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Work Load and Pre Employment • Astrand ch. 13 p 453-472 • 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 – % of individual max requires determination of – individual VO2 max – VO2 of imposed load – assessment of muscle groups and the % of max strength - fatigue onset 1 Assessment • Maximal aerobic power – direct - VO2 max test – estimation - predictive tests • Assessment of Workload • measure O2 uptake during work – validity and assumption around indirect calorimetry addressed last week • Fig 13-2 O2 uptake vs bike/work – portable devices, rapid analysis of VCO2 and VO2 - large data base – field studies - collect expired air – or - flow meter and sampling of air • Fig 13-3 commercial fisherman • subject often affected - test atypical 2 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,emotional stress • Fig 13-4 - criterion of HR response • 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 13-5 - fisherman 3 Comparison studies • Fig 13-6 - strong day - day consistency - mean HR • Fig 13-7 comparison of direct vs indirect measurement +/- 15 % • Fig 13-8 arm vs leg work – strong discrepancy - difficult and inaccurate comparison • O2 uptake for work load must be expressed as % max of individual • indicates relative degree of exertion 4 Nervous Response • Inc sympathetic tone - inc HR – influence linear relationship – HR vs workload • Hormonal response – total stress reflected by sympathetic response – measured with urinary excretion or ep and nor ep (blood samples) • Fig 13-9, 13-10 – Catecholamines - inc with standing, phsyical exertion, cold,emotional factors – inc with duration and severity of muscular exertion 5 Energy Expenditure • Practical limits • 30 - 40% VO2 max for 8 hour day • 40 % of max strength in repetitive muscular work ; rest:work ; 2:1 • physiological and psychological responses influenced by – – – – individual max aerobic power size of muscle being engaged working position type of activity (intermittent or continuous) – environmental conditions • Classification - O2 uptake and HR • Table p 462 6 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 13-1 (*1964*) • Fig 13-11 - work expenditure 7 Energy expenditure • Manual labour • technique can influence significantly – inc energy with speed, 10X with stairs • only rough estimate of efficiency – driving nails eg. – Bench, wall, ceiling – same O2 consumption, difference in efficiency – same HR as cycling - different O2, different BP response • * variability in work output and physiological response with tools and position – even at same energy expenditure 8 Strength • Gallegher - OEH ch 21 • Strength - capacity to produce a force or torque with a voluntary muscle contraction • Measurement of human strength – at interface between subject and device – influences measurement • Fig 21.1 Biomechanical eg. – Q = (F * a)/b or c or d – results specific to set of circumstances, force from muscle is always the same • dynamic - motion around joint – variability - speed - difficult to compare • static - isometric- no motion – easy to quantify and compare - not representative of dynamic activity 9 Strength • Isometric strength • 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 (isotonic) • isoinertial - mass properties of an object are held constant • Psychophysical - subject estimate of (submax) load - under set conditions • isokinetic strength – through ROM at constant velocity 10 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 11 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 12 Measurement for Job Design • 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 13 Worker selection and Placement • General recommendations • Key principles – job relatedness • 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 strong and less strong • Isometric analysis fig 21.5 – for each task - posture of torso and extremities is documented (video) • recreate posture - software – values compared to population norms industrial workers – estimate % capable of level of exertion – predict forces acting on lumbar spine 14 Job placement • 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 - 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 15 Job placement testing • Isokinetic testing – humans do not move at constant velocity – isokinetic tests usually isolated joint movements – may not be reflective of performance ability • attempts to redesign - multi joint simulation tasks for industry – fig 21.8 – core stability required – still in progress, limited validity 16 Pre Employment Physical Evaluation • Physical ability test - negative impact on females (.80) – strength, VO2 max, % body fat • integration of psychometric measurement theory, biomechanics, ergonomics and work physiology • Injury rates – some jobs - high low back injuries – not serious but prevalent - 80% – lifting, twisting, bending, pulling • approaches – redesign job – pre employment testing – education and training 17 Job Analysis • essential component in developing pre employment test • 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 • Biomechanical methods – heights and weights of objects lifted – forces - opening, pulling, pushing – evaluate potential stress on lower spine • Physiological - CV components - O2 – HR - actual/simulated - estimate work 18 Validation Strategies • Determine accuracy with which test measures important work behaviors • 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 • Construct validity – link important constructs and multiple indicators of job performance 19