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Biomechanics
Rebecca W. Boren, Ph.D.
IEE 437/547
Introduction to Human
Factors Engineering
November 16, 2011
Occupational Biomechanics
Plays a major role in studying and
analyzing human performance and
musculoskeletal problems.
Human musculoskeletal system is a
mechanical system that obeys the laws
of physics – Newton’s Laws.
One technique for studying
biomechanics is the use of a pressure
platform.
Disclaimer: this is not an endorsement for Fitflop shoes,
merely an illustration.
http://www.fitflop.com/
The main functions of the
musculoskeletal system are
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To support and protect the body and body
parts
To maintain posture and produce body
movement
To generate heat and maintain body
temperature.
Musculoskeletal system is comprised of
• Bones
• Muscles
• Connective tissue
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Ligaments
Tendons
Fascia
Cartilage
Bones
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Bones form the rigid skeletal structure.
Two or more bones are linked with each
other at joints
Synovial joints (e.g. shoulder, elbow)
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Most joints are in this category.
No tissue exists between the two bones.
The joint is highly lubricated with synovial fluid.
Fibrous joints (e.g. bones of the skull).
Cartilaginous joints (e.g. spine)
Joints can be classified by range of
motion
No-mobility joints
(e.g. seams of the
skull)
Joints can be classified by range of
motion
Hinge joints
(e.g. elbow, knee)
Joints can be classified by range of
motion
Pivot joints
Joints can be classified by range of
motion
Ball and
socket joints
Connective Tissue
• Tendons are dense fibrous connective tissues that
attach muscles to bones and transmit the forces
exerted by the muscles to the attached bones.
• Ligaments are also dense fibrous tissues, but their
function is to connect the articular extremities of
bones and help stabilize the articulations of bones at
joints.
• Cartilage is a translucent elastic tissue that can be
found on some articular bony surfaces and in some
organs such as the nose and the ear.
• Fascia covers body structures and separates them
from each other.
Connective Tissue
• Tendons are dense
fibrous connective
tissues that attach
muscles to bones and
transmit the forces
exerted by the muscles
to the attached bones.
Connective Tissue
• Ligaments are also
dense fibrous tissues, but
their function is to
connect the articular
extremities of bones and
help stabilize the
articulations of bones at
joints.
Connective Tissue
• Cartilage is a translucent elastic tissue that
can be found on some articular bony surfaces
and in some organs such as the nose and the
ear.
Connective Tissue
• Fascia covers body structures and separates
them from each other.
Fascia is a three
dimensional, web
like, network of
connective tissue
which wraps all of
the muscles, bones,
organs, and brain.
Muscles
• Muscles make up about 40-50% of body weight
• Muscles consume almost half of the body’s
metabolism, which supplies the energy for
maintaining body posture , producing body motion,
generating heat, and maintaining body temperature.
• Composed of bundles of muscle fibers, connective
tissue, and nerves.
• Connective tissue provides a channel through which
nerves and blood vessels enter and leave the muscle.
Muscles
Muscles
• Muscle activities are regulated by motor nerve
fibers, which transmit impulses from the
central nervous system to the muscles.
• Each motor nerve fiber regulates a group of
related muscle fibers.
• Motor nerves fire to activate the muscles as an
all or nothing function.
Structure of Skeletal Muscle
Muscle Activity
• No device to measure tension within the
muscle directly. We can measure the amount
of force or torque it exerts. We can measure
electrical activity of the muscles
(electromyography or EMG).
• In performing physical work, excessive
loading can cause musculoskeletal problems
such as bone fracture and muscle fatigue.
EMG (Electromyography)
• EMG can be a useful tool in the evaluation of
work performance.
• Ergonomic analyses often include use of EMG
when comparing the specific MS stress in
given muscles associated with various work
positions or activities.
EMG setup
Measuring muscle activity in the flexor muscle.
Static and Dynamic Strength
Measurement
• To measure static elbow
flexion strength as a
function of the angle of
elbow rotation.
• To measure dynamic elbow
flexion strength as a
function of the velocity of
flexion.
• To compare maximum
dynamic strength to
maximum static strength at
a given angle.
Motion Analysis
• Motion of limbs and the
hammer used as a
representative task.
• Calculate the angles of
deviation.
• Calculation the relative
position, velocity, and
acceleration of the
hammer.
The force plate system is used
both independently to measure
parameters such as static balance
as well as in conjunction with the
motion capture system in order
to determine lower extremity
kinetics such as ground reaction
forces and moments.
Click here for a demonstration.
Biomechanical Models
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Mathematical models that represent the mechanical
properties of the human body.
The musculoskeletal system is analyzed as a system
of mechanical links; and, the bones and muscles act
as a series of levers.
Allows one to predict the stress levels on specific
musculoskeletal components quantitatively.
Serve as an analytical tool to help job designers
identify and avoid hazardous job situations.
The designer must fit the physical
dimensions of the products & workplaces
to human characteristics.
The consequences of a mismatch.
• Worker assumes awkward
positions.
• May not be able to use the
product at all.
• Worker is at risk for injury.
• The two most prevalent
injuries are low back pain
and upper-extremity
cumulative trauma
disorders.
Incorrect hand position.
Work-related Problems: Low-back Pain
• One of the most costly and prevalent workrelated musculoskeletal disorder in industry.
• Accounts for one-third of all workers'
compensation payments.
• Prevalent in two common situations
– Manual material handling
– Sedentary work environments requiring a
prolonged static sitting posture.
Work-related Problems: Low-back Pain
Manual material handling
Proper Lifting
Improper Lifting
Seated Work and Chair Design
Low-back pain caused by prolonged static sitting
posture.
Designers must pay particular attention
• Lumbar support should
be adjustable.
• Arm rests support the
body weight and
reduces the load on the
spine, if they are
adjusted correctly.
Backrest Inclination Angle
• Backrest inclination angle (1100 is preferred over
900 because it retains the body’s natural state of
lordosis).
Reducing Back Stress
• Other factors can help
reduce back stress, but are
not as critical (tiltable seat
surface, properly adjusted
seat height, use of cushioned
seat surfaces, and adequate
leg space)
• Slouching is OUT. It
causes kyphosis.
• Workers must take frequent
breaks.
Upper-extremity cumulative trauma
disorders (CDTs)
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Also called repetitive motion injuries or repetitive
strain injuries (RSIs).
Can be even more costly than low-back problems.
Examples
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CDT’s of the fingers: caused by vibrating hand tools;
can cause numbness, tingling, or pain when the hands
are exposed to cold. Also trigger finger and
gamekeeper’s thumb.
CDTs of the hand and wrist (carpal tunnel syndrome)
CDTs of the elbow: tennis or golfer’s elbow.
Carpal Tunnel Syndrome
Signs and Symptoms
• CTS is characterized by pain, numbness and tingling
sensations in the proximity of the median nerve’s
entrance into the wrist.
• Onset is gradual
• Localized pain and numbness of thumb and first 2 1/2
digits
• As disorder progresses, pain worsens at night and
grip strength is diminished
Carpal Tunnel Release Surgery
• Carpal Tunnel bounded by carpal bones on
bottom and transverse carpal ligament on top
• Surgeon transects ligament to increase size of
carpal tunnel
• Relieves pressure on median nerve
Carpal Tunnel Release Surgery Types
Open
Endoscopic
Carpal Tunnel Release Surgery
Carpal Tunnel Syndrome
• Surgery is not the only option and may be
ineffective or cause further damage.
Avoiding Carpal Tunnel Syndrome
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Fit your workstation to your size.
Use proper posture.
Avoid awkward wrist positions.
Take frequent breaks.
Stretch and strengthen muscles.
In extreme cases avoid
keyboarding altogether.
• Wear a wrist brace at night, not
while you are working.
Other Upper-Extremity Injuries
• Excessive and repetitive use of tendons can cause
inflammation of tendons.
• One example is lateral epicondylitis (tennis elbow).
– Can also be caused by painting with a brush or
roller, running a chain saw, and using many types
of hand tools.
Other Upper-Extremity Injuries
• Shoulder pain can be caused by rotator cuff injuries,
swimmer’s shoulder, or pitcher’s arm.
Other Injuries
• Vibration injuries:
whole body, hand,
finger, or shoulder.
Hand Tool Design
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Keep a neutral position
Do not bend at the wrist
Shape tool handles to assist grip
Provide adequate grip span
Provide finger and gloves
clearance
Keep a neutral hand position
Avoid these movements
• Flexion/extension
• Radial/ulnar deviation
How to Avoid or Minimize RSIs from
Computer Work
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Avoid awkward wrist positions!
Relax your muscles while you work!
Fit your workstation to your size
Use proper posture
Take frequent breaks
Stretch and strengthen muscles
In extreme cases avoid keyboarding altogether
Questions?