque

This is the measure of the force that causes rotation. If the force is applied outside of the enter of the mass it will translate.

Rigid body

rigid bodies have all of its mass evenly distributed around it. If we send a force through this body's center of mass it will then translate forward
The rotational equivalent of the linear force. The muscles must generate some sort of torque, muscles generate force. Muscles can cause talk, they can cause rotational action.

Inverse Dynamics

Calculates joint power, the energy generation of a single joint and its contribution to location. Typically using a force plate.

Measuring Force

Kinetic measuring systems can measure ground reaction force. Saddle pressure, rein tension are measurement of how ground force is translated from the horse to the rider.

  • Pressure: Force over area (Force/Area)

Rein Tension

A simplest form of a strain gauge. It should fluctuate with gait, with peek occurring with most ground reaction force in the training forelimb.