Comparative Features of Locomotor Apparatus and Their Application in the Biomechanics of Mammalian B | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Introduction

• In the effort to survive in their natural habitats, animals adopt diverse adaptive strategies, some of which are mechanical in nature. Predators, for instance, rely on well-developed sense organs, claws, and dentition, coupled with specific requirements for circulatory, respiratory, excretory, and neurohormonal systems to ensure successful actions. 
• Additionally, mechanical adaptations in the locomotor system are crucial for achieving the speed needed to catch prey. The examination of forces and accelerations acting on living organisms through specialized techniques is known as Biomechanics.

Relevance to Veterinary Practice

• Biomechanics plays a significant role in veterinary practice, serving as a potent tool for analyzing animal movements and anatomical features. 
• A fundamental understanding of the mechanical processes in living animals is essential, as many disorders and traumatic conditions in the locomotor system have a mechanical basis. 
• Animals, governed by the same physical laws as inanimate objects, lead to the division of biomechanics into two subdisciplines: Biodynamics and Biostatics.

Biodynamics Subdivisions

• Biokinematics: Focuses on analyzing motions without considering the forces causing them. An example is the cinematographic analysis of limb movements in a walking horse.
• Biokinetics: Studies changes in motion caused by an unbalanced system of forces and determines the force required for specific motion changes. An illustration is the analysis of forces in the legs of a running dog.

Biostatics

• Deals with forces and their equilibrium acting on animals and their organs, either at rest or in motion at a uniform velocity in a straight line. 
• For instance, studying the forces acting in a standing horse is an example of a biostatic approach.

Fundamental Terms in Biomechanics

• Force: Any factor causing or tending to cause a change in the existing state of rest or uniform motion in a straight line of a body. The nature of force depends on its magnitude, point of application, and line of action.
• Equilibrium: A body is in equilibrium when the applied forces balance each other. If two opposite and equal forces act at one point in the same straight line, they create equilibrium.
• Reaction: The resistance offered by a body to which a force is applied, such as the reaction of the ground when an animal's weight is transmitted through its legs.
• Resultant: The single force that can replace a given number or system of forces without altering the effect on the body.

Centre of gravity

• The center of gravity in the animal body is located approximately 42.86 percent of the trunk length behind the cranial surface of the shoulder or about 38 cm behind the elbow point. While carnivores and pigs share a similar center of gravity position, heavy ruminants and horses have theirs farther back. 
• This center's location is not fixed; it shifts backward when the head is raised, abdominal organs are distended, or the tail is extended. Conversely, it moves forward when the head and neck are lowered or when the abdominal organs are empty. 
• The center of gravity's position is crucial in saddling animals, seating riders, and determining the proper load placement on beasts of burden. It is closer to the front feet than the back ones, making it easier to unbalance a standing animal by pushing forward or sideways than pushing backward, which is significant in dynamics.

Construction of this trunks

• The structure of the trunk in quadrupeds resembles that of a "lowstring" or arched bridge. In the animal's body, this entire bridge system is characterized not only by its capacity to bear weight but also by its elasticity and flexibility. 
• The arch of the bridge is represented by the thoracic and lumbar vertebral column, along with the pelvis and its associated muscles and ligaments. The head and neck are positioned in the front, while the sacrum and tail at the rear function as a 'cantilever' to counteract and balance centrifugal thrust during the animal's movement, aiding in maintaining equilibrium. 
• The majority of the weight is borne by the forelimbs, and the forward movement is typical, leading to a greater impact on the forelimbs that varies with the speed of motion. Consequently, the front cantilever is larger, designed to be mobile and attached to the trunk, while the hind one is smaller and firmly anchored.

Construction of the limbs

• The construction of the limbs follows distinct static principles in the two pairs, influenced by the position of the center of gravity and the resulting distribution of the body's weight. 
• The forelimbs can be likened to a vertical supporting column or a check lever system, handling the weight propelled onto them from the rear. In contrast, the hindlimbs, with their acute angle, resemble catapult lever systems that primarily contribute to forward propulsion. 
• The variation in the functions demanded of the fore and hindlimbs is evident in how they connect to the trunk and the greater muscular mass present in the hindlimbs.

Stress and Strain in locomotor system

• Concerning stress and strain in the locomotor system, the work (K) of a muscle is the product of muscular force (P in Newton) and tendon excursion (S) in meters, expressed as K = F x S joules. 
• Power output (P) is the work done in one contraction divided by the time taken for shortening, measured in joules per second (Watt). The muscle can be parallel-fibered, parallel-pinnate, or radially-pinnate. 
• The locomotory system experiences various stresses and strains, including tensile stress, shear stress, and compressive stress. 
• Consequently, the system adapts to distribute stressing forces as close as possible to the central axis of bones, minimizing the impact on a larger area.

Muscle Influence on Bones

• Muscles not only induce movements but also regulate bone stress based on their place of origin and insertion.
• Minimization of bending tendency in bones can be achieved by the muscle's insertion towards the base or by having two muscles passing through the same joint.

Kinks in Long Bones

• Distinct kinks in the central axis at the ends of long bones are due to maximum stress intensity at these points.
• Adapted muscular activity and differential growth help minimize the effects of deforming couples, but most long bones endure a moderate bending tendency.

Bone Structure and Economy

The hollow construction of bones is the most economical in terms of minimizing bending tendency.

Morphology of Bones and Joints

• Morphological features adhere to mechanical principles for optimal benefit and comfort.
• Strain from weight and muscular contraction is transmitted through contact areas on articular surfaces.
• Articular surfaces, covered with cartilage and lubricated by synovial fluid, follow biomechanical principles for force transmission.

Preventing Shearing Forces

• Limiting the inclination of the force's line of action prevents shearing forces between articular surfaces.
• Ligaments, joint capsules, and central condylar prominences prevent dislocation of joint components.

Position of Load and Equilibrium

• Equilibrium requires the resultant of all operating forces to pass through the momentary center of rotation.
• Tolerance for deviation of force line in joints is proportional to the curvature degree of articular surfaces.

Stress Distribution

• Understanding stress distribution over articular surfaces is crucial for investigating the relationship between abnormal working conditions and functional disorders in articular cartilage.

Muscle Influence on Bending Movement

• The number of muscles passing a joint influences bending movement and stress in bones.
• Minimizing the bending tendency in bones can be achieved through various means, such as the occurrence of two muscles passing through the same joint, fan-shaped muscle insertion, and the curvature of the bone shaft coupled with muscle action to distribute stress over a larger area.

Biomechanics and Newton's Laws

The biomechanics of the mammalian body, including the architecture of the trunk, legs, head, and vertebral locomotion, can be understood based on Newton's three laws of motion.

Newton's First Law

• States that a body remains at rest or in uniform motion in a straight line unless compelled by applied forces to change its state.
• Example: A standing horse stays at rest unless a muscular force is applied to initiate movement.

Application of First Law

• Illustration with a jumping dog over a fence.
• In the absence of air resistance and gravitation, the body would continue in a straight line, but external forces result in a parabolic path due to gravitation and air resistance.

Newton's Second Law

• States that the change of momentum per unit time is proportional to the applied force and occurs in the direction of that force.
• Biomechanical implication: When a horse exerts a propulsive force (F), the resulting velocity (V) is proportional to the force's magnitude, the time (T) it acts, and inversely proportional to the body mass (M).

Acceleration Calculation

The velocity increases by F/M for each second the force is applied, leading to acceleration (A) given by F/M.

Restraint and Kinetic Energy Dissipation

• If a horse comes to rest under a constant restraining force (friction between feet and ground), the kinetic energy is dissipated over a distance (s) traveled.
• The equation for kinetic energy dissipation: M x v^2 / 2F.

Newton's Third Law

• States that forces occur in pairs, with each pair consisting of two equal and opposite forces.
• Implication in animal locomotion: When an animal applies a forward propulsive force, the ground exerts an equal but opposite backward force, resisting limb movements relative to the body.

Ground Reaction Forces

• Clarifies that for every action (animal applying a force), there is an equal and opposite reaction (ground resisting the animal's movements).
• This principle is crucial for analyzing animal locomotion and understanding how forces influence motion.