NMSK - biomechanics

what do bios and mekhaniki mean?

what is biomechanics

bios = life

mekhaniki = mechanics

biomechanics = applying mechanical laws to living things by studying the mechanics of a living body the forces and their effects on and within the body

what does biomechanics act on?

inside the cell to the whole animal

what is classical mechanics

how bodies (solids/fluids) or systems of bodies responds to external forces

what is meant by a system of levers

• bones of the skeleton form a system of levers

• lever = rigid object rotates around a fixed point

• most muscles act on tendons to change joint angles

what is functional anatomy?

applying principles of biomechanics to work out how structures function

how can we infer functional anatomy

• examine muscle action relative to one joint axis of rotation

• infer joint action based on orientation and pull of muscle in anatomical position

• some are easier than others

outline material properties of muscles

• ability to shorten: to 50% of original length, dertermined by muscle length)

• parallel formation of fibres: greater range of motion (ROM), less force developped

• amount of tendon and tendon:muscle ratio: affects force and ROM potential of a muscle

• muscle power: determined by cross-sectional area of muscle, determined where fibres join tendons at an angle

outline muscle architecture

Variation in fibre arrangement in muscle belly:

• strap muscle (e.g. infrahyoid muscle)

• fusiform muscles (e.g. spindle shaped)

• pennate

what does ACS and PCS stand for?

ACS = widest part of muscle belly

PCS = total area of muscle at right angle to muscle fibres

what is a compromise made in pennate muscle?

Creates a large cross-section and force

BUT
less range for shortening

what is hysteresis and where do we find it in this graph?

hysteresis = energy dissipation during loading and unloading

found in the space of the lines.

explain this graph

Loading to failure

1. force (N) against deformation (mm)

2. linear region = elastic deformation = reversible

3. yield region = plastic deformation = irreversible

4. 3 regions: toe region, linear region, yield and failure

5. green line (line at 45 degrees), young’s modulus linear gradient.

what are 4 ways force can be applied

1. tension

2. compression

3. shear

4. torsion

What is wolff’s law for bone?

1. bone acts as a piezoelectric crystal

2. bending generates microcurrents

3. osteocytes sense currents

compression = negative charge = lay down bone

tension = positive charge = remove bone

4. eventually bone adapts to the forces applied

why is wolff’s law important

• fracture healing and rehabilitation

• slower healing than soft tissue adaptions

• stress cracks are evident the animal is being exercised in excess of current fitness

what will compression cause

• negative charge

• lay down bone

what will tension cause

positive charge

removal of bone

name 2 important characteristics of bone

1. not homogenous (mechanical behaviour is influenced by direction of loading relative to its orientation) anisotropic

2. bone is most resistant to compression in all directions

outline the forces acting on the bone in this image - when are each of these forces occuring:

torsion, bending, tensile and axial compression

1. Torsion - occurs when mass of body changes direction while limb weight bearing

2. Bending - occurs when bone at angle on weight bearing/greater pull of muscles on one side

3. tensile force = stretching forces

4. axial compression = when weight bearing, muscle contractions contribute

what are stress and strain?

stress = magnitude of this resistance

strain = amount the object deforms

what do fractures often form from?

Caused by tension

configuration results of:

• type of loading

• rate the load is applied

what fracture lines may appear as a result of different types of loading?

1. tension - transverse lines

2. compression - oblique lines (shears at 45 degrees to long axis)

3. rotation - spiral fracture

4. bending - butterfly fragments

• as bone bends, convex side is under tension, concave under compression. Transverse fracture but shears at 45 degrees proximal and distal to form a butterfly fragment

outline fracture formation based on rate of loading in cortical bone

SLOWLY:

• fracture starts at weakest point, follows weakest path

• single like of fracture is influenced by load applied and any weakness present

RAPIDLY:

• energy stored in structure causes multiple sites of disruption

• not necessarily along weakest plane

what may high energy trauma lead to?

high degree of comminution

outline fracture formation due to rate of loading in cancellous bone

• follows some fracture patterns as seen in cortical bone

• compressive force = collapse and compaction

• typically seen in vertebrae

outline bone physiology in fractures

• bone has capacity to heal

• inflammation - stages of soft to hard callus - eventually forms bone of original tissue

what do we want to optimum healing

some movement (wolff’s law) to stimulate bone formation = promotes callus formation and increases rigidity.

excessive movement prolongs fracture healing

outline the 4 stages of bone healing

1. fracture

2. haematoma forms

3. hard callus forms

4. bone remodelling

what is the purpose of fracture fixation

1. minimise strain of fracture callus

2. allow healing to occur

3. spatial realignment

4. takes load until bone is strong enough

5. weight bearing on limb

6. reconstruct the original structure

7. neutralise forces acting on fracture site.

what 5 options do we have for fixation options?

1. splints and casts

2. external fixator

3. plate and screw

4. intramedullary rod

5. interlocking nails

why do we use splints and casts?

1. immobilise fractures

2. do not directly contact bone

3. rigid material

what are external fixators used for

• complex fractures

• used where there is high risk

why and how are plates and screws used?

• resist all 3 forces

screws:

• apply friction between plate and bone

• apply compression between fragments

plate:

• internal splint, hold fractures parts in alignment

• allows compression between fracture ends and load transfer

long bones loaded eccentrically:

• results in tension side and compression side

• position plate on tension side helps counteract load compressing on opposite side

what are intramedullary rods used for

• little resistance to axial compression/torsion

• resist bending forces

how are interlocking nails used

• resist all 3 forces

• central nail resists bending

screws/bolts:

• prevent collapse under compressive force

• prevent rotation with torsional force

• place away from fracture to avoid weakening it

give an example of a surgical intervention and how they assessed its functionality

tibiotarsal bone plate

Aim: improve surgical repairs

Validate surgical procedure: increase fatigue strength of surgically repaired tibiotarsal jionts

Biomechanical study on the hindlimb: - strain gauges mounted to bone plates, test efficacy of added IM rod in this procedure

Tested by compressive loading noted: increase in fatigue life via reduction in plate strains and increase in structural stability due to IM rod in tibia.

what 5 aspects do we consider with conformation?

1. structural arrangement

2. species/breed standards

3. indicator of performance and soundness

4. subjective/objective assessment

5. implications?

what guidelines do we use for conformation?

• reference points identified

• look at length and angles

what is conformation and why is it important

1. alignment of bones and joints

2. critical to performance

what can conformation cause/lead to

1. determination of foot shape

2. how food wears over time

3. how leg moves during locomotion

4. some abnormalities simply reduce visual appeal, others can lead to lameness

5. knowing proper conformation can help us recognise problems before lameness occurs

what is a problem with breeding

breeding for looks at the compromise of performance

give 4 examples of breeding conformation complications

1. daschund = disc problems

2. bulldog = respiratory and limb problems

3. pig = conformation problems, accentuated by farming demants

4. GSD = low hips, back problems

give 3 key ideas for conformation

1. solid structures last for years

2. weaker structures aren’t able to withstand much

3. always exceptions to the rules

identify the following points on this image of the horse:

1. wing of atlas

2. proximal end of spine of scapula

3. prosterior part of greater tubercle of the humerus

4. elbow joint

5. lateral tuberosity of distal end of radius

6. articulation of carpus and metacarpal bones

7. distal end of third metacarpal bone

8. articulation of first pastern joint (between the proximal and middle phalanges) and phalanx.

Label these parts of the horse

1. proximal end of ileum

2. greater trochanter of femur

3. stifle joint

4. calcaneus bone

5. articulation of tarsus and tarsal bones

6. articulation of fetlock joint and metatarsal bone

7. articulation of pastern joint

on this horse draw the following lines between::

1. shoulder inclination

2. shoulder joint

3. elbow joint

4. fetlock joint

5. pelvis inclination

6. femur inclination

7. stifle joint

8. hock joint

Outline 7 key facts about greyhounds

1. third fastest land animal

2. second fastest acceleration

3. body mass range 22-40kg

4. females = 27.8 and males = 32.7kg average

5. races range from 250-1000m, 470m average

6. 17s for 280m

7. average race is 30.5s for 480m

outline the muscle fibres found in canines and specifically greyhounds

• type 1

• type 2: IIA and IIX and IID (IID only found in dogs)

• more fat

• better adapted for endurance

IID:
- a mix of type 1 and type 2 fibres which enables the dog to be goodo at both endurance and sprinting.

Why do greyhounds need energy?

• ATP used for myosin-ATPase from actin

• sources overlap: aerobic respiration, glycolysis, creatin phosphate

• as greyhounds exit the trap, they’re at full power, need to produce ATP very very quickly

What happens in the absence of O2 during respiration (i.e. during those first moments when they greyhound exits the trap)

• no aerobic respiration reaction can take placce that quickly

Lactate:

• lactate is produced from pyruvate during glycolysis (an anaerobic reaction)

• allows NAD+ regeneration

• H+ are consumed

• keeps glycolysis going when no O2 is available

• H+ produced by ATP hydrolyssi

• lactate and H+ are handled differently

• produced in parallel by separate processes (glycolysis and hydrolysis reaction)

Outline exercise physiology in greyhounds in sprint races

sprint races rely on interconversion of AMPADP/ATP and creatine phosphate

• totally anaerobic in muscles

• all about acceleration

• better in males

• tends to be more in large male greyhounds

2ADP —> ATP + AMP

Creatine + ATP —> Creatine-P + ADP

why is creatine phosphate so useful?

• quick regeneration

• anaerobic

• enables quick production of energy

outline exercise physiology in standard races

• switches to glycolysis for 15-30s

• glycolysis = major factor in performance

• aerobic processes dominant after 30s

• acidosis due to ATP hydrolysis

• post-race panting

why is there post race panting?

• acidosis

ATP + H2O —> ADP + Pi + H+ + Energy

Outline exercise physiology in long distance races

• aerobic after 30s

• suits smaller females

• fading towards end of race due to metabolic acidosis

What is the Duty Cycle

Running forces are cyclical

Duty cycle/factor:

• equal to contact time/total stride time

• expressed as proportion (0-1)

• as duty cycle falls impact force increases

• impulse of contact time has to propel body in a leap

Deceleration of a limb at contact:

• F=ma

• as deceleration decreases so does force.

Explain F1 x T1 = F2 x T2

When standing still, the force applied and the duration it’s applied for is equal on all 4 feet. (F1 x T1)

If we reduce the time each foot spends on the ground e.g. when running, the force each foot must overcome and experiences on contact with the ground increases

• because T2 x F2 must be equal to F1 x T1.

• If T2 decreases, F2 must increase

When an animal goes from standing still to walking to running, why does the force applied to each limb increase?

• need to overcome gravitational force

• feet spend less time on the ground

• force increases

If we have a 30kg greyhound that goes from standing still to running with 10% of limb contact with ground, what is the new force applied to each limb?

F1 x T1 = F2 x T2

F1 = 30kg (should be in Newtons)

T1 = 100s (for ease)

T2 = 10% therefore 10s

F2 = ?

30 × 100 = 3,000

3,000/10

300kg per limb.

Outline the force of balance in relation to moving in a curve

• any object moving in a curve experiences centripetal acceleration

• in unstable models, lean-in balances this with gravity

• independent of mass

• only velocity, radius and g are important

what forces are there on a bend?

• When leaning in, G increases

• when on a 45 degree tangent, + ~1 G on top of normal body weight

• This is because even more weight is placed onto the inner limbs

If we have a 30kg greyhound running with each limb experiencing 300kg (N). What force is applied on each limb when running round a curve with a 45 degree tangent?

• +1G, therefore now experiencing around 2G on the bend

• this is equal to around 1.5 x bodyweight

• therefore 300 × 1.5 = 450kg

Why are there so many hindlimb retractors?

• muscle force-length curves

• muscles grouped so that Force-length curves peak at different joint angles

• produces even force over a joint range of motion

• different lever arms also alter force-angular velocity relationship

• e.g. gluteal muscles vs caudal thigh muscles

Outline passive and active tension

Hindlimbs = power and acceleration

Forelimbs = breaking and steering/spring

• Red line (upside down U shaped curve) = as muscle starts short, gets longer and longer, tension increases then decreases

• Can’t be sustained

• As active tension starts to decrease, passive tension increases

How do we overcome the length-tension curve of a muscle in the hindlimbs?

• large range of motion in the hindlimbs that can’t be controlled by one muscle alone

• set to act at different times of the hindlimb swing

• each muscle pulls hard as limb swings past it’s optimal point (top of the red line, the upside down U curve)

• enables most muscles to be working at optimal tension.

Outline muscles and links

in cursorial animals tend to cross 2 joints

work in antagonistic pairs

• link is made, flexion and extension of joints is coordinated without multiple muscle contractions

• single extensor can affect many joints distally

examples:

• elbow-shoulder

• elbow-carpus

• hip-stifle

• stifle-tarsus-digits

why are bone clinically important in greyhound racing?

• dog is producing centripetal acceleration

• loads the OUTSIDE limbs

• hard track = medial aspect of OUTSIDE limb receives most load

• 60-70% more BW (body weight) on hind limbs, more on right hind

which bones can over-stress in greyhound racing?

cuboidal tarsal bones

• central and third tarsals

• metatarsals/metacarpals

• sesamoid bones within interosseus muscle tendons

• phalanges

How does force applied change based on ground type?

soft ground = equal loading on both sides of the limb

Hard ground = more loading on the inside of the limb - unshared proportion of load of what we calculated

If a greyhound has a stress fracture, how do we treat it?

• lag time between repeated stresses and increase in strength = danger period

• bone remodelling can’t keep up with stress fracture formation

• Wollf’s law - want to reduce exercise so bone can heal but not stop exercise because remodelling needs to be stimulated.

why is remodelling a good thing?

• bone strengthens and becomes more rigid

• peak force can be increased

• failure occurs at higher forces

Downside:

• fewer warning signs due to lack of micro-fractures causing lameness

Outline tendon injury clinical relevance in greyhound racing

• tendons that take bodyweight and deal with propulsive forces = more prone to injury

• e.g. carpal flexors, tarsal extensors, deep and superficial digital flexors

• sometimes tendon of origin for biceps brachii

Rare = triceps brachii tendon of insertion, quadriceps (patellar) tendon of insertion

what can greyhounds to at highspeed that they can’t when at rest and what does this indicate?

• bend the carpal joint to 90 degrees, parallel to ground

• injury isn’t often due to inherent strength and design of the skeletal system

what is the role of a track vet?

• check all dogs for injury, lameness, season (females can’t race if in season), general appearance

• maintain welfare standards

• first aid

• minor sales - vaccines, wormers

• GBGB official and Veterinary surgeon

• no vet = no race

what do we need to consider for a greyhound to be considered fit to race

• lameness

• wounds

• in season? (pro-oestrus and oestrus)

• general well-being

what are we looking for to identify injuries in greyhounds during the race?

classic in their site and how they occur = indirect injury

• gracilis - out of traps and on first bend

• tarsus, carpus, triceps - on bends

• watch for tail lifting, whirling

• stumble - one stride missed

• cramp

• running wide/uneven cornering

• listen for squeaking/yelping

• 0.3% injury rate

what do we need to consider about injuries as a track vet and what do we provide?

• some injuries aren’t apparent until next day

• basic first aid

• fracture stabilisation

• wounds

• euthanasia

• arranging further vet consultations

define biomechanics

the study of mechanics of a living body, especially of the forces and effects of these forces on and within the body