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'''What this machine is made of: passive and active tissues'''  

'''What this machine is made of: passive and active tissues'''  

[[File:Fig 2.1.png|thumb|'''2.1 Tissues of the musculoskeletal system.''']]

[[File:Fig 2.1.png|thumb|'''Fig. 2.1 Tissues of the musculoskeletal system.''']]

The tissues of the musculoskeletal system are bone, cartilage, ligament, tendon, fascia and muscle (Fig. 2.1).

The tissues of the musculoskeletal system are bone, cartilage, ligament, tendon, fascia and muscle (Fig. 2.1).

The non-muscular parts support the body passively by resisting forces that are applied to the body from outside (extrinsic) or those that are created intrinsically within the body.  The most significant extrinsic forces are the earth's gravitational force on the mass of the animal, and the reaction of the ground.  An animal, once it is born, must spend most of its life battling with gravity and the problems of contact with ground surfaces.  

The non-muscular parts support the body passively by resisting forces that are applied to the body from outside (extrinsic) or those that are created intrinsically within the body.  The most significant extrinsic forces are the earth's gravitational force on the mass of the animal, and the reaction of the ground.  An animal, once it is born, must spend most of its life battling with gravity and the problems of contact with ground surfaces.  

Animals also derive support from the activity of their muscles, as evidenced from the inability of an anesthetised animal to stand. This active type of support will be discussed in Sections 4 and 5.  

Animals also derive support from the activity of their muscles, as evidenced from the inability of an anesthetised animal to stand. This active type of support will be discussed in Sections 4 and 5.  

::2.1 Tissues of the musculoskeletal system.  

::Fig. 2.1 Tissues of the musculoskeletal system.  

::This is a stylised view of the lateral aspect of the proximal left forelimb of the sheep.  The skeleton is made of bones and cartilages.  A combination of ligaments, muscles & tendons, and sheets of fascia, examples of which are shown, hold these together. Without these soft tissues around them, the jointed bones will not support the weight of the sheep.  

::This is a stylised view of the lateral aspect of the proximal left forelimb of the sheep.  The skeleton is made of bones and cartilages.  A combination of ligaments, muscles & tendons, and sheets of fascia, examples of which are shown, hold these together. Without these soft tissues around them, the jointed bones will not support the weight of the sheep.  

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Elasticity is measured by determining the effect of stress, defined as force per unit area, on the shape of the material.  When this stress acts in only one direction, this shape change is measured as strain, the distortion per original unit distance, expressed as a percentage.  

Elasticity is measured by determining the effect of stress, defined as force per unit area, on the shape of the material.  When this stress acts in only one direction, this shape change is measured as strain, the distortion per original unit distance, expressed as a percentage.  

[[File:Fig 2.2.png|thumb|'''2.2  Stress, strain and elastic energy''' ]]

[[File:Fig 2.2.png|thumb|'''Fig. 2.2  Stress, strain and elastic energy''' ]]

At a particular percentage of strain, ρ, a stiffer material will store more elastic energy than a more elastic tissue.  Suppose that at this strain percentage, the material can no longer be deformed without damage.  Fracture will occur, and because the stiffer material releases more energy it will shatter rather that suffering a simple break.  Bone fractures are discussed later in Chapter 3.  

At a particular percentage of strain, ρ, a stiffer material will store more elastic energy than a more elastic tissue.  Suppose that at this strain percentage, the material can no longer be deformed without damage.  Fracture will occur, and because the stiffer material releases more energy it will shatter rather that suffering a simple break.  Bone fractures are discussed later in Chapter 3.  

::2.2 Stress, strain & elastic energy

::Fig. 2.2 Stress, strain & elastic energy

::Two linearly elastic tissues A and B have stress - strain curves as shown.  At any given value of stress, ρ, the energy absorbed by A, represented here by the area beneath the curve, is less than that absorbed by B.  A is stiffer than B. If A and B were bones, A would be the more mineralised.

::Two linearly elastic tissues A and B have stress - strain curves as shown.  At any given value of stress, ρ, the energy absorbed by A, represented here by the area beneath the curve, is less than that absorbed by B.  A is stiffer than B. If A and B were bones, A would be the more mineralised.

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[[File:QMSection2.3.png|thumb|'''Fig. 2.3  Elasticity''']]

[[File:QMSection2.3.png|thumb|Add caption here]]

When an elastic material is strained, work is done on it and energy is stored in it.  Elastic energy is a form of potential energy.  A rubber ball falling on to a hard surface converts what was previously kinetic energy into elastic energy on impact.  This elastic energy is released again as kinetic energy when the ball bounces.  The ball does not, however, attain the same height as previously.  The elastic framework of an animal’s body behaves similarly, as it bounces up and down during running (Fig. 2.4).  Some of the kinetic energy is lost as heat.  The less energy lost in this way, the greater the elastic resilience of the material.  Resilience is the work recovered from a material in elastic recoil, expressed as a percentage of the work previously done in straining it (Fig. 2.5).  Passive musculoskeletal tissues should be as resilient as possible, to conserve energy.  

When an elastic material is strained, work is done on it and energy is stored in it.  Elastic energy is a form of potential energy.  A rubber ball falling on to a hard surface converts what was previously kinetic energy into elastic energy on impact.  This elastic energy is released again as kinetic energy when the ball bounces.  The ball does not, however, attain the same height as previously.  The elastic framework of an animal’s body behaves similarly, as it bounces up and down during running (Fig. 2.4).  Some of the kinetic energy is lost as heat.  The less energy lost in this way, the greater the elastic resilience of the material.  Resilience is the work recovered from a material in elastic recoil, expressed as a percentage of the work previously done in straining it (Fig. 2.5).  Passive musculoskeletal tissues should be as resilient as possible, to conserve energy.  

:2.5 Elastic resilience 

::Fig. 2.3 Elasticity

:The work done in deforming the material is Fd, the product of force and distance.  This is the area under the curve made during the application of the force (horizontal hatching).  The work done by the elastic restoring force is the area under the curve made during the removal of the deforming force (vertical hatching).  In this example, these two areas are not the same.  The difference in area is the energy lost as heat.  The resilience is the vertically hatched area as a percentage of the horizontally hatched area.   

::The force F tenses a block of material of transverse sectional area A and length l, producing a deformation e.  In Figure 2.3A, stress is proportional to strain.  The slope of this line is the elastic modulus. 

:Indented line

::Upon removal of the stress, the block returns to its original shape.  The block is perfectly elastic.  In Figure 2.3B, the elastic modulus is not constant; with more stress, a disproportionate deformation is produced. 

::For this reason, and also because when the stress is removed some of the deformation remains, the block in this instance is imperfectly elastic.  Such a residual deformation is not useful in animal mechanics.

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