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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

• 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– 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– 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• Work expenditures• Most light work < 5 kcal /min• Some physical jobs 7.5 - 10.5 kcal/min• Permissible limits for daily work 2000-

2500 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– Cycling race 13 kcal/min

– Golf 6 kcal/min

– Skiing uphill fast 21 kcal/min

– Swimming -fast 13 kcal/min

– Running 5:30 mile 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• 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 - – 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 pre-employment 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)

– 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.