Manual Work Design: Principles and Musculoskeletal System Analysis

1. ‐ Manual Work Design‐ END202 END 202 – Work analysis anddesign

2. The design of manual work was introduced by the Gilbreths through motion study and the principles of motioneconomy • The principles are broken down into 3 basic subdivisions: • The use of the humanbody • The arrangement and conditions of theworkplace • The design of tools andequipments Manual workdesign • The principles are based on anatomical, biomechanical and physiological principles of the human body. They form the scientific basisfor ergonomics and workdesign. END 202 – Work analysis anddesign

3. Musculoskeletal system of thearm Musculoskeletalsystem • The human body is able to produce movements because of a complex system of muscles andbones • The muscles are attached to the bones by ajoint • Agonists act as the prime activators of themotion • Antagonists counteract the agonists and oppose the motion END 202 – Work analysis anddesign

4. There are 3 types of muscles in the human body • Skeletal: attached to the bones, approx. 500 in the body • Cardiac muscles: foundin theheart • Smooth muscle: found in the internal organs and the walls of the bloodvessels Musculoskeletalsystem END 202 – Work analysis anddesign

5. Skeletalmuscles • Myofibrils are subdivided into myofilaments • Myofilaments: thick (myosine) and thin filaments(actine) • Filaments slide over one another to contract and extendmuscles • Complete contraction=50%of the restinglength • Complete extension=180% of the restinglength Musculoskeletalsystem END 202 – Work analysis anddesign

6. Principles of work design:Human capabilities and motioneconomy END 202 – Work analysis anddesign

7. Force‐length relationship of skeletalmuscle Principles of workdesign A task requiring an important muscle force should be performed at the optimumposition. END 202 – Work analysis anddesign

8. Principles of workdesign • Maximum forcestate • resting length: optimal bonding between thick (myosin) and thin (actin)filaments • stretched state: minimal overlap, decreased muscleforce • contracted state: interference between filaments, decrease muscleforce END 202 – Work analysis anddesign

9. 1. Achieve the maximum muscle strength at the midrange ofmotion Midrange ofmotion (relaxedposture) The posture assumed by an astronaut in weightless conditions when both agonist and antagonist muscles are relaxed. Typical relaxedposture Principles of workdesign END 202 – Work analysis anddesign

10. 2. Achieve the maximum muscle strength with slowmovements • The faster the molecular bonds are formed, broken, reformed, the less effective is the bonding and the less muscular force isproduced • Slow movements arethe mostefficient Force‐velocity relationship Principles of workdesign END 202 – Work analysis anddesign

11. 3. Use momentum to assist workers wherever possible; minimize it if it is counteracted by musculareffort • Faster movements produce higher momentum and higher impact forces in the case ofblows • Downwardmotionsare more effectivethan upward motions because of the assistance fromgravity Principles of workdesign END 202 – Work analysis anddesign

12. Design tasks to optimize human strengthcapability • Human strength capability dependson • – the type ofstrength: • Dynamic strength(isotonic) • Static strength(isometric) • Psychophysical strength (extendedtime) • the muscle or joint motion beingutilized • posture • Maximum acceptable load is 40 to 50 percent less than a one‐time static exertion Principles of workdesign • Extensive tables for psychophysical strength of various frequencies and postures (pg 134, Tables4.2,4.3,4.4) Static strengthpositions END 202 – Work analysis anddesign

13. 5. Use large muscles for tasks requiringstrength • Muscle strength is directly proportional to the size of the muscle • For example, leg and trunk muscles should be used in heavy load lifting, ratherthan weak armmuscles • Static strengthpositions Principles of workdesign END 202 – Work analysis anddesign

14. 6. Stay below 15% (even below 10% or 5%) of maximum voluntaryforce • The non‐linearrelationship: • at a maximal contraction a very short endurance time (6sec) • indefinite endurance time at approx. 15% of a maximalcontraction Principles of workdesign T  1.2 /( f 0.15)0,6181.21 T:Endurancetime(min) f:Requiredforce,expressedasafraction of maximum staticstrength Ex: T = 1.09 min for f =50% Static muscle endurance‐exertion levelrelationship END 202 – Work analysis anddesign

15. 7. Use short, frequent, intermittent, work/rest cycles • A fast initial recovery period, which then tends to level off with increasingtime • Most of the benefit is gained in a relatively shortperiod • A higher % of max strength canbe maintained if the strength is exerted as a series ofrepetitive Principles of workdesign contractions rather thanone sustained staticcontraction Percentage of max static strength that can be maintained in a steady state during rhythmiccontractions END 202 – Work analysis anddesign

16. 8. Design tasks so that most workers can dothem • Individual factors affect strength performance: gender, age, handedness, andfitness/training • Muscle strength appears to peak in the mid‐20s, then decreases linearly by themid‐60s. • Handedness: non‐dominant hand produces about 90% of dominant hand’sgrip Principles of workdesign END 202 – Work analysis anddesign

17. Use of low force for precise movements or fine motor control • When the force increase, muscle controldecreases • Do not attempt precise movements or fine control immediately after heavywork • Ex: Assembly operations after lifting heavy parts. Use different workers forlifting. • Begin and end motions with both handssimultaneously • Workstations can be designed to do “two at atime” • Move the hands symmetrically and simultaneously to and from the center of thebody • Deviations from symmetry result in slow, awkwardmovements Principles of workdesign END 202 – Work analysis anddesign

18. 13. Use the natural rhythms of thebody • Optimum work tempo for sometasks: • Filing metal: 60‐78 strokes perminute • Chiseling: 60 strokes perminute • Arm cranking: 35 rpm (number of full rotations over 1min) • Shoveling: 14‐17 tosses permin Principles of workdesign END 202 – Work analysis anddesign

19. 14. Use continuous curved motions • Straight‐line motions involving sudden and sharp changes in direction require more time and are lessaccurate • Continuous curved motions donot require deceleration to make a directional change and performed faster per unit ofdistance • Pivot around a joint(elbow) • Forearm motionisbest while pivoting onelbow Principles of workdesign END 202 – Work analysis anddesign

20. 15. Use the lowest practical classification ofmovement • Always utilize the lowest possible:finger • → wrist → forearm → shoulder →body • Additional time is required for the central nervous system to process additional joints andmuscles • 16. Work with both hands and feetsimultaneously Principles of workdesign • Relieve the hands of work that can be done by the feet if this work is performed while the hands areoccupied Classifications ofmovements END 202 – Work analysis anddesign

21. 17. Minimize eyefixations • The location of the primary visual targets should be optimized with respect to theoperator • Within the area in the figure no head movements are needed and eye fatigue isminimized Principles of workdesign END 202 – Work analysis anddesign

22. Human capabilities and motioneconomy Achieve the maximum muscle strength at the midrange of motion Achieve the maximum muscle strength with slowmovements Use momentum to assist workers wherever possible; minimize it if it is counteracted by muscular effort Design tasks to optimize human strengthcapability Use large muscles for tasks requiringstrength Stay below 15% (even below 10% or 5%) of maximum voluntary force Use short, frequent, intermittent, work/restcycles 1. 2. 3. 4. 5. 6. 7. Principles of work design –Summary Design tasks so that most workers can dothem Use of low force for precise movements or fine motorcontrol Do not attempt precise movements or fine control immediately after heavywork Begin and end motions with both handssimultaneously Move the hands symmetrically and simultaneously to and from the center of thebody Use the natural rhythms of thebody Use continuous curvedmotions Use the lowest practical classification ofmovement Work with both hands and feetsimultaneously Minimize eye fixations END 202 – Work analysis anddesign

23. The principles of human capabilities and motion economy are based on an elementary understanding of humanphysiology • The analyst need not be an expert in human anatomy • The motion economy checklist summarizes most of the principles in a questionnaire format (Figure 4.16, pg 146: Course webpage) Principles of work design –Summary END 202 – Work analysis anddesign

24. MotionStudy END 202 – Work analysis anddesign

25. Motion study is the careful analysis of body motions employed in doing ajob • The purpose is to eliminate or reduce ineffective movements and facilitate and speed effective movements • Gilbreths pioneered the study of manual motionand Motionstudy • developed basic laws of motioneconomy • Gilbreths concluded that all work, productive or non‐ productive, is done by using combinations of 17basic motions calledtherbligs: – Effective therbligs: directly advance the work progress,they can be shortened but not completelyeliminated – Ineffective therbligs: do not advance the work progress, should beeliminated END 202 – Work analysis anddesign

26. Effectivetherbligs END 202 – Work analysis anddesign

27. Ineffectivetherbligs END 202 – Work analysis anddesign

28. The two‐hand process chart (operator process chart) shows all movements and delays made by the right and left hands, and the relationships between them • Analyst can determine what improvements can beintroduced • Therblig analysis checklist (Figure 4.18, pg 151: Course webpage) The two‐hand processchart END 202 – Work analysis anddesign

29. Manual work and designguidelines END 202 – Work analysis anddesign

30. Energy expenditure and workloadguidelines • Heart rateguidelines • NIOSH liftingguidelines • Multitask liftingguidelines • General guidelines: Manuallifting Manual work and designguidelines END 202 – Work analysis anddesign

31. Energy is required for musclecontraction • ATP (adenosine triphosphate) molecule: immediate energy source but very limited, lasting only fewseconds • ATP must be replenished from CP (creatine phosphate) molecule, lasting for less than 1min • CP is regenerated from the basic foods: carbohydrates, fats,proteins • Two different modes for CPgeneration: • Aerobic (requires oxygen): much more efficient, generates 38 ATPs for each glucose molecule, but it isslow • Anaerobic (without oxygen) : very inefficient, generates only 2 ATPs for each glucose molecule, but muchquicker. • (Glucose molecule is only partially broken down into 2 lactate molecules, in watery environment of the body they form lactic acid which causesfatigue) Energy expenditure and workloadguidelines END 202 – Work analysis anddesign

32. Energy expenditure and workloadguidelines Sources of energy during the first few minutes of moderately heavywork END 202 – Work analysis anddesign

33. By warming up and starting heavy work slowly, the worker can minimize the amount of anaerobic metabolism and the buildup of lactic acid associated with feelings offatigue. • The delay of full aerobic metabolism is termed oxygen deficit • The energy expended on a task can be estimated by assuming that the energy is produced through aerobic metabolism and measuring the amount of oxygen consumed by theworker Energy expenditure and workloadguidelines END 202 – Work analysis anddesign

34. The amount of inspired air is assumed to contain 21% oxygen • Typically 4.9 kcal of energy is produced for each liter of oxygen used inmetabolism • Energyexpenditure: • E (kcal/min) = 4.9 x oxygenconsumption • = 4.9 * V * (0.21 –EO2) • V = volume of air inspired, L/min (can be measured by aflowmeter) • EO2 = fraction of oxygen in expired air (can be measured by an oxygenmeter) Energy expenditure and workloadguidelines END 202 – Work analysis anddesign

35. The energy expended on a task varies by the type of task, the posture and the loadcarriage. • Data on several hundred different types of tasks have beencollected • For manual material handling, the manner in which the load is carried is mostcritical • Balanced loads carried closest to the center of gravity of the body demand lowestenergy • – Ex: backpack supported by the truck muscles iseasierthan two suitcases of equal weight in eacharm • Posture also plays an important role, with less energy for supportedpostures Energy expenditure and workloadguidelines END 202 – Work analysis anddesign

36. 5.33 kcal/min is the limit of acceptable energy expenditure for an 8h workday for aman, • This number corresponds to 1/3 the maximum energy expenditure of the average U.S.Male • 16 kcal/min x 1/3 = 5.33kcal/min • For females:12kcal/min x 1/3 = 4kcal/min Energy expenditure and workloadguidelines END 202 – Work analysis anddesign

37. Energy expenditure and workloadguidelines Examples ofenergy costs of various types of human activity(kcal/min) END 202 – Work analysis anddesign

38. If the overall workload is exceeded (more than 5.33 kcal/min for males and 4 kcal/min for females) rest is required to allow the body to recover from fatigue and recycle the lacticacid • Guideline for rest allocation: • R = (W – 5.33) / (W –1.33) • R: time required for rest, as percent of totaltime • W: average energy expenditure during work,kcal/min • 1.33 kcal/min: energy expenditure duringrest • Example: Shoveling coal W=9.33kcal/min R =0.5 • For an 8h of work spend 4h forresting Energy expenditure and workloadguidelines END 202 – Work analysis anddesign

39. The duration of the work cycle is important for an optimal recovery • For heavy works, without resting, the recovery process tends to be exponential • Short bursts of heavy works (1/2 to 1 min) followed by short rest periods provide maximumbenefit • Micropauses of 1 to 3s are alsouseful • Active breaks, during which the worker alternates hands or uses other muscles, relieves fatiguedmuscles • It is best for workers to decide when to take breaks, whenever they feel the need for rest as opposed to prescribedbreaks • The use of frequent short work/ rest cycles is highlyrecommended Energy expenditure and workloadguidelines END 202 – Work analysis anddesign

40. The measurement of oxygen consumption can be cumbersome and expensive (equipment costs several thousand dollars and interferes with theworker) • An alternative way to measure the energy expenditure is the heart ratelevel • The higher the energy expenditure the higher the heartrate • Less expensive (100 $ for a visual readout, severalhundred Heart rateguidelines dollars for a PCinterface) END 202 – Work analysis anddesign

41. The analyst must becareful: • This measurement is most appropriate for dynamicwork • Can vary considerably between individuals, depending on their fitness levels andage • Can be confounded by other stressors including heat, humidity, emotional levels, and mental stress Heart rateguidelines END 202 – Work analysis anddesign

42. Heart rateguidelines • Average working heart rate = 40 beats/min + resting average heart rate (≈ 72beats/min) • average increase in heart rate / increase in energy expenditure (slope) = 10 beats/min per 1kcal/min • A 5.33 kcal/min workload (4 kcal/min above the resting levelof • 1.33 kcal/min) produces a 40 beats/min increase in heartrate END 202 – Work analysis anddesign

43. Heart rate between 1/2 to1 • min after cessation:HR1 • Heart rate between 2.5 to 3 min after cessation: HR2 • Acceptable heart rate recovery: • HR1 ≤ 110beats/min • HR2 – HR1 ≥ 20beats/min • The increase in the heartrate Heart rateguidelines during steady‐state work, called heart rate creep, indicates an increasing buildup of fatigue and insufficient recovery during rest pauses and must beavoided Average heart rate measurement for two differentworkloads END 202 – Work analysis anddesign

44. Subjective ratings of perceivedexertion • The scale developed by Borg(1967) • Ratings through 6 to20 Heart rateguidelines • correspond to the heart rates divided by10 • The ratings should be used with cautionand normalized to each individual’s maximum rating Borg Rating of Perceived Exertion(RPE) END 202 – Work analysis anddesign

45. 40% of work accidents are caused during material handling • 70% of manual material handling accidents concern lowback • Low back accidents and illnesses consists 25% of the totalcompensations • The average cost of low back accidents is60,000$ Low back compressiveforces END 202 – Work analysis anddesign

46. Vertebrae is divided into: cervical, thoracic, lumbar and sacrumregions • Spinal cord is protected by vertebrae, spinal nerve roots are separated from the spinal cord to achieve internal organs andextremities • Vertebral bones are separated by a softer tissue: intervertebraldisks. • They serve as joints, allowing a large range of motion, and cushions betweenvertebral • Most trunk flexion occurs in the two lowest joints: L5/S1 veL4/L5 • Anatomy of the humanspine Low back compressiveforces END 202 – Work analysis anddesign

47. Low backproblems • Aging, heavy manual work exposure: the disks can weaken • Cartilage end plate can suffer microfractures, releasing some gelatinous material, the center starts dryingup • The disk space narrows, vertebral bones come closer and touch: irritation and pain, motorimpairments • Disk herniation (slippeddisk) • Soft tissue injuries (ligaments, muscles,tendons) Low back compressiveforces END 202 – Work analysis anddesign

48. Normalstate Narrowing of the disk space, allowing the nerve root to bepinched Herniated disk, allowing the gel material to extrude and impinge upon the nerveroot Low back compressiveforces Anatomy of a vertebra and the process of diskregeneration END 202 – Work analysis anddesign

49. Some causes for low‐backproblems • Heavywork • Frequent lifting of largeloads • Forward‐bending trunk postures for longperiods • Long periods of immobility, even in sittingpostures • Whole bodyvibration • Genetic predisposition (weaker connective tissues, disks, ligaments,etc.) • Personal lifestyle conditions (smoking, obesity,etc.) Low back compressiveforces END 202 – Work analysis anddesign

50. Analogy between L5/S1 disk and first‐classlever Low back compressiveforces • Center of the disk acts as thefulcrum • Find the muscle force FM and the total compressive force Fcomp • 2 x FM = 30 x 50 => FM =750 lb (341kg) • Fcomp = 750 + 50 = 800 lb(364kg) • A compressive force of 770 lb (350 kg) is considered the danger threshold END 202 – Work analysis anddesign