ECE 796/896: Human Factors Engineering

1. ECE 796/896:Human Factors Engineering Chapter 8: Physical Work and Manual Materials Handling

2. Physical Work • Back Injuries from overextension accounts for about 25% of all occupational injuries • Nurses experience more back injuries than any other occupation • Stress: Refers to some undesirable condition, circumstance, task, or other factor that impinges on the individual • Strain: The effect of the stress on the individual

3. Muscle Physiology • Three types of Muscles • Striated - Skeletal • Cardiac - Heart • Smooth - Walls of the internal organs and blood vessels

4. Nature of Muscles • Over 600 muscles in human body • Over 400 are skeletal muscles that appear in pairs on both sides of the body • Skeletal muscles are attached to bones by way of tendons • Constructed by muscle fibers • Length ranges from .2” to 5.5” • Muscles have up to several hundred thousand fibers • Tiny blood vessels carrying oxygen and nutrients to the muscle form a complex capillary network

5. Contractibility of Muscles • Contracting muscles is the only active action a muscle can perform • Myofibrils - Contractile units of a muscle fiber which is composed of two types of protein filaments • Myosin and Actin • When muscle contracts, actin filaments slide together between the myosin filaments • Recording the electrical impulses is called an electromyography (EMG)

6. Control of Muscle Action • Muscle fibers are enervated by motor nerves emanating from the spinal cord • A single motor nerve fiber enervates a group of muscle fibers called a motor unit

7. Muscle Metabolism • Energy for muscle action is provided by the foods we eat and digest, primarily the Carbs and Fats • Carbs are converted to glucose for energy. Takes place in the liver • Muscle can not directly use the energy liberated by the breakdown of glucose or glycogen

8. Muscle Metabolism Cont. • Energy is stored in molecules of Adenosine Triphosphate (ATP) • ATP breaks down into Adenosine Diphosphate (ADP) plus a phosphate radical - causing the release of energy for the cell to use • ADP then captures more energy and recombines into ATP. • ATP is somewhat like a spoon, feeding the cell. It takes a load of energy (ADP to ATP) and unloads it (ATP to ADP) for cell to use

9. Muscle Metabolism Cont. • Another source of immediate energy is Creatinine Phosphate (CP) • Serves as an energy store for ATP generation • Only available for a short period of time

10. Anaerobic Glycolysis • Initial breakdown of glucose does not require oxygen • Glucose is broken down into pyruvic acid • If there is insufficient oxygen to break down pyruvic acid, it is transformed into lactic acid • As severity of work increases, body may not be able to produce enough oxygen for all the pyruvic acid, causing a lactic acid buildup

11. Aerobic Glycolysis • If oxygen is available, pyruvic acid is oxidized into carbon dioxide and water • Liberates about 20x the energy liberated by anaerobic glycosis • After strenuous exercise, lactic acid can be converted back to pyruvic acid • Used to build up ATP and CP stores for future muscle activity

12. Basal Metabolism • Metabolism is the chemical process by which foodstuffs are converted into heat or mechanical energy • 30% of muscles produce mechanical energy, 70% give off as heat • Body requires energy just to stay alive • Laying down and not moving still utilizes energy • Amount of energy needed per unit time to sustain life is called Basal Metabolic Rate (BMR)

13. BMR • BMR depends on: • Body size (large people => higher BMR) • Age (young people => higher BMR) • Sex (male => higher BMR) • For 155lb male: 1.2 kcal/min • For 133lb female: 1.0 kcal/min

14. Work Physiology:Respiratory Response • Additional oxygen is needed by working muscles for aerobic glycolysis • Respiratory response is to increase rate of breathing and the volume of air inspired with each breath • Due to the lag between intense work and when heavy breathing starts, anaerobic glycolysis takes place and depletion of ATP and CP stores provide energy to muscles • This causes the body to experience oxygen debt

15. Respiratory Response • Oxygen Debt - Amount of oxygen required by the muscles after the beginning of work, over and above that which is supplied to them during the work activity • The oxygen debt is “repaid” after heavy work is complete and heavy breathing continues • During this time, pyruvic and lactic acid is broken down • ATP and CP stores are replenished

16. Cardiovascular Response • Heart must pump more blood per unit time (known as cardiac output) to get more oxygen to muscles • Can be increased in two ways: • Increasing number of pumping action per minute (heart rate) • Increasing volume of blood pumped during each beat (stroke volume) • At about 40% of a persons work capacity, stroke volume stabilizes while heart rate increases

17. Blood Pressure • Increased cardiac output = increased blood pressure • Because the heart must pump against the increased pressure, the strain on the heart is greater • Increased blood pressure is necessary so that sufficient quantities of blood returns to the heart to fill it back up between beats • 120BPM means heart has .5sec to fill back up

18. Blood Flow • Because there is more blood flowing per unit time, heart receives more blood, although the percentage is the same • Most blood goes to the muscles being worked • Volume of blood going to skin increases during work to help dissipate heat generated by metabolic process

19. Measurements of Strain • Force - Measured in Newtons • Work or Energy - Measured in Joules or kilocalories • Power - Measured in Joules/sec or Watts • 1 kcal = 1Calorie (Food Energy) • 1 kcal = 4.1868 kJ • 1 kcal = 3087.4 ft-lbs

20. Oxygen Measurement (Uptake) • By measuring oxygen uptake, we can estimate energy consumption • Two measurements are taken: • Inspired air per unit time - Amount of oxygen the individual is inhaling and exhaling • Expired air per unit time - Amount of oxygen present in the air • Two measurements are compared to determine what the oxygen uptake is

21. Maximum Aerobic Power • Increasing rate of work, linearly increases oxygen uptake, to a point! • Where it plateau’s is known as Maximum Aerobic Power (MAP) or Maximal Oxygen Uptake • MAP - Highest oxygen uptake an individual can attain during exercise while breathing air at sea level • Women typically have 65-75% of that of men • 65 year old male has same MAP as 25 year old female

22. Heart Rate • Linear relationship between heart rate and oxygen consumption • Can be used to determine energy expenditure • Some issues: • Different linear relationships for different people • Oxygen consumption is more predictable after the stroke volume of the heart has been stabilized (happens at about 40% of MAP) • Stress, fatigue, and heat stress affect heart rate but not oxygen consumption

23. Local Muscle Activity • Measure overall level of physical work and stress placed on the body • It is desirable to measure the physiological strain of individual muscles or muscle groups • Electromyography (EMG)

24. Measures of Exertion • Standard: Borg-RPE (Ration of Perceived Exertion) • Rated from 6 - 20 • 6 = No exertion at all • 20 = Maximal Exertion • Intended to rate exertion during dynamic work • Motivation and Previous Experience affect results

25. Physical Workload • Body’s level of energy output: • Nature of work • Somatic factors • Training • Motivation • Environmental Factors

26. Work Efficiency • Not all energy expended is useful work • 70% dissipated as heat • Some of it expended as unproductive static efforts Efficiency (%) = [(Work Output)/(Energy Consumption)] * 100

27. Energy Consumption • Factors • Methods of Work • Energy cost for certain types of work can vary with the manner in which the work is carried out • Work Posture • Posture of workers while performing some tasks can influence energy consumption • Work Rate • For any particular task, at some specific pace, the task can be carried out over an extended time without any appreciable physiological cost • Tool Design • Influence energy expenditure as well as amount of work accomplished

28. Work/Rest Cycles • Total amount of rest required for a given period of work is important, however, the actual duration of work before a rest period is even more important • Shorter work periods with shorter rest periods results in better physiological recovery

29. Exercise Training • Increases MAP • Reduces Heart Rate • Reduces Blood Pressure • Increases Muscle Strength and Endurance

30. Strength and Endurance • Work capacity is limited by the cardiovascular system’s ability to deliver fuel and oxygen to the muscles • Strength and Endurance are also limiting factors in many tasks and activities

31. Strength • Two conditions in which strength is important: • Dynamic: Body member is actually being moved (a.k.a. Isokinetic) • Static: Force is applied against a fixed object with no body member movement (a.k.a. Isometric) • Definition: Maximal force muscles can exert isometrically in a single voluntary effort • Refers to the muscles’ capacity to exert force under static conditions

32. Measurement of Strength • Static: Assessed by having a subject exert maximum force against an immovable object • Factors: Posture, angle of joints, motivation, and manner in which force is exerted • Dynamic: Testing devices have been developed that control the speed of the movement to a preset value, regardless of the effort exerted • Need to do this due to changes in acceleration and joint angles

33. Factors Affecting Strength • Factors: • Genetics • Body Dimensions • Physical Training • Motivation • Sex • Age • Peak strength 25-35 years old

34. Sex and Strength • Of all factors, sex accounts for largest differences • Females mean strength is approx. 2/3 that of mean males strength • Could be higher or lower, depending on muscle group • Lower extremity strengths are more comparable to male than upper extremity

35. Endurance • Endurance time is a function of static forcerequirements of the task • If it is necessary to require individuals to maintain static force over a period of time, the force required should be well below each individuals own static force capacity • Dynamic work: the combination of force and frequency of repetition determines the length of time that the activity can be endured. • When subjects perform rhythmic, maximal isometric contractions in time to a metronome, the force generated gradually decreases because of fatigue, but levels off at a value that can be maintained for a long time

36. Manual Materials Handling (MMH) • Various short term and long term health effects can be attributed to MMH. • Account for 25% of all industrial injuries and result in 12 million lost workdays per year • Estimated that 70 million Americans have suffered back injuries and that number is expected to increase 7 million annually • 50% of these injuries occur while lifting objects

37. Assessing MMH Capabilities • Three Methods: • Biomechanical Approach • Physiological Approach • Psychophysical Approach

38. Biomechanical Approach • View body as a system of links and connecting joints corresponding to segments of the body • Principles of physics are used to determine stresses and strains • In many MMH tasks (lifting, pushing, carrying, and pulling) significant forces are produced in lower back (L5/S1 disc) • Has been limited to analyzing infrequency MMH tasks • Infrequent lifts and pushes are what create excessive force on lower back

39. Physiological Approach • Best suited to analyze MMH tasks that are done frequently and over some period of time • Concerned with energy consumption and the stresses acting on the cardiovascular system • Takes into account: • Body weight, weight of load, gender, vertical start and end positions of lift, dimensions of load, and frequency of handling

40. Psychophysical Approach • Integrations of biomechanical and physiological stresses • Maximum Acceptable Weight of Load (MAWL): the maximum amount a person can sustain without strain or discomfort and without becoming unusually tired, weakened, overheated, or out of breath • Disadvantage: People tested must be representative of the population to which the data will be applied

41. Lifting Tasks • Make up a large proportion of MMH tasks • Involved in far more back injuries than any other type • Variables: • Horizontal Position of Load • Height and Range of Lift • Method of Lifting from the Floor • Frequency of Lifting • Object Characteristics

42. Horizontal Position • Horizontal Position of load in relationship to the L5/S1 disc is one of the most significant factors affecting the compressive forces experienced on that disc • Force = 400lbs when 44lb weight is held 8 inches in front of disc • Force = 750lbs when 44lbs weight is held 30 inches in front of disc

43. Height and Range of Lift • Categorizing height of lift: • Floor to knuckle; knuckle to shoulder; should to reach • Reaches about shoulder are more demanding in terms of physiological • Lifting objects 0 - 20 inch. requires half as much energy than lifting same object from 20 - 40 inch. • Most efficient range is between 40 - 60 inch.

44. Method of Lifting from Floor • Positions: • Squat Lift (Straight back, bent knee) • Load needs to be small enough to fit between knees otherwise it causes to much stress on L5/S1 • Stoop Method (Bent back, straight leg) • Less stress than squat lift • Some people can’t lift both load and body weight • Free Style (Semi-squat posture with load resting on legs) • Requires least amount of energy and least stressful

45. Frequency of Lifting • Occasional exertion is more tolerable than frequent exertion • Standard is 1 lift/min • 12 lifts per min => MAWL is 70 - 80% that of 1 lift/min • Significant decline in endurance time as the frequency of lifting increases • 9lb load, 4 lifts/min, Endurance time = 292 min • 1171 lifts are made • 9lb load, 10 lifts/min, Endurance time = 27 min • 272 lifts are made

46. Object Characteristics • ObjectSize: • The length and width of the object being lifted influence the MAWL, energy expenditure, and spinal stresses • Increase height, then width, then length • ObjectShape: • MAWL is higher with bagged containers rather than box containers. Bags can be carried closer to body, decreasing stress on L5/S1

47. Object Characteristics • Load Distribution and Stability: • Always carry loads along centerline of body to decrease lateral bending moment on lumbar spine • Handles: • Less stressful and safer than loads without handles

48. Recommended Limits for MMH Tasks • Infrequent Tasks: • Use recommended limits based on biomechanical criteria • Frequent/Longer/Repetitive Tasks: • Use recommended limits based on physiological criteria • Psychophysical approach can be used in either situation depending on how the data was gathered

49. Biomechanical Limits • Focuses on: Strength and Compressive Forces on spine • Strength of the vertebrae to resist compressive forces is in large part a function of age, cross-sectional area of the vertebrae, and the bone-mineral content • NIOSH guidelines: • Takes into account: Horizontal position of the load, vertical distance traveled of the load, average frequency of lift • Two Limits: Action Limit(AL) & Maximum Permissible Limit (MPL = 3 * AL) • Recommends that no one work above MPL

50. Physiological Limits • 33% of maximum aerobic power (MAP) as maximum for 8-h level of energy expenditure • Frederick Lifting Equation • Energy expenditure should not exceed 3.33 kcal/min E = F * a * W * C/1000 E = Energy Expenditure (kcal/hr) F = Frequency of Lifting (lifts/hr) a = Vertical Lifting Range (ft) W = Weight to be Lifted (lbs) C = Energy Consumption (g*cal/ft*lb)