Equine Biomechanics and Locomotion

Biomechanics of the Equine Locomotion

Introduction

Biomechanics involves analyzing movement patterns to understand optimal and suboptimal movements, particularly in horses.
The initial video illustrates differences in male and female gait, demonstrating how much information can be gathered from movement patterns alone. These differences can highlight variations in muscle usage, joint flexibility, and overall efficiency of movement.

Evolutionary Design of the Horse

The horse's primary evolutionary purpose is to escape predators, influencing its biomechanical design. This need for speed and energy conservation has shaped its anatomy. The musculoskeletal system is optimized for efficient locomotion over long distances.
Horses have evolved to have minimal muscle mass in the lower limbs, with muscles concentrated higher up, connected to the lower limb via tendons and ligaments. This design reduces the energy needed to swing the lower limb. This arrangement allows for rapid limb movement with less fatigue.

Skeletal Structure and Joints

Bones are connected to each other by ligaments. Ligaments provide stability and limit excessive joint movement.
Muscles are connected to bones by tendons. Tendons transmit the force generated by muscles to the bones, enabling movement.
The carpal joint is not a single joint but a complex of multiple bones. This complex structure allows for a wide range of motion and shock absorption.
Muscles generate the force required for movement via tendons attached to bones. The efficiency of this force generation and transmission is crucial for locomotion.

Muscle Function

Muscles provide support and, most importantly, create movement. They stabilize joints and control the range of motion.
Locomotion is defined as managing the trajectory of the center of mass. Efficient locomotion minimizes energy expenditure while maintaining stability.

Center of Mass

The center of mass is the point around which an object's mass is equally balanced. It is not stationary and can move depending on body position. Changes in body posture and limb position affect the location of the center of mass.
Raising arms shifts the center of mass upwards. This principle applies to both humans and horses, influencing balance and stability.
In horses, approximately 58% of the mass is distributed on the front end and 42% on the hind end. This is largely due to the weight of the head. This weight distribution affects the horse's balance and maneuverability.
The center of mass in a horse is located approximately under the 13th thoracic vertebra (T13). This location is critical for understanding how forces are distributed during movement.
The rider typically sits around the T30 location. The rider's position can significantly influence the horse's center of mass and overall balance.

Locomotion and Muscle Mechanics

The goal of locomotion is to manage the trajectory of the center of mass. This involves coordinating muscle activity to maintain balance and forward motion.
Movement is achieved through muscle contractions that rotate joints. The coordinated action of multiple muscles is required for smooth and efficient movement.

- Muscles often work in pairs (biceps and triceps), with one muscle shortening (concentric contraction) while the other lengthens (eccentric contraction). This reciprocal action allows for controlled movement and stability.

Concentric Contraction: Muscle shortening. This type of contraction generates force to overcome resistance.

Eccentric Contraction: Muscle lengthening. Eccentric contractions control movement and absorb impact.

Isometric Contraction: Muscle holding a position without movement. Isometric contractions provide stability and maintain posture.

These muscles are co-activated, meaning they work together to control the movement. Co-activation ensures smooth and coordinated joint movement.
Agonist and antagonist muscles work in opposing pairs to control movement; their roles reverse depending on the direction of motion. This dynamic interaction allows for precise control and efficient energy usage.

### Types of Motion

Translation: All parts of a body move in the same direction at the same distance simultaneously. A horse on a lorry is translating. Translational movement involves no change in orientation.

Rotation: A body moves around an axis. Rotational movement involves a change in orientation.

In biomechanics, these motions are measured separately to simplify analysis. Breaking down complex movements into translational and rotational components allows for a more detailed understanding.
Complex movements are a combination of translation and rotation. Most real-world movements involve both types of motion.

Measurement Techniques

Spinal and limb movements can be measured independently to assess their influence on each other. This helps in understanding how different body segments contribute to overall movement.
Muscle activation can be measured separately and related to movement. Electromyography (EMG) is a common technique for measuring muscle activation patterns.
Dynamometric horse shoes can measure hoof forces. These specialized shoes provide data on the forces exerted by the hoof during different phases of the stride.
Instrumented treadmills measure force during locomotion. There is only one in the world; located at RVC. Instrumented treadmills provide detailed information on ground reaction forces and limb kinematics.

Stride Cycle

The stride cycle is a cyclical pattern measured from the point of impact of one foot to its next impact. This helps standardize measurements. Standardizing measurements allows for comparison between different gaits and horses.

#### Phases of the Stride

Stance Phase: The period when the limb is in contact with the ground. The stance phase provides support and propulsion.

Impact: Initial contact with the ground. The impact phase involves significant force absorption.

Mid-Stance: The point when the leg is vertical and bears the most load. Mid-stance is a critical phase for stability and balance.

Push-Off: The final phase of stance, where the limb propels the body forward. The push-off phase generates the force needed for forward movement.

Swing Phase: The period when the limb is not in contact with the ground. The swing phase allows the limb to reposition for the next stride.

Early Swing: Immediately after the limb leaves the ground. Early swing involves rapid limb flexion.

Mid-Swing: The point of maximal limb flexion. Mid-swing minimizes limb inertia and facilitates rapid protraction.

Late Swing: The limb extends in preparation for ground contact. Late swing prepares the limb for impact and weight bearing.

Protraction and Retraction

- Protraction and retraction refer to the extreme points of a limb's swing.

Protraction: The most cranial (forward) position of the limb at the end of the swing phase, measured as the angle of the leg relative to a vertical line dropped from the scapula. Protraction determines the length of the stride.

Retraction: The most caudal (backward) position of the limb, measured similarly relative to a vertical line from the tuber ischii. Retraction contributes to propulsion and efficiency.

Retraction is particularly desirable in dressage horses. It allows the horse to engage the hindquarters and increaseCollection and impulsion.

Models of Locomotion

Inverted Pendulum Model

Applies to walking, where the center of mass moves over a fixed limb. The body vaults over the stance limb, conserving energy.
The leg acts as a pivot point as the body's center of mass passes over it. This pivoting action minimizes vertical displacement of the center of mass.
Center of mass is high at mid stance. The high center of mass position maximizes potential energy.
This model is not energy-efficient. It involves significant energy expenditure due to the vertical displacement of the center of mass.

Spring Mass Model

Applies to running gaits with suspension phases. The elastic properties of muscles and tendons are utilized to store and release energy.
The limb acts as a spring, compressing upon impact to store energy and then releasing it to propel the body forward. This spring-like action reduces the metabolic cost of locomotion.
Tendons play a crucial role in storing and releasing energy. The elastic recoil of tendons contributes significantly to the efficiency of running.
The center of mass is low at midstance. The low center of mass position minimizes vertical displacement and conserves energy.
This model is an efficient method of travel. It allows for energy conservation and reduced muscle work.

Practical Application

Marker placement and palpation exercises are essential for accurately measuring angles and movements in biomechanical assessments. Accurate marker placement ensures