Transcript Work Load and Pre Employment

Work Load and
Pre Employment
• Astrand ch. 17 p 503-540
• Gallagher and Moore - Occupational
Ergonomics Handbook Ch 21 p 371-383
• Jackson p53, 58-70
• Outline
• Assessment of workload
– Relative VO2, HR, Hormonal response
• Energy expenditure over workday
– Energy systems, work rest ratio
• Strength testing
– Onset of fatigue
– Isoinertial, isometric, isokinetic,
psychophysical
• Job analysis
– validation
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Work Load and
Pre Employment
• Astrand ch. 17 p 503-540
• Gallagher and Moore - Occupational
Ergonomics Handbook Ch 21 p 371-383
• Jackson p53, 58-70
• Assessment of work load in relation
to work capacity
– variability in capacity
– variability in response
• expression of workload by absolute
VO2 alone is almost meaningless
• Need work load as % of individual max
• Assessment requires the determination of
– individual VO2 max
– VO2 requirement of imposed load
– assessment of muscle groups being utilized, and
the % of their maximum strength -to determine
fatigue onset
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Assessment
• Maximal aerobic power
– direct - VO2 max test
– estimation - predictive tests
• Assessment of Workload
– measure O2 uptake during work
• Fig 17-2 O2 uptake vs bike/work
– portable devices, rapid analysis of
VCO2 and VO2 - large data base
– field studies - collect expired air
• Douglas bag
– or - use flow meter to determine
volume of air, and take samples of air
for content analysis
• Fig 17-3 commercial fisherman
– subjects often affected - test atypical
– Eg. Breathing through mouth not nose
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Indirect assessment
• Recall linear relationships between
HR and VO2, VO2 and work rate
– HR may be used to estimate workload on individual basis
– same muscle groups environmental
temperature, and emotional stress
• Continuously recorded HR
– provides general picture of overall
activity level during entire day
– along with time activity studies
collected by observers
– possible to separate different activities
with respect to HR
– Fig 17-5 - fisherman
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Comparison studies
• Fig 17-6 - strong day - day consistency
• Computer analysis of HR data provides;
– mean values, peak values, distribution
and HR variability
• Fig 17-8 comparison of direct vs
indirect measurement +/- 15 %
• HR is good estimate of workload
when work uses large muscle groups
• Fig 17-9 arm vs leg work
– HR is higher in arm work than leg work
for the same work load.
• O2 uptake for work load must be
expressed as % max of individual
– indicates relative degree of exertion
• HR reserve (HR max - HR rest)
– Circulatory strain is best expressed as a
percentage of an individuals HR reserve
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Stress of Work
• The total stress imposed on the organism
by a given work situation (physical or
psychological) is generally reflected by
nervous and hormonal stimulation
– Proportional to the degree of stress
• Nervous Response
• Inc sympathetic tone - inc HR
– influence linear relationship
– Eg HR vs workload
• Hormonal response
– total stress reflected by sympathetic
response
– Measure ep and/or nor ep with urinary
excretion or blood samples
• Fig 1716, 17-17 - Catecholamines - inc
with standing, cold and emotion
– Also inc with duration and severity of
muscular exertion
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Energy Expenditure
• Need to establish practical limits for
physical work loads
• Type of work and work/rest cycles
are important
• Large individual differences in
physical work capacity
– 30 - 40% VO2 max for 8 hour day
– 40 % of max strength in repetitive
muscular work;
• rest:work ratio of ; 2:1
• physiological and psychological
responses influenced by
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individual max aerobic power
size of muscle being engaged
working position
intermittent vs continuous activity)
environmental conditions
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Daily Energy Expenditure
• Important for
– calculation of energy needs
– determine physical activity of groups
– role of physical activity in health
• Methodology
– 24 hr recorded HR
– time activity data
– assessment of daily energy intake to
maintain body weight
– all fairly accurate +/- 15%
• show large individual variability
– 1300-5000 kcal /day
• Table 17-1, 17-2
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Energy expenditure
• O2 uptake and HR - Table 17-1
• Important for ;
– Calculation of energy needs
– Determine physical activity of groups
– Role of physical activity in health
• Methodology All fairly accurate +/- 15%
– 24 hr recorded HR ; Time activity data
– Assessment of daily energy intake to
maintain body weight
• Wide individual variability in energy
output - Table 17-1,2
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Occupation
Leisure activity / Physical activity
Environmental temp
Daily rate 1300 - 5000 kcal
• Reg active male 2900 kcal/day
• Reg active female 2100 kcal/day
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Energy Expenditures
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Work expenditures
Most light work < 5 kcal /min
Some physical jobs 7.5 - 10.5 kcal/min
Permissible limits for daily work 20002500 kcal
Limits are difficult due to individual
differences in work capacity or fitness
Individuals will usually self regulate the
rest pauses
Peak load is more important than mean
energy expenditure
You can attain a higher 8 hour energy
expenditure if the work is consistent and
does not have peak loads
Basal Metabolic rate (BMR) - rate of
energy metabolism in a resting individual
14-18 hours after eating
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Energy Expenditure
• Recreational activities
• McArdle, Katch and Katch
– Appendix C (on reserves)
• Different activities have different
energy expenditures
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Cycling race
Golf
Skiing uphill fast
Swimming -fast
Running 5:30 mile
13 kcal/min
6 kcal/min
21 kcal/min
13 kcal/min
22 kcal/min
• Individuals do activities at different
intensities
• Must take body weight into account
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Strength
• Gallagher - OEH ch 21
• Muscle strength is a complex function that
can vary with the methods of assessment
• Strength - capacity to produce a force or
torque with a voluntary muscle contraction
• Measurement of human strength
– Cannot be measured directly
– interface between subject and device
influences measurement
– Fig 21.1 Biomechanical eg.
• Q = (F * a)/b or c or d
• force from muscle is always the same
• results are specific to circumstances
• dynamic strength - motion around joint
– variable speed - difficult to compare
• static or isometric strength- no motion
– easy to quantify and compare
– not representative of dynamic activity
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Strength Testing
• Isometric strength testing
• standardized procedures
– 4-6 sec, 30 sec to 2 min rest
– standardized instruction
• postures, body supports, restraint systems,
and environmental factors
– worldwide acceptance and adoption
• Dynamic strength
– isoinertial (isotonic)- mass properties of an
object are held constant
– Psychophysical - subject estimate of
(submax) load - under set conditions
– isokinetic strength
• through ROM at constant velocity
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Factors Affecting Strength
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Gender
Age
Anthropometry
Psychological factors - motivation
– table 21.1
• Task influence
– Posture
• fig 21.2 angle and force production
– Duration
• Fig 21.3
– Velocity of Contraction
• Fig 21.4
– Muscle Fatigue
– Temperature and Humidity
• inc from 20-27 C - dec 10-20% in capacity
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Strength Measurement
• Strength assessment for job design
• psychophysical methods
– workers adjust demand to acceptable
levels for specified conditions
– provides ‘submax’ endurance estimate
• Procedure –
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subject manipulate one variable-weight
two tests : start heavy and start light
add or remove weight to fair workload
without straining, becoming over tired,
weakened, over heated or out of breath
• large #’s of subjects
– evaluate / design jobs within capacity
– 75% or workers rate as acceptable
• over this; 3 times the injury rate
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Psychophysical (cont)
• Summary
– Table 21.2 (Snook and Cirello)
• advantages
– realistic simulation of industrial tasks
– very reproducible - related to incidence
of low back injury
• Disadvantages
– results can exceed “safe” as determined
through other methodology
– biomechanical, physiological
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Worker selection and
Placement
• Used to ensure that worker can
tolerate physical aspects of job
• Key principles
– Strength test employed must be directly
related to work requirements
• must be tied to biomechanical analysis
– use of strength tests only to identify
workers at high risk of injury
• similar rates of overexertion injuries for
stronger and weaker workers
• Isometric analysis fig 21.5
– for each task - posture of torso and
extremities is documented (video)
• recreate postures using software
– values compared to population norms
• industrial workers
– estimate % capable of level of exertion
– predict forces acting on lumbar spine
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Isoinertial Testing
• SAT - strength aptitude testing
– air force standard testing
– preselected mass - increase to criterion
level - success or failure
– found incremental weight lifted to
1.83m to be safe and reliable
• PILE - progressive inertial lifting
evaluation
– lumbar and cervical lifts -progressive
weight - variable termination
• voluntary, 85 % max HR, 55-60% body
weight
• standards normalized for age, gender and
body weight
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Isokinetic Testing
• Isokinetic testing
– Evaluates muscular strength throughout
a range of motion at a constant velocity
– However;
• humans do not move at constant velocity
• isokinetic tests usually isolated joint
movements
• may not be reflective of performance ability
• attempts to redesign isokinetic
testing
– multi joint simulation tasks for industry
• fig 21.8
• core stability required
• still in progress, therefore limited validity
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Pre Employment Physical
Evaluation
• Early tests focused on cognitive abilities
• Physical ability tests have adverse
impact on female applicants
– Due to differences in size, strength,
VO2 max, % body fat
• Improvements attempt to integrate
– psychometric measurement theory,
biomechanics, ergonomics and work
physiology
• Employers must show that the preemployment screening test measures
only those abilities required to
perform essential job functions
– (*Assignment two *)
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Injury Rates
• Pre-employment tests are used to
reduce work related injuries
– some jobs - high low back injuries
– not serious but prevalent - 80%
– lifting, twisting, bending, pulling
• 3 Ergonomic approaches to reduce
industrial back injuries
– job Redesign
• Engineering approach to reduce exposure to
risk factors
– pre employment testing
• Preferred approach for physically
demanding jobs that cannot be redesigned
• Select individuals with capacity to safely
perform job
• Relationship between probability of injury
and the % of capacity to perform task
– education and training
• Train to lift properly - not successful
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Fitness
• Fitness level appears to be related to
injury
– High levels of aerobic fitness, strength
and flexibility were inversely related to
workers compensation costs of fire
fighters
• Fitness programs should reduce
musculoskeletal injuries
– Only 20-40 % of employees will use
work site exercise facilities
– Only 33% of these will train at the
appropriate frequency, duration and
intensity
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Job Analysis
• Goal is to identify the important
work behaviours demanded by job
• Reduce job actions to words
• essential component in developing
pre employment test
• Job analysis methods can use a
combination of;
• Psychophysical, biomechanical and
physiological approaches
• Workers rate tasks (psychophysical)
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RPE - rate of perceived exertion
Borg, Likert scales
compare to text book ratings
.8 correlation - not biased by gender or
experience
– components - strength, CV endurance and
movement quality
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Job analysis methods
• Biomechanical methods
– heights and weights of objects
lifted
– forces - opening, pulling, pushing
– evaluate potential stress on lower
spine
• Physiological - CV components
– O2, HR - actual or simulated to
estimate work load
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Validation Strategies
• Determine accuracy with which test
measures important work behaviors
• Validity depends on reliability and
relevance
– Reliability - ability to differentiate among
true levels of performance
– Relevance - defining qualities being tested
• Criterion related validity
– significant correlation between pre
employment test and job performance
– Concurrent / predictive
• Content validity
– work sample or simulation of work
– Test represents the content of the job
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Construct Validity
• Establish that a construct is required for
job success
• The test measures that same construct
• The degree to which a test measures an
attribute that cannot be directly measure
(coordination)
• link important constructs and multiple
indicators of job performance
• Tests theoretical and empirical
relationships
• Relationship between physical ability and
job constructs
• FDNY physical ability test was found to
discriminate against women as constructs
were not related to the job
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Physical Ability Test Selection
Content validation studies
 Tests that simulate work tasks
 Advantages
Simulate actual working
conditions
- fire fighter drags a dummy
- lineman climbs a telephone pole
 Disadvantages
safety (applicants are highly
motivated)
do not measure maximal capacity
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Physical ability (cont)
Motor ability and fitness test items
static strength (exert maximal force)
dynamic strength
balance
flexibility
Cut Score
the test score that an applicant must
obtain to be considered for a job.
difficult to establish a cut score.
should you hire someone that just meets
the minimal requirement?
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Summary
• Employers are using pre-employment tests
to ;
– enhance worker productivity
– to minimize the threat of litigation for
discriminatory hiring practices
– and to reduce the risk of
musculoskeletal injuries.
• The most common reason for ruling an
employment practice invalid is the failure
to show that the test measured important
job behaviours.
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