Bio Mechanical Web

8/8/2019 Bio Mechanical Web

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Biomechanical approach

How does the body perform activities if it is free

to choose different options? Biomechanical modeling treats the human body

as a mechanical system of linkages and

masses, activated by muscles than span joints.

Strain tolerance limits, in muscles and tendons,

joints and joint - enclosing ligaments are of

particular biomechanical interest in work

activities.


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Introduction

The biomechanical approach is

concerned with determining the forces

exerted upon the musculoskeletal systemduring the performance of a task.

Therefore, the biomechanical approachwill attempt to determine tolerable forces

and aim to predict maximal and low

frequency capacity for individuals.


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The aims of the biomechanical

approach are:

To minimise the reaction force.

To minimise the moment force

To minimise the compression force

To minimise the shear forces


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Muscle strength

Muscle strength is the maximal tension or force that a

muscle can develop between its origin and insertion.

Internal transmission is the manner in which muscle

tension is transferred inside the body along links and

across joints as torque to the point of application to a

resisting object.

Body segment strength is the force or torque applied by

a body segment to an external object.


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Length-strength relationships

In engineering terms skeletal muscles exhibitviscoelastic behaviour.

Viscosity depends upon the amount by whichthe muscle is deformed and the rate ofdeformation.

Elasticity is the return of the muscle to itsoriginal length and shape after deformation.


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Tension -muscles

At rest/ under no load: smallest possiblemuscle length (Z - lines are as close aspossible) - about 60% of the musclelength.

At about 120-130% of muscle length theactin and myosin rods are in an optimalposition to generate a contractile force.

No force can be generated internally atabout 160% of resting muscle length.


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Static and dynamic strength Static in physiological terms is an isometric muscle

contraction- muscle length remains unchanged.

Newtons first law applies, as all forces in the systemmust be in equilibrium.

Most available information on human strength is derivedfrom static muscular effort.

Dynamic activities describe muscle length changes,

and, therefore, the body segments move. Thus, displacement is present in dynamic

movement, and its time derivatives (velocity,acceleration and jerk) must be considered.


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Force transmission in the body

Force or torques, are transmitted along

the bodys links.

An external force will create force and

torque vectors internally in the body that

can be separated into their componentdirections.


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Designing for body strength The engineer in considering human strength

needs to decide the following:

Is strength used mostly statically or

dynamically?

Which part of the body is performing thestrength action?

Is a maximal or minimal strength exertion

the critical design factor?


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Biomechanical models-static

Static models

Single body-segment model

Two body-segment static model

Static planar model of nonparallel forces

Planar static analysis of internal forces

Multiple-link coplanar static modeling

T

hree-dimensional modeling of staticstren th


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Biomechanical models-

dynamic

Dynamic models

Single body-segment dynamic

biomechanical model

Multiple-segment biodynamic model of

load lifting

Modeling of muscle strength


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Dynamic modeling

Single segment dynamic model at the

elbow

The sum of moments at the elbow:

ME = Mstatic + M(dynamic tangential) + M(dynamic rotational)

Explain using mechanical reasoning -

why a worker lifting a weight quickly

could potentially be very dangerous to


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Biomechanical approach

How does the body perform activities if it is free

to choose different options? Biomechanical modeling treats the human body

as a mechanical system of linkages and

masses, activated by muscles than span joints.

Strain tolerance limits, in muscles and tendons,

joints and joint - enclosing ligaments are of

particular biomechanical interest in work

activities.


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Outline Introduction

Biomechanical aims

Muscle strength and work

implications

Designing for body strength

Biomechanical models

Practical


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Aim

Minimise forces

Maximise strength potential

Identify static or dynamic strength scenarios

Understand for design purposes the direction of

force vectors: internally and externally to thehuman body.

Build a model to determine the force vectors


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Solving for a static load

Sign direction

R Elbow

ME

17.2 CM

35.5 CM

W LOAD = 49 NW Forearm+hand = 15.8 N

F

BD of forearm holding a weight in the horizontal


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Solve for weight of box

10 kg boxs weight is 98 N

Static situation. Therefore, F = 0

- 98 N + 2 R hand = 0

R hand = 98 N/ 2 = 49 N (upward)

Note CM is located between hands, thusweight is equally divided.


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Solve for forces

Assuming:

The forces act in a single plane.

Weight acts through CM of the hand

Forearm and hand are one segment

Relbow force acts at the elbow


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Solution

ForcesE = 0

-49 n 15.8 N + Relbow = 0

Relbow = 64.8 N (upward)

The reactive force at the elbow worksagainst the weight of the box and forearm+ hand.


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Solve forMoment

The reactive force is able to stop the weight fromtranslation but not from rotation.

The second condition of equilibrium is;

moments = 0

Downward forces are negative (following convention):

17.2 cm (-15.8 N) + 35.5 cm (49 N) + ME = 0

(-271.8 N cm) + (-1739.5 N cm) + ME = 0

ME = 20.\11.3 N cm or 20.113 N m (positivecounterclockwise)


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Concluding thoughts

Force

We know the force is 64.8 N.The reactiveforces are important at a joint:

Determine magnitude of the tensile forces in

ligaments and muscles to hold joint together.

Magnitude of shear and compressive forces

acting on surfaces that contact the joint.


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Concluding thoughts

Moments

The reaction moments are important

because they represent the strength

required for specific muscle actions to

maintain posture or impart motion.


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Using the same example

Using the same example, solve for the

arm at 30 degrees below the horizontalor extension of 150 degrees.

0 = cos [17.2 (-15.8) + 35.5 (-49)] + ME


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Static Single body-segment

model- percentile ranges

If the weight that can be held in the palm is 15

N- the distance ofb is normally distributed with

a mean of 38 cm and a SD of 3 cm. Ifa is 5 cm,

determine the tension of the muscles of the

upper arm at a 90 degree angle for the

following:

5th percentile person



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