Transcript Slide 1

Ergonomics
An ergonomics approach to
designing for disabled workers
Agenda
•
•
•
•
•
•
•
•
•
Ergonomics & disability
Design implications
Consequences of adaptation
Reducing risk: Posture
Dimensional factors in work posture
Practical exercise
Force, repetition, vibration
RULA/REBA
Ergonomics Design Guide
Ergonomics
• Ergo = work nomos = knowledge
• Rational approach to optimizing work efficiency,
minimizing health risk.
• Based on:
–
–
–
–
–
–
–
Anatomy
Physiology
Psychology
Biomechanics
Work study/Task analysis
Epidemiology
Systems Design Engineering
What is ‘Disablement?’
Disablement
“Disablement is the loss or limitation
of opportunities to take part in the
normal life of the community
[including employment] on an equal
level with others, due to physical and
social barriers." Disabled People's International 1981
Disability
Which is more rational for us to say;
“a disabled person” or “a person
with a disability?”
Disability ‘Models’
‘Person With a Disability’ =
Medical Model
 18th Century ‘scientific/rationalist’
approach.
 Assesses impairment from the
point of view of what a person
cannot do, instead of what they
can do.
 Sees people as having to be
adapted to fit the ‘normal’ world.
 The emphasis is dependence; its
focus is upon the impairment,
rather than the needs of the
person.
 Attributes problems arising from
the built environment to a lack of
rehabilitation of people with
disabilities.
‘Disabled Person’ =
Social Model






20th/21st Century
social/contextual approach.
Starts by looking at the
capabilities of the person.
Defines 'impairment' and
'disability' as different things.
Suggests disabled people's
disadvantage is due to
institutional discrimination.
The focus is on independence.
The view is that the built
environment should be made to
suit all possible users.
Disability
It is the built environment that is the source of
the problem, not the person.
By falsely attributing ‘fault’ to the disabled
person, equipment designers may not feel it
necessary to accommodate their needs as a
matter of course, only perhaps as ‘special
cases.’
Ergonomics and Disablement
• Ergonomics - recognizes that people are
routinely disabled by barriers presented in a
poorly designed built environment.
• This environment includes tasks that do not
take account of individual, natural,
predictable variability.
• Further recognizes that capacity and
capability vary not only between, but also
within individuals, due to accident, illness
and age.
Inclusive Design Steps
• Designing for a disabled worker logically
requires the following steps.
• Determine their needs now and (if feasible) in
the future, based on:
– task demands (task analysis, task measurement)
– worker capacity (worker measurement, static and
dynamic).
Inclusive Design Steps
• Verify that meeting the disabled worker’s needs
will not force others to adapt and suffer injury.
• If others might have to adapt, design the task to
meet the needs of these other users also.
• Or specify removable adaptations
• Or specify which users may and may not safely
use the adapted equipment.
• This will minimize risk of secondary and primary
injuries and hence risk of personal injury
litigation.
Primary and Secondary Injuries
• Primary injuries (1): due to the poor fit of a
task or ‘tool’ to the capacities or
capabilities of the operator.
• Primary injuries (2) Harm caused by poor
fit of an adaptive aid to the capacities or
capabilities of an unintended user.
• Secondary injuries: Harm caused by the
poor fit of an adaptive aid to the capacity
or capability of an intended user.
Reducing Injury Risk
Risk reduction begins with determining a user
population’s variability in parameters related to
adaptation such as:
Size (physical dimensions of body parts)
Gender
Age
Weight
Strength
Handedness
Mental capacity.
Fit-Adaptation
•
•
•
If we do not fit tasks to operators’ parameters, we
force operators to adapt to their tasks.
Hidden costs in terms of ill-health (latent injury).
Reduces efficiency/productivity.
Worker - Job Match<<<<<>>>>> Worker - Job Mismatch
Decreasing Adaptation<<<<<>>>>>> Increasing Adaptation
Enabling, Safe Work<<<<<>>>>> Disabling, Injurious Work
14
Adaptation to Task:
Health Consequences
Pathological spine deformations in different occupations as
determined by x-ray examination.
Occupation
% with Spine Deformations Average Age (years)
Truck drivers
80.0
-Tractor drivers
71.3
26
Miners
70.0
51
Bus drivers
43.6
40
Factory workers
43.0
45
Construction workers
37.0
51
Source: Rossegger, R., and S. Rossegger. 1960. Health effects of tractor driving. Journal of Agricultural
Engineering Research 5(3): 241.
15
Reducing Risk (Example)
• Back disorders are the most common
debilitating musculoskeletal disorders
affecting people who work in farming –
adults and children.
• Need rational risk assessment - important
not to overlook risk factors.
• Example: postural risk assessment for
back injury.
Compressive loading at L5 disk:
reference posture
100%
Compressive loading at L5 disk:
Sitting & Reaching:
C
180%
B
175%
A
125%
Biomechanical load
(backrest angle)
N
500
Height of backrest
Giving lumbar support
(cm)
400
Disc Pressure
0
300
3
200
5
90
100
110
Backrest Inclination (Degrees)
120
Intradiscal pressure - influence of backrest angle
The ability to adopt reclined
postures is mainly affected by:
1.Reach demands of the task
2.Visual demands of the task
Now consider one of these factors
– reach in a short practical session.
Practical
• Maximum Forward Grip Reach variability
in this audience.
• Based on the measurements we have
taken, at what distance would we position
a lever hand grip, assuming the lever
might be operated by any of you as part of
your job?
Calculating Percentiles
1. Find the mean (total measurements /number of
measurements taken ‘n’ ) (so if n=10, with total of 250” then:
250/10 = mean is 25”)
2. Calculate the Standard Deviation:
1. Deduct the mean from each measurement (e.g. 26.5 – 25
= 1.5, 23 - 25 = -2)
2. Square each difference (all ‘-’ become ‘+’) e.g 1.5x1.5 =
2.25, -2x-2 = 4);
3. Add together all of the squares (2.25+4+,…,)
4. Divide this sum by n-1
5. The figure you now have is the ‘standard deviation’
(shown as σ² or s²). Let’s assume σ² is 1.5 in this
case.
Calculating Percentiles
1. Now consider your measurement…say 23 inches.
Deduct the mean from this figure: 23 - 25 = -2
2. Divide -2/σ² = -2/1.5 = -1.333 (This is known as a ‘zscore’)
3. Now look up -1.333 in a table of z-scores:
4. We see that -1.34 = p=9 and -1.28 = p=10
5. These ‘p’s are percentiles. So, -1.33 is just over the
9th percentile. That means that in our sample of 10
measurements, we expect 9% to be 23 inches or
shorter.
6. Therefore, we expect 91% to be longer than 23
inches.
Using %iles to Calculate
Dimensions
1. We can also calculate this ‘backwards’ allowing
us to find out the size we need to make
something to fit a desired percentage of a user
population.
2. This approach is essential when designing
equipment or tools that unknown people (or
changeable people, e.g. due to aging, loss of
functionality etc) will possibly use.
3. Commonly use the 5th and 95th percentiles
depending on which is critical.
Using %iles to Calculate
Dimensions
1.
2.
3.
4.
Assume an anthropometric table is available:
Forward grip reach (mm…we’ll convert shortly)
Females 5th percentile = 655mm (25.79 inches)
Note that if you had made this for the 95th
percentile US male the distance would have
been 33.27 inches!
5. What would be the effect on posture and safety
of a 5th percentile woman using a lever placed
7.48 inches beyond her maximum reach?
Percentiles in Design
1. 5th%le US female for:
1. Reach
2. Strength/Force requirement
3. But N.B. Consider correct %ile for guarding: e.g. to
prevent slim or long fingers from being injured
2. 95th%le US male for:
1. Clearance
2. Weight bearing (plus adequate safety margin – e.g.
could use 99.9th% and add 15-20%)
Source:
Pheasant, S. and Haslegrave, C.M. (2006)
“Bodyspace” 3rd Edition.
Taylor and Francis
Limits of Usability
• Designing for all possible users is ideal, but
typically has practicability and affordability
issues. Consider 5th-95th percentiles, extend
either direction as dictated by the task or user
group (e.g. 1st to 95th percentile).
• If designing for one user, you must calculate and
state what other users’ parameters must be.
• Potential users - consider future changes in work
force, addition of family members, gender, age,
functional capacity, etc.
Static Anthropometry
• Useful starting point for design of tasks
and task environments.
• Age, gender, racial factors.
• Clearance generally >95th percentile male,
plus clothing allowance;
• Clearance in guarding components (e.g. to
prevent access of body parts) may need to
be <1st percentile.
• Reach generally <5th percentile female.
Dynamic Anthropometry
• The dimensions of the body in motion.
• Range of motion (joints)
• Age factors.
Force
• Amount of safe force we can apply is influenced
by:
–
–
–
–
–
–
Frequency
Posture
Maximum strength (for that movement)
Climatic factors
Health status
Fatigue
• Always aim to minimize force (but not so much
that a device will be activated unintentionally).
Repetition
• High force, high repetition most dangerous
• High force, low repetition dangerous
• High repetition, moderate or low force can
be dangerous also.
• Figures differ for different tasks.
• Always try to minimize repetition.
Vibration
• Segmental: especially upper-limbs. Power
tools, machinery etc.
– Gloves with absorbing pads can help. Foam
wrapping may also be useful in some cases.
– Avoid gripping too firmly any hard surface or
handle that is vibrating. Look at alternative
methods of mounting the device.
– Eliminate mechanical sources of vibration if
possible – e.g. servicing equipment.
• Whole body: driving off road
RULA-REBA
• RULA (Rapid Upper Limb Assessment)
• REBA (Rapid Entire Body Assessment)
• Systematic tools to quickly assess risk
from posture, force and repetition.
• Reduce scores through task redesign
and/or behavior guidance.
• Re-assess after intervention.
Ergonomics Design Data
• Extracts from Woodson (1981) are a
useful field reference/guide.
• However, anthropometric data are old and
generally based on military personnel –
• New civilian anthropometric data are
available but expensive (c. $30,000).
• Useful: Bodyspace (Pheasant, S. &
Haslegrave, C.M. 2006).
Questions