10. Anthropometry and Work-Space Design

1. 10. Anthropometry and Work-Space Design • Anthropometry – the study and measurement of human body dimensions • HUMAN VARIABILITY AND STATISTICS • Human Variability • Age Variability • Sex Variability • Racial and Ethnic Group Variability • Occupational Variability • Generational or Secular Variability • Transient Diurnal Variability • Statistical Analysis • Normal Distribution • Percentiles • ANTHROPOMETRIC DATA • Measurement Devices and Methods • height, breadth, depth, distance, circumference, curvature • Civilian and Military Data • civilian -- out-dated and limited

2. Structural and Functional Data • structural data (static data) • taken with the body in standard and still position • functional data (dynamic data) • taken when the body adopts various working postures • Use of Anthropometric Data in Design • determine the user population (the intended users) • determine the relevant body dimensions • determine the percentage of the population to be accommodated • design for extremes • design for adjustable range • design for the average • determine the percentile value of the selected anthropometric dimension • lower-limit dimension • upper-limit dimension • make necessary design modifications to the data from the anthropometric tables • use mock-ups or simulators to test the design

3. GENERAL PRINCIPLES FOR WORK-SPACE DESIGN • Clearance Requirement of the Largest users • lower-limit dimension, for the largest users (start with 95 %tile) • Reach Requirement of the Smallest Users • upper-limit dimensions, for the smallest users (start with 5 %tile) • reach envelop (area) – the 3D space in front of a person without leaning forward or stretching • Special Requirement of Maintenance People • AdjustabilityRequirements • adjusting the workplace • adjusting the worker position relative to the workplace • adjusting the workpiece • adjusting the tool • Visibility and Normal Line of Sight • normal line of sight – the preferred direction of gaze when the eyes are at a resting condition • about 10 to 15°below the horizontal plane • Component Arrangement • increase overall movement efficiency and reduce total movement distance • frequency of use principle • importance principle

4. sequence of use principle • consistency principle • control-display compatibility principle of colocation • clutter-avoidance principle • functional grouping principle • functionaland sequence more critical than importance in positioning controls and displays • subjective judgment, link analysis, optimization approach • DESIGN OF STANDING AND SEATED WORK AREAS • Choice Between Standing and Seated Work Areas • standing • frequent movements in a large work area • heavy or large objects or exert large forces with their hands • use of floor mats and shoes with cushioned soles • seated • long-duration jobs • allows for better controlled arm movements, provides a stronger sense of balance and safety, improves blood circulation • leg rooms or leg and knee clearance • adjustable chairs and footrests

5. seat-stand • Work Surface Heights • 5-10 cmbelow elbow level for standing and at elbow level for seated – fig 10.9 • Work Surface Depth • normal work area – a sweep of the forearm without extending the upper arm – fig. 10.10 • maximum – a sweep of the arm by extending the arm from the shoulder • Work Surface Inclination • slightly slanted surfaces (about 15°) for reading • less trunk movement, less bending of the neck • horizontal desk for writing

9. 11. Biomechanics of Work • awkward postures and heavy exertion forces – musculoskeletal problems • low back pain and UECTDs • THE MUSCULOSKELETAL SYSTEM • support and protect body and body parts, maintain posture and produce body movement, generate heat and maintain body temperature • Bones and Connective Tissues • protect internal organs – skull, rib cage • support body movement and activities – long bones of the upper and lower-extremities • Connective Tissues -- tendons, ligaments, cartilage, fascia • joints -- synovial joints, fibulous joints (skull: fibulous tissues), cartilaginous joints (vertebral bones) • no mobility joints,hinge joints, pivot joints, ball and socket joints • Muscles • 400 muscles, 40 – 50% of BW • supply energy and produce body motion • generate heat and maintain body temperature • muscle fibers, connective tissues and nerves • a motor unit – “all-or-none” • concentric contraction – concentric (isotonic), eccentric, isometric contraction • no measuring device for tension in the muscle for muscle strength  torque or moment • static/dynamic muscle strength (isokinetic equipment, psychophysics)

10. +y +x +M • BIOMECHANICAL MODELS • musculoskeletal system as a system of mechanical links • bones and muscles act as a series of levers • Newton’s law • Single-Segment Planar, Static Model • Two-Segment Planar, Static Model • Assumption • pin centered joint • sign convention • neglect segment weight • joint center doesn’t move • rigid body • friction negligible • planar activity • LOW-BACK PROBLEMS • Low-Back Biomechanics of Lifting • the most vulnerable link because of most distant from the load • L5/S1 • normal range of strength capability of the erector spinal muscle at low back is 2,200 – 5,500N • compression force on L5/S1 4 5 3 2 1

11. Seated Work and Chair Design • LBP is common – loss of lordotic curvature in the spine  increase in disc pressure • lordosis and kyphosis • seating – pelvis rotated backward  lumbar lordosis into kyphosis • backrest inclination angle – 110 to 120° • lumbar support – a pad in the lumbar region – thickness of 5cm • arm rest, tiltable seat surface • UPPER-EXTREMILTY CUMULATIVE TRAUMA DISORDER • Common Forms of CTD • Tendon-Related CTD -- tendon pain, inflammation of tendon, tendonitits • Neuritis – tingling and numbing • Ischemia – tingling and numbing at the fingers • Bursitis – inflammation of a bursa • CTDs of the Fingers – vibration-induced white fingers (cold), trigger finger • CTDs of the hand and wrist -- CTS (carpal tunnel syndrome) • CTDs at the elbow -- Tennis elbow (lateral epicondylitis), golfer’s elbow (medial epicondylitis) • CTDs at the shoulder -- Rotator cuff irritation, swimmer’s shoulder, pitcher’s arm • Causes and prevention of CTDs • Repetitive motion, excessive force application, unnatural posture, prolonged static exertion, fast movement, vibration, cold environment, pressure of tools or sharp edges of soft tissues

12. Hand-tool Design • Do not bend the wrist • shape tool handles to assist grip • provide adequate grip span • provide finger and gloves clearances

18. 12. Work Physiology • MUSCLE STRUCTURE AND METABOLISM • Muscle Structure • primary function – generate force and produce movement • smooth muscle – digestion of food and regulation of the internal environment – no conscious control • cardiac muscle – no conscious control • skeletal muscle – the largest tissue in the body – 40% of body weight • direct conscious control, physical work possible • muscle fibers>myofibrils>sarcomeres (fig 12.1) • sarcomeres – myosin and actin • the sliding filament theory of muscle contraction • Aerobic and Anaerobic Metabolism • Phosphorylation – from ATP and CP to create high energy phosphate compounds through aerobic and anaerobic metabolism (fig 12.2) • Anaerobic • Phosphagen (ATP - CP) System • ATP  ADP + P + Energy • CP  C + P + Energy (rebound ADP and P to ATP)

19. Anaerobic Glycolysis System – oxygendebt, not efficient • Glucose (C6H12O6)n Lactic acid (2C3H6O3) + Energy • Energy + 3ADP + 3P  3ATP • Aerobic Reaction – steady state • C16H32O2 + 23O2  16CO2 + 16H2O + Energy • 130 ADP + 130P + Energy  130ATP • THE CIRCULATORY AND RESPIRATORY SYSTEMS • The Circulatory System • The Blood • red blood cells • transport oxygen and remove carbon dioxide • formed in bone marrow and carries the Hb • white blood cells – fight germs and defend the body against infections • platelets – stop bleeding • plasma • The Structure of the Cardiovascular Systems • the heart – four-chambered (atrium and ventricle, atrioventricular valves) – fig 12.3 • arteries and veins (one-way valves)

20. the systemic circulation • the left ventricle  aorta  arteries  arterioles  capillaries • venules  veins  superior vena cava (inferior v.c.)  the right atrium • the pulmonary circulation (oxygenation) • the right ventricle  pulmonary arteries to the lung  arterioles  capillaries • venules  veins  pulmonary veins  the left artium • Blood Flow and Distribution • the resistance to flow – blood vessel’s radius and length • systolic pressure – the maximum arterial pressure • diastolic pressure – the minimum • arterioles are the major source to blood flow • cardiac output (Q) – the amount of blood pumped out of the left ventricle per minute • influencedby physiological, environmental, psychological, individual factors • 5 L/min for rest to 25 L/min for heavy work • to increase the cardiac output -- increase HR or stroke volume (SV) • Q (L/min) = HR (beats/min) * SV (L/beat)

21. The Respiratory System • The Structure of the Respiratory System • the nose, pharynx, larynx, trachea, bronchi • lungs – alveoli (200 mil to 600 mil) • alveolar ventilation – the amount of gas exchange per min. in the alveoli • the muscles of the chest, diaphragm • Lung Capacity • total lung capacity (fig. 12.4) • minute ventilation (volume) – tidal volume and frequency • increasing the tidal volume is more efficient than increasing the breathing frequency • ENERGY COST OF WORK AND WORKLOAD ASSESSMENT • Energy Cost of Work • basal metabolism – the lowest level of energy expenditure to maintain life; a resting person under dietary restrictions for several days and no food intake for 12 hours – 1600 to 1800 kcal/day or 1 kcal/kg/hour • 2400 kcal/day for basal metabolism and leisure and low-intensity everyday nonworking activities • Working metabolism (metabolic cost of work) – increase in metabolism from the resting to the working level • metabolic or energy expenditure rate during physical activity = working metabolism rate (metabolic cost of work) + basal metabolism rate – fig. 12.5

22. physical demand of work • Light – smaller than 2.5 kcal/min – oxidative metabolism • Moderate – 2.5 to 5.0 kcal/min – oxidative metabolism • Heavy – 5.0 to 7.5 kcal/min – only physically fit workers through oxidative metabolism, oxygen deficit incurred at the start of work cannot be repaid until the end of the work • very heavy ( 7.5 to 10 kcal/min), extremely heavy (greater than 10 kcal/min) – even physically fit workers cannot reach a steady state condition during the period of work – oxygen deficit and lactic acid accumulation • Measurement of Workload • Physiological and subjective methods • energy expenditure rate is linearly related to the oxygen consumption rate and to HR • Oxygen Consumption • Energy expenditure rate (kcal/min) = 4.8 kcal/liter * oxygen consumption rate (l/min) • Oxygen consumption = aerobic metabolism during work + anaerobic metabolism during recovery • static work not well reflected in O2 measure • Heart Rate • indirect measure of energy expenditure, not as reliable as O2 consumption rate • resting HR – 60 to 80 beats/min • increase from the resting to the steady state is a measure of physical workload • max HR = 206 – (0.62*age) • max HR = 220 – age

23. Blood Pressure and Minute Ventilation • BP -- not used as often as O2 consumption and HR but more accurate for awkward static posture • minute ventilation (minute volume) – the amount of air breathed out per minute – measured in conjunction with O2 consumption and used as an index of emotional stress • Subjective Measurement of Workload • Borg RPE (Ratings of Perceived Exertion) Scale of 6 to 20 (beats/min) • PHYSICAL WORK CAPACITY AND WHOLE-BODY FATIGUE • Short-Term and Long-Term Work Capacity • Physical work capacity -- a person’s maximum rate of energy production during physical work • the short-term maximum physical work capacity (MPWC) or aerobic capacity – VO2max – heart cannot beat faster and the cardiovascular system cannot supply oxygen – 15kcal/min for healthy male and 10 kcal/min for healthy female • long-term maximum physical work capacity • for continuous dynamic work, 5 kcal/min for male and 3.5 kcal/min for female • Causes and Control of Whole-Body Fatigue • experienced whole-body fatigue around 30 to 40% of maximum aerobic capacity • certainly feel fatigued if the energy cost exceeds 50% of the aerobic capacity because the body cannot reach the “steady state”

24. Causes of fatigue  Accumulation of lactic acid in prolonged heavy work but not found with prolonged moderate work; depletion of ATP and CP, symptom of disease or poor health • engineering methods to reduce the risk of whole-body fatigue – redesign the job and provide job aids • administrative methods(work-rest scheduling) without heat stress • rest period = (PWC – Ejob)/(Erest – E job) • with heat stress • Static Work and Local Muscle Fatigue • Static muscle contractions impede or even occlude blood flow to the working muscles • Rohmert curve – the relationship between endurance and %MVC • EMG and psychophysical scales • Engineering and Administrative methods

30. BORG’S RATED PERCEIVED SCALE 6 7 VERY, VERY, LIGHT 8 9 VERY LIGHT 10 11 FAIRLY LIGHT 12 13 SOMEWHAT HARD 14 15 HARD 16 17 VERY HARD 18 19 VERY, VERY, HARD 20