Great deal of our work activities require physical effort and the manual handling of materials, supplies and tools.

Transcription

1 Physical Workload

2 Introduction Great deal of our work activities require physical effort and the manual handling of materials, supplies and tools. 2

3 Stress & Strain Stress Undesirable condition, circumstance, task, or other factor that impinges upon the individual Strain Effect of the stress on the individual 3

4 Muscle Physiology Nature Contractibility Control of Action Metabolism Anaerobic Glycolysis Aerobic Glycolysis Basal Metabolism 4

5 Nature > 600 muscles > 400 skeletal muscles (in pairs) Skeletal muscles are attached to bones by tendons Vigorous Activities < 80 pairs Contraction causes movement 5

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7 Nature Several hundred thousands of muscle fibers Fibers connected by tissues nerve impulses entrance, carried through fibers Oxygen & nutrients thru blood vessels permeating muscle tissue 7

8 Contractibility Muscle fiber Contractile unit (Myofibril) Myosin & Actin (arranged in bands) Slide between each other (half length) Force = f (# activated muscle fibers) 8

9 Contractibility 9

10 Contractibility 10

11 Control of Action Activation Motor nerves emanating from the spinal cord Conscious vs. Reflex Motor Unit Motor-nerve fiber + group of muscle fibers 11

12 Metabolism Energy to slide actin over myosin Food (carbs & fat) Carbs to Glucose (in the liver) Energy stored in ATP Adenosine Triphosphate ATP to ADP Creatinine Phosphate (CP) Immediate energy for a muscle Energy store for ATP (short time) 12

13 Anaerobic Glycolysis Initial breakdown of glucose and liberation of energy does not require oxygen Glucose into pyruvic acid Pyruvic acid into lactic acid Fatigue 13

14 Aerobic Glycolysis Pyruvic acid into CO 2 & H 2 O 20 times the energy by anaerobic glycolysis Strenuous bout of physical activity converts lactic acid into pyruvic acid + O 2 (pyruvic acid undergo aerobic glycolysis) Energy used to build up ATP and CP stores 14

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16 Basal Metabolism Food Heat + Mech. Energy 70% 30% large muscles BMR: basic metabolic rate: amount of energy per unit time needed to sustain life Body size Gender Age 16

17 Work Physiology Respiratory Response Additional O 2 for aerobic glycolysis At Rest < 0.5 L/min Extremely heavy work 5 L/min breathing rate volume of inspired air Lag >> anaerobic glycolysis ATP & CP stores depletion (debt) 17

18 Work Physiology Respiratory Response Oxygen Debt: O required by the muscles 2 after beginning of work over and above supplies during work activity Has to be repaid 18

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20 Work Physiology Cardiovascular Response O 2 + glucose Blood Muscles 25 [O 2 + glucose] Blood Cardiac Output 5 Cardiac Output heart rate stroke volume 20

21 Work Physiology Cardiovascular Response Cardiac Output Rest 5 L/min Heavy work > 25 L/min At 40% of max Capacity Stroke volume stabilizes HR continue to increase 21

22 Work Physiology Cardiovascular Response Catecholamines Adrenal Glands Strengthen heart beat blood pressure Greater strain on the heart blood pressure blood to the heart 22

23 Work Physiology Cardiovascular Response Redistribution of Blood Flow Blood Flow Distribution (%) Part of the Body Resting Heavy Work Muscles Skin 5 10 Brain Bones Kidneys Digestive System Heart Muscle

24 Measures of Physiological Strain Overall Level Physical work & Stress placed on the Body Local Strain Oxygen Consumption (uptake) Max. Aerobic Power (MAP) Heart Rate Catecholamines Muscle Activity Subjective Measure of Exertion Intrusive & Expensive 24

25 Oxygen Consumption 25

26 Oxygen Consumption 1 liter of O 2 liberates 5 kcal of energy Oxygen Energy Inspired air per minute Air contains 21% Oxygen Analyze expired air for % O 2 O 2 uptake is determined 26

27 MAP O 2 levels off at maximum O 2 uptake Energy metabolism is largely anaerobic Lactic acid builds up in blood and muscles tissues 27

28 MAP The Higher person s MAP the greater the efficiency of the cardiovascular system Age, Sex, Genetics, and Physical Training Measuring MAP 28

29 Age, Gender and MAP 29

30 Heart Rate O 2 consumption and HR Directly related (linear) Varies with people O 2 consumption Energy is determined 30

31 Heart Rate Each person should be calibrated Dynamic workload Static workload (higher HR at any level of O consumption) 2 HR is best used as O 2 consumption predictor after stroke volume stabilizes (moderate to heavy work) 31

32 Heart Rate other influences Emotional stress Fatigue Heat stress 32

33 Heart Rate Application Continuous sampling of HR over a workday or task General indicator of physiological stress w/o reference to O consumption or 2 energy expenditure HR recovery curve 33

34 Heart Rate 34

35 Catecholamines Secretion by adrenal gland Epinephrine & norepinephrin Every two hrs Measure of Occupational Stress 35

36 Local Muscle Activity Physiological strain of individual muscles or muscle groups Electromyography (EMG) 36

37 EMG Electrodes Data Logger Recording electrical activities EMG signal analysis Amplitude (% of MVC EMG) Frequency 37

38 EMG 38

39 EMG Higher correlation between EMG activity and muscular force Fatigue Increased activity in the low frequency range Reduced activity in the high frequency range 39

40 Subjective Measure of Exertion Borg Scale RPE (rating of perceived exertion) Dynamic Work Ratings 6 to 20 are linearly related to Heart Rate HR = 10 * rating 40

41 Borg-RPE Scale 6. No exertion Extremely light 9. Very light Light Somewhat hard 41

42 Borg-RPE Scale Hard (heavy) Very hard Extremely hard 20.Maximal exertion 42

43 Major Factors Influencing Power & Capacity for Physical Activity Somatic Factors Sex & Age Body dimensions Health Training adaptation Psychic Factors Attitude Motivation Workload Nature Intensity Duration Technique Position Rhythm Schedule Service Functions 1. Fuel 2. Oxygen Energy yielding processes Environment Altitude High gas pressure Heat Cold Noise Air pollution Physical Performance 43

44 Work Efficiency Energy expended Much of it heat ( 70%) Some unproductive static efforts (e.g. holding) Useful work Efficiency (%) work output energy consumption

45 Work Efficiency Activity Efficiency (%) Shoveling (stooped posture) 3 Shoveling (normal posture) 6 Using heavy hammer 15 Going up & down stairs (no load) 23 Pulling a cart 24 Pushing a cart 27 Cycling 25 Walking on level (no load) 27 45

46 Effect of Equipment on Efficiency 46

47 Efficiency as Energy Consumption per unit of Walking Effort 47

48 Energy Consumption Activity Physiological Cost (kcal/min) Sleeping 1.3 Sitting 1.6 Standing 2.25 Walking (level) 2.1 Cleaning / ironing Cycling (16 km/h)

49 Energy Consumption Occupation Energy Expenditure (kcal / day) Mean Min Max Laboratory technicians Males Females University students Males Females Males only Construction workers Steel workers Coal miners Housewives (middle age)

50 Energy Consumption 50

51 Grades of Work Grade of work Energy expenditure, (kcal / min) Energy expenditure, 8 h (kcal / d) HR, (beats / min) Oxygen consumption, L / min Rest (sitting) 1.5 < Very light work Light work Moderate work Heavy work Very heavy work Unduly heavy work > 12.5 > 6000 > 180 >

52 Grades of Work Most occupations light or moderate work maximum values mostly at heavy work 52

53 Factors Affecting Energy of Work Methods of Work Work Posture Work Rate Tool Design 53

54 Methods of Work Good posture balance Minimal effect on body s center of gravity 54

55 Methods of Work 55

56 Work Posture 56

57 Work Rate 57

58 Tool Design 58

59 Recommended Limits 35 % of MAP < 5 kcal / min over 8-h workday males < 3.35 kcal / min over 8-h workday females Max 6.25 kcal / min over 4-h workday males Max 4.2 kcal / min over 4-h workday males 115 beat / min 112 beat / min leg work 99 beat / min arm work 59

60 Work-Rest Cycles Workload > limits Compensation (rest) Grade of Work & Duration Charts % of work time e.g. very heavy task for 10 min 80% rest = 8 min 60

61 Work-Rest Cycles 61

62 Work-Rest Cycles Work period before rest Shorter work periods with shorter rest periods Better physiological recovery and lower stress level 62

63 Work-Rest Cycles 63

64 Strength & Endurance Limiting factors in many tasks and activities Endurance: Manifestation of cardiovascular system capability 64

65 Strength Maximal force muscles can exert isometrically in a single voluntary effort Muscle tension is not constant Static (isometric change with time) Dynamic (isokinetic change with movement) 65

66 Dynamic Strength 66

67 Static Strength 67

68 Maximum Static Strength 68

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70 Personal factors Affecting Strength Sex Age 70

71 Examples of Strength Data Body joint / description Female (%ile) Male (%ile) Elbow Flexion (90 arm at side) extension (70 arm at side) Knee Flexion (135 seated) Extension (120 seated) Torso Flexion (100 seated) Extension (100 seated)

72 Endurance Maintain a given muscular force Repetitive dynamic work Frequency Force 72

73 Strength & Endurance Correlated 73

74 Strength & Endurance 74

75 Strength & Endurance 75

76 Manual Material Handling (MMH) Lifting Carrying Pushing Pulling 76

77 Manual Material Handling (MMH) Health Effects Lacerations, Bruises, Fractures Cardiovascular strain Muscular fatigue Chronic bronchitis Musculoskeletal injuries Back pain 77

78 Approaches to Assessing MMH Biomechanical Physiological Psychophysical Each result in different recommended load limits 78

79 Biomechanical Approaches Human body system of links and connecting joints corresponding to body segments Mechanical stress and muscle force Models have been developed to determine forces and torques acting on the body during MMH 79

80 Biomechanical Approaches 80

81 Biomechanical Approaches Significant forces in the lower back Compressive and Shear forces Most MMH injuries in the low back area 81

82 Physiological Approaches Frequent tasks Over some duration of time 8-hour shift Energy consumption Stresses on the cardiovascular system 82

83 Psychophysical Approaches Subjective evaluation of perceived biomechanical and physiological stresses Max load sustained w/o: Strain or discomfort Unusually tired, weakened, overheated, or out of breath 83

84 Psychophysical Approaches Appropriate for setting criteria for MMH tasks Maximum acceptable weigh of load (MAWL) Validation required Representation of population of workers Repetition Training Reduce visual cues 84

85 Lifting Tasks Most of the MMH Most back injuries Variables influence stress during lifting Horizontal position of load Height and range of lift Method of lifting Frequency of lifting Load characteristics Size, shape, handles, and distribution and stability 85

86 Horizontal Position of Load 86

87 Method of Lifting 87

88 Method of Lifting 88

89 Frequency of Lifting 89

90 Frequency of Lifting 90

91 Recommended Limits for MMH Tasks Biomechanical Physiological Psychophysical 91

92 Biomechanical Recommended Limits Based on compressive forces on the low back Action limit (AL) Maximum Permissible limit (MPL) MPL < 3 * AL 92

93 Biomechanical Recommended Limits 93

94 Physiological Recommended Limits 33% of maximum aerobic power (MAP) 5 kcal/min 94

95 Physiological Recommended Limits 95

96 Psychophysical Recommended Limits MAWL is the recommended load limit for a specific task 96

97 Reducing Risk of MMH - Job Design Decrease weight Use help Change activity Minimize horizontal distance Stack no higher than shoulder Keep heavy objects at knuckle height Reduce frequency of lifting Rest Job rotation Containers with handles 97

98 Reducing Risk of MMH Worker selection Pre-employment screening Training Controversial 98