exam 1 Flashcards
What does “ergonomics” mean in Greek?
ergon - work
nomos - law
ergonomics
the process of designing or evaluating products, tasks, environments, and systems to improve performance and/or reduce the risk of injurt
industrial ergonomics
application of ergonomics and human factors methods to the design, evaluation, improvement of work tasks, tools, environments to improve performance, productivity, quality or decrease injury, fatigue, waste
What are side effects of ignoring ergonomics?
- compromises worker and company performance
- lower quality work and productivity
- absenteeism
- turnover
- training
- morale
- accidents
- acute of chronic musculoskeletal disorders
- occupational injuries and illnesses in the US is $150b annually
3 areas of ergonomics
- physical: human anthropometry, musculoskeletal, metabolic, cardiovascular, environmental issues; working posture, MH, repetitive movements, workplace layout, health and safety
- cognitive: mental processes as they affect interactions among humans and other elements of the system; mental workload, decision making, HCI, work stress, training
- organizational (marcoergo): sociotechnical systems, including organizational structures, policies, and processes; shift work, scheduling, job satisfaction, motivational theory, teamwork, ethics
4 types of physical ergonomics
- anthropometric: concern physical dimensional conflicts between functional space geometry and the human body
- musculoskeletal: concern forces, moments, postures, and mechanical stress on muscles and skeletal system
- metabolic/cardiovascular: concern stress on metabolic energy and cardiovascular system (heart rate, breathing rate, etc.)
- environmental: concern exposure to excessive environmental stress (heat, vibration)
What is the ergonomic process?
- characterize existing or potential problems
- perform job analysis
- implement controls
- evaluate effectiveness of controls; educate employees on ergonomics
What is the overarching goal of ergonomics?
prevent disorders through proactive design and vigilance
What is the guiding principle of ergonomics?
D = task demand C = human capability or capacity
maintain D < C
6 types of interaction
- **human with machine (H->M): control actions provided by human
- **machine with human (M->H): information feedback and forces generated by machine
- human with environment (H->E): effect of human on local environment
- **environment with human (E->H): influence of environmental factors on human performance
- machine with environment (M->E): alteration of work environment by machine
- environment with machine (E->M): machine function altered by environment
What are indicators for the need of ergonomics?
- waste production is too high
- too many rejects
- production accidents too frequent
- higher medical costs
- absenteeism is high due to injuries
- frequent complaints about job requirements
- production output or efficiency is low
5 components of the muscle system
- connective tissue: bones, ligaments, tendons, fascia, cartilage
- joints: unions with more than 1 degrees of freedom
- skeletal muscle: muscle fibers, connective tissue, nerve excitation
- nerves: afferent (sensory) and efferent (motor)
- brain: central and peripheral nervous systems
4 functions of bone
- support and motion
- protection of organs
- mineral storage (calcium and phosphorus)
- formation of blood cells
2 types of bone
- cortical (compact): dense compact shell provides strength and impact resistance; very strong
- cancellous or trabecular (spongy): organization of trabeculae tend to be distributed along lies of principal loads of stresses and strains
2 mechanical properties of bone
stress: force/initial cross-sectional area
strain: change in length/original length
What type of material is bone?
anisotropic
Wolff’s Law
bone adapts to its mechanical environment: it will be deposited where needed and be reabsorbed where not needed; bone growth stops but thickness and diameter can change
ligament
a connective tissue that binds bone to bone
tendon
a connective tissue that binds muscle to bone
cartilage
a connective tissue that is a smooth elastic tissue that covers the ends of long bones at joints to provide a low-friction surface for movement
can be a structural component
fascia
a band or sheet of connective tissue - primarily collagen - beneath the skin that attaches, stabilizes, encloses, and separates muscles and other internal organs
surrounds muscles or other structures
viscoelasticity
material response to force/displacement depends not only on force/displacement but also time
creep
change in strain for a constant stress
load (stress) relaxation
change in stress for a constant strain
What is the relevance of viscoelasticity?
- joint stiffness and load distribution change over time
- repeated loads or movements can accumulate over time to have larger effects than just one repetition
3 types of joints
- synovial: common, large motion, synovial fluid forms interface; elbow, knee, shoulder
- fibrous: relatively immobile, fibrous connective tissue bridges the joint; ribs, pelvis, skull
- cartilage: bridges the joint; lumbar motion segment
What is the function of joints
motion between bones
pulleys for tendons
synovial joint: the knee
- capsule includes a synovial membrane that produces synovial fluid
- some joints contain menisci to protect articular cartilage and distribute loads
- VERY LOW coefficient of friction
cartilage joint: intervertebral discs
composition:
- nucleus pulposus: gel-like material
- annulus fibrosus: layers of fibrous connective tissues with varying orientation
- disc height decrease throughout the day in response to loading
- discs have no direct blood supply which makes them extremely slow to heal
What are potential causes of back pain?
- prolonged static loading -> decreased disc height -> misalignment of facet joints -> pain
- weakened annulus (disc degeneration)
- disc bulging, compression of spinal nerve roots; can lead to disc rupture
skeletal muscle
- constitutes 50% of body weight
- ~400 skeletal muscles in the body
- basic function: generate moments about joints and help maintain joint stability
- can only actively contract
muscle names
- origin: bony end at proximal end of muscle
- belly: thicker middle region of muscle between origin and insertion
- insertion: bony attachment at distal end of muscle
5 structural units of skeletal muscle
(in decreasing order) muscle fascicle muscle fiber/cell myofibril myofilaments
sarcomere
the basic contractile unit of muscle
delineated by its z-lines, thick filaments, and thin filaments
How does a muscle contract?
- contract by sliding
- the amount of overlap determines the amount of force that can be generated
- a sarcomere with vertical z discs has horizontal thin filament perpendicularly attached; thick filament lies between the thin
- when thick filaments are pulled between thing –> contraction
- more cross bridges formed = more force
WMSD
Work-related MusculoSkeletal Disorders
- involve injury to soft tissues of the body and joints
- tend to develop gradually/cumulatively over time
- work performance and environment contribute significantly to the condition; likely make it worse or persist longer
- injury event or exposure to the event involves: bodily action
- NOT sudden injuries from slips, trips, falls, or MV accidents
What are examples of WMSDs?
- tendonitis
- low back pain
- sprains, strains, tears
- carpal tunnel syndrome
- hernia
- bursitis
How prevalent and significant are WMSDs?
- account for 1/3rd of all worker injury and illness cases
- difficult to diagnose because seen as normal aches and pains
- 1/2 of US workers are employed in occupations with high risk for WMSDs
- ~$20b annually in direct costs; worker compensation, legal and medical expensive
- 5xs direct costs = indirect costs; training, accident investigation, lost productivity, repairs, lower morale, absenteeism
risk factor
an individual characteristic or exposure associated with an increased risk of a disease or injury
workplace risk factors
- high force exertions
- highly repetitive or static work
- awkward/extreme postures
- vibration
- mismatched anthropometry
- psychosocial job factors
2 types of mental risk factors (psychosocial job factors)
influence the mental state of the individual (mental stress, supervisory control, autonomy, job security, interactions with coworkers, anxiety)
- psychological factors: individual level
- social factors: organizational level
individual risk factors
age sex obesity (anthropometry) systemic diseases (diabetes) acute trauma congenital defects pregnancy
What are industries and jobs with high WMSDs prevalence?
- industry: agriculture, construction, manufacturing, mining, transportation
- jobs: meat packers, keyboard entry work, sewing operations, packing operations
2 categories of WMSDs
- by tissue or bodily system; muscular, tendon, neurovascular, joint, spinal disorders
- by body location; neck, back, elbow, wrists, fingers, shoulders
tendon disorders
WMSD characterized by tissue or bodily system
result from repetitive or prolonged activities, forceful exertion, awkward and static posture, vibration, localized mechanical stress
two types: tendinitis, tenosynovitis
carpal tunnel syndrome
WMSD characterized by tissue or bodily system
tendinitis of the flexor tendons of the fingers which leads to compression of the median nerve
contributing factors: repetitive forceful wrist extension/flexion, high speed finger movements, pressure on base of hand
neurovascular disorders
WMSD characterized by tissue or bodily system
result in compression or damage to both nervous and vascular tissues; increase inadequate blood supply and localized muscle fatigue
neck WMSD
WMSD characterized by body location
contributing factors: prolonged static and restricted posture, prolonged unnatural posture of the head/neck/shoulder
back WMSD
WMSD characterized by body location
contributing factors: prolonged static load on the upper torso musculature, non-neutral posture, constant lifting of objects from the floor
shoulder WMSDs
WMSD characterized by body location
contributing factors: prolonged shoulder flexion/abduction, frequent high reaching, prolonged load, repetitive throwing
What are commonalities seen among contributing factors to WMSDs?
repetitive movements extreme movements prolonged postures vibration forceful extertions
What are commonalities seen among prevention and control of WMSDs?
vary jobs/movements minimize extreme movements vary postures protective materials to absorb vibration reduce forceful exertions
ergonomic process relating to WMSDs
- characterize existing or potential problems
- identify jobs with high risk factors
- tracking and quantifying injuries - perform job analysis
- **goal is to identify and quantify risk factors - implement controls
- **goal is to reduce exposure to risk factors
- hierarchy of controls: elimination, substitution, engineering controls, administrative controls, PPE
- **engineering controls: change the task/environment
- **administrative controls: change the work procedures or methods (exposure to the task) - evaluate effectiveness of controls and educate employees on ergonomics
- job analyze to ensure elimination or reduction of risk factors
- ensure no new risk factors were created
3 types of occupational risk factors
- workplace: associated with the work task and work area
- individual: associated with an individual and may predispose them to risk
- psychosocial: psychological factors (individual) and social factors (organizational) that influence the mental state of the individual
3 job analysis quantifiers
frequency, duration, magnitude
posture
includes all of your joint and body segment angles
5 classifications of repetition rates
- very high
- body parts in constant motion
- difficulty maintaining pace - high
- hands in constant rapid motion
- wasted motions or difficulty with equipment would result in worker immediately falling behind - medium
- hands in steady motion
- no difficulty keeping up
- worker may pause or rest if necessary - low
- obvious pauses in each work cycle
- worker may wait for equipment to cycle
- hand used only to remove occasional defects - very low
- hands are idle most of the time
constant mechanical stress
stress or pressure directly on nerves and tendons can aggravate existing injuries and possibly lead to WMSDs
increase applied force area; less stress when using a power grip (whole hand) vs. a pinch grip (small area)
What factors should be considered when selecting tools?
posture
forceful exertions
repeated/sustained exertions
contact mechanical stress
What should hand tools be designed to fit?
- minimize wrist angular deviations
- minimize hand force requirements
- minimize force concentrations or pressure points
- maximize grip strength capability
- accommodate anthropometric variability
2 types of occupational risk factors
temperature
- cold: work has higher incidences of WMSDs, decreased circulation
- hot: fatigue quickly
vibration
-causes reflex reduction, decrease circulation, loss of sensations, unconscious increases in force level
challenges of WMSD prevention
- multi-factorial nature
- complex unknown interrelationships between risk factors
- long development cycles
- inconsistency amongst workers with and without WMSD
- reducing injury risk and maintaining/improving productivity may be in conflict
anthropometry
the study of the physical dimensions of the human body
What is anthropometry data for?
design of dimensions of seating, furniture, tools, workspaces, and many places that humans occupy
in biomechanical models to:
- predict human reach, force, and space requirements
- analyze work demands
2 types of anthropometric data
static anthropometry: physical dimensions of the body -body segment length -body segment mass -body segment COM body segment moments of inertia
functional/dynamic anthropometry: physical dimensions for completing particular activities or tasks
- reach distance
- motion envelop
center of mass
the point at which all mass of a body can be concentrated so that its results in forces and moments equivalent to the actual distributed mass
3 design strategies for anthropometric variance
average
- emphasize the center of a population distribution
- both extremes of users won’t be accommodated
- minimizes manufacturing costs
extremes (max or min)
- emphasizes one tail of distribution
- clearance: 95th male
- reach: 5th female
safe: accomodate > 99% of population - must select to account for min or max
adjustability
- emphasize that all potential users are “equal”
- preferred method
- expensive and involved
- range and degrees of adjustment can be difficult to specify
- must select “acceptable” range
- a wider the range means the process/tool/task is more important
standard normal distribution equation
X = mean +/- Z * std
X = value of interest mean = mean of distribution Z = standard deviations above (+) or below (-) the mean std = standard deviation of the distribution
procedure for anthropometric design
- identify key anthropometric measurements
- determine the population and/or percentiles to target
- develop a scaled drawing (often side view) where body parts could be locate with adjustments
- from 2D mannequins, develop initial estimates of population effects
- use functional anthropometric data or computer based human models to verify predictions
- build mock-up with 3D mannequins and test with extremes to verify and validate
What are limitations of anthropometric design?
- population based data can have selection bias
- averages and proportions do not represent individuals; people are not proportional
- muscle strength not considered; capable of reaching but not moving or moving safely
- functional data may vary with clothing, protective equipment, etc
- even if guidelines, standards are followed, the workstation may not be completely functional, usable, or comfortable
Why evaluate ROM and strength?
- both are components of worker capacity; goal is to design or improve tools, tasks, and environments so that demands are less than or equal to capacity
- identify high risk workers
- asses “return to work” status
- understand how individual factors affect worker capacity
range of motions (ROM)
the maximum angular deviation of segments forming a joint
2 types of ROM
passive
- external force used to reach ROM limits
- more dependent on passive elasticity of MLTs
active
- muscle force used to reach ROM limits
- dependent on muscle contraction
active ROM < passive ROM
anatomical position
universally accepted reference position
standing, feet together, arms at sides, palms facing forward
anatomical planes of motion
sagittal: divides left and right sides of the body
frontal: divides front and back portions of the body
transverse: divides upper and lower portions of the body
What movements occur in the sagittal plane?
flexion and extension
What movements occur in the frontal plane
abduction: away from mid sagittal plane
adduction: toward mid sagittal plane
What movements occur in the transverse plane?
rotation
What methods are used to measure join ROM?
manual goniometer
electrogoniometer
inclinometer
How should ROM data be presented?
- use accepted terminology to describe posture or motion
- describe subject population
- describe the measurement method
What factors affect ROM?
age sex training (stretching and changes in flexibility) genetics 2 joint muscle effect
muscle strength
the maximum force/moment a group of muscles can develop under prescribed conditions
4 types of strength
isometric: no motion
isokinetic: constant velocity
free dynamic: nonconstant force/motion
psychophysical: the maximum load that can be handled for an extended period of time without overexertion; subjectively determined
What are the pros and cons of static strength measurements?
reliant on posture due to:
- force-length relationship
- changes in tendon moment arm with respect to the joint
- changes in weight moment arm
- two-joint muscles
pros:
- simplicity
- person stands in fixed positions
- instrumentation is relatively cheap and easily used
cons:
-poor association with dynamic performance capability
What are the pros and cons of dynamic strength measurements?
isokinetic is the most common dynamic strength measurement
pros:
-more realistic to real tasks
cons:
-testing is difficult given the range of potential postures and velocities
What are the pros and cons of psychophysical strength measurements?
lab simulation of task
pros:
- dynamic trial using the actual task
- measures worker preference
- expectation that design based on preferred limits will minimize injury
cons:
- 30 min session does not always correlate to 8h workday
- workers may not effectively judge risl
- expensive
- very sensitive to variables; instructions, procedures, motivation
What factors affect muscle strength?
sex
age
obesity (1/3 obsese, 1/3 overweight)
5 types of muscle contractions
isometric: constant length
concentric: muscle shortening
eccentric: muscle lengthening
isokinetic: constant velocity
isotonic: constant force
3 muscle groupings
co-contraction: two or more muscles at a joint contracting at the same time
agonist: contributes to desired effort at a joint
antagonist: opposes desired effort at a joint
3 mechanical properties of muscle
force-length (length-tension)
force-velocity (velocity-tension)
force-activation
force-length relationship
the amount of force a muscle can produce depends upon its length and passive tension
muscle length changes with posture -> the amount of force a muscle can produce depends on posture
force-velocity relationship
the amount of force a muscle can produce depends upon its velocity (rate of change of length)
concentric contractions: increasing velocity decreases the amount of force a muscle can produce
eccentric contractions: increasing velocity increases the amount of force a muscle can produce
force-activation relationship
the amount of force a muscle can produce depends upon its activation level (0-100%)
motor unit
a single motor neuron and all of the muscle fibers it innervates
ratio (nerve:muscle fibers)
- 1:5 precise control (eye)
- 1:1000 gross control and high forces (thigh and back muscles)
What stimulates a motor unit?
a motor unit potential elicits a twitch of tension in the motor unit’s muscle fibers
3 types of muscle fibers
slow: Type I
fast: Type IIa and IIb
Type I:
- more fatigue resistance
- oxidative (aerobic)
- “postural” muscles
Type 2:
- larger cross-sectional area
- higher maximum tension and rate of force development
- glycolytic (anaerobic)
- “working” muscles
What is the effect of aging on muscles?
- decrease of strength with increase of age
- loss of muscle mass and number of muscle fibers
- conversion of type II to type I
What is the effect of sex on muscles?
females show higher or equal endurance than males
Why is awkward/extreme posture bad?
physical risk factor
- more effort is required to generate same force
- can cause pressure between adjacent anatomic structures
minimize frequency, duration, and magnitude
Why are high force exertions bad?
physical risk factor
- cause large deformation of tissue
- reduce/completely stop circulation
- increase localized fatigue
minimize frequency, duration, and magnitude
Why are high repetition and sustained exertions bad?
physical risk factor
- affected by other factors: force, contact stresses, posture, vibration, temperature
- 5 classifications of rates
minimize frequency, duration, and magnitude
