1 Work Load and Pre Employment Astrand ch. 13 p Gallagher and Moore - Occupational Ergonomics Handbook Ch 21 p Jackson p53, 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

2 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

3 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 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 fisherman

4 Comparison studies Fig 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

5 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, –Catecholamines - inc with standing, phsyical exertion, cold,emotional factors –inc with duration and severity of muscular exertion

6 Energy Expenditure Practical limits % 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

7 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 – kcal /day Table 13-1 (*1964*) Fig work expenditure

8 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

9 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

10 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

11 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 C - dec 10-20% in capacity

12 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

13 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

14 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

15 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

16 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

17 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

18 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

19 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