Industrial Ergonomics Unit 1 Flashcards
What is Ergonomics?
Is the process of designing or evaluating products, tasks, environments, and systems to improve performance and/or reduce the risk of injury.
Acute or Chronic Musculoskeletal Disorders (MSDs)
Present in 12% of Population
32 million MS injuries per year
Nearly 2% of workers have work-related back problems each year
National systems markedly underestimate magnitudes
Cost of MSDs
The total cost of occupational injuries and illness in the US is >$150 Billion (nearly 3% of GDP)
More than AIDS, nearly equal to costs of cancer or heart disease
Injuries account for 85% of these costs, with MS injuries generating a large majority
Work-related MSDs account for ~1/3 of all workers compensation costs
Types of Physical Ergonomics Problems
Anthropometric problems
Musculoskeletal problems
Metabolic/Cardiovascular problems
Environmental problems
The Ergonomic Process
- Characterize existing or potential problems
- Perform job analysis
- Implement controls
- Evaluate the effectiveness of controls, and educate employees on ergonomics
Guiding Principle of Ergonomics
D (Demands) < C (Capacity or Capability)
6 Types of Interactions
Human with Machine Machine with Human Human with Environment Environment with Human Machine with Environment Environment with Machine
Stress Formula
Stress = Force/ Initial cross-sectional area
Strain Formula
Strain = Change in length/ Original length
Mechanical properties of bone
- Hooke’s law
- Young’s modulus (E)
- Yield stress
- Yield strain
- Ultimate stress
- Ultimate strain
Wolff’s Law
Bone (or other material) adapts to its mechanical environment: it will be added where needed and reabsorbed where not needed
Ligaments
Ligaments connect bone to bone
Tendons
Tendons connect muscles to bones
Structural differences between ligaments and tendons
Tendon fibers are parallel to one another due to the push and pull function of a tendon, while ligaments fibers are cross-sectional due to the variety of ways a ligament is pulled.
Cartilage
A smooth elastic tissue that covers the ends of long bones at joints to provide a low-friction for movement.
Fascia
A band or sheet of connective tissue, primarily collagen, beneath the skin that attaches, stabilizes, encloses, and separates muscles and other internal organs.
Viscoelasticity
Material response to force (or displacement) depends not only on force (or displacement), but also on time.
Creep
A change in strain for constant stress, and over time will reach a constant displacement
Load (stress) relaxation
A change in stress for constant strain, and over time will reach a constant force
Muscle Structure Units
(Largest) Muscle -> Fascicle -> muscle fiber -> myofilament (Smallest)
Myofilaments
Thin (actin) filaments
Thick (myosin) filaments
How does a muscle contract
Muscles contract not because of shortening of individual myofilaments, but because they slide
Types of muscle contractions
Isometric- constant length (or joint angle)
Concentric- muscle shortening
Eccentric- muscle lengthening,»_space;> external load (plyometric)
Isokinetic- constant velocity (or joint angular velocity)
Isotonic- constant force (or moment)
Muscle groupings
Co-contraction- two or more muscles contracting at the same time
Agonist- contributes to the desired effort
Antagonist- opposes desired effort
Force-length Relationship
The amount of force a muscle can produce depends upon its length. Muscle force also depends upon passive tension
Force-Velocity Relationship
The amount of force a muscle can produce depends upon its velocity (rate of change of length)
Force-Activation Relationship
The amount of force a muscle produces depends upon its activation level (0-100%)
The Motor Unit
A single nerve and all of the muscle fibers it innervates
3 Types of Muscle Fibers
Slow (Type I) and fast (Type IIaand Type IIb ) fibers
Muscle Spindles
Are specialized muscle fibers that provide CNS feedback on muscle strain (length) and strain rate (velocity)
Golgi Tendon Organs
Golgi tendon organs provide CNS feedback on muscle force. Oriented in series with muscle, act as a force gauge
WMSDs
Work tasks and environment contribute significantly to the condition, and likely make it worse or persist longer
Involve injury to soft tissues of the body and joints
They develop gradually over time
The injury event, or the exposure leading to the event, involves bodily action, overexertion, or repetitive motion.
WMSDs do not include sudden injuries such as those from slips, trips, and falls, or motor vehicle accidents
What is a risk factor?
A risk factor is an individual characteristic or exposure associated with an increased risk of a disease or injury.
Individual risk factors for WMSDs
Age Sex Obesity (anthropometry) Systemic diseases (e.g. diabetes) Acute trauma Congenital conditions
Task-related risk factors for WMSDs
High force exertions Highly repetitive work or static work Awkward or extreme postures Vibration Contact mechanical stress Environmental stress
Psychosocial risk factors for WMSDs
Psychological factors (individual-level) and social factors (organization-level) that influence the mental state of the individual Examples: mental stress, supervisory control, autonomy, job security, interaction with coworkers, anxiety, etc.
Tendon Disorders
Tendonitis: Inflammation of the tendon
Tenosynovitis: Inflammation of tendon sheath
Hierarchy of Controls
Elimination, Substitution, Engineering Controls, Administrative Controls, PPE
Characterize existing or potential problems
Quantify injuries or performances issues
Identify tasks that may have risk factors
Evaluate the effectiveness of controls, and educate employees on ergonomics
Provide employees with a basic awareness of ergonomics principles.
Involve workers in the process. They have the most knowledge about their jobs. They may already know how to improve them.
Awkward or Extreme Posture
Awkward posture- requires excessive effort against gravity or other issues
Extreme posture- at or near a joint’s limits of motion
Awkward or Extreme Posture: Job Analysis
Determine frequency, duration, and/or magnitude of awk/extreme postures
ErgonomicsHigh Force Exertions: Job Analysis
Determine frequency, duration, and/or magnitude of forceful exertions
Contact Mechanical Stress: Job Analysis
Determine frequency, duration, and/or magnitude of contact force
Anthropometry
Is the study of the physical dimensions of the human body
Two types of anthropometric data
- Static anthropometry: Physical dimensions of the body
2. Functional or dynamic anthropometry: physical dimensions for completing particular activities or tasks
Units of mass conversions (lbs)
1 kg = 2.2 pounds
1 slug = 32.2 pounds
Center of mass
COM is the point at which all the mass of a body can be concentrated so that it results in external forces and moments equivalent to the actual distributed body
General Design Strategies to account for anthropometric variance
Design for average
Design for extremes (maximum or minimum)
Design for Adjustability
Limitations of using Anthrodata to Design
Population-based data can have selection bias
Averages and proportions don’t represent individuals; people are not proportional
Functional data may vary with clothing, protective equipment, etc.
Even if guidelines, standards, etc. are followed, the workstation may not be completely functional, useable, or comfortable!
Why evaluate ROM and strength?
Identify high-risk work tasks
Identify high-risk workers –Workers with limited capacities may be at elevated risk for WMSD, etc.
Assess “Return to Work” status -Determine physical capacity after an injury to evaluate how much recovery has occurred.
More broadly, help improve understanding of how individual risk factors affect worker capacity
Two types of ROM measurement
Active ROM: muscle force used to reach ROM limits, so also dependent upon muscle contraction
Passive ROM: external force used to reach ROM limits; more dependent upon passive elasticity of MLTs
Active ROM < Passive ROM
Describing Joint Motions
Anatomical Position and Anatomical Planes of Motion
Anatomical Terms Related to Movement
Flexion and extension movements occur in the sagittal plane
Abduction and adduction movements occur in the frontal plane
Rotation movements occur in the transverse plane
Methods to measure joint ROM
goniometer: measures the joint angle
Inclinometer: measures the angles of slope, elevation, or depression of joint
Individual Factors Affecting ROM
Age
Sex
Training
Joint or muscle pathology, genetics
Muscle Strength
Defined as the maximum force/moment a group of muscles can develop under prescribed conditions
Types of Muscle Strength Measurement
Isometric (static): no motion
Dynamic: strength test allowing motion
Factors affecting strength: Posture
Force - length relationship
Changes in tendon moment arm with respect to the joint
Changes in weight moment arm
Two-joint muscles(muscle length depends upon two joint angles)
Factors affecting strength: Sex
Women are generally weaker than men by an average of about 35% to 85%
The magnitude of sex difference is task-dependent
When corrected for muscle x-section and lean body mass, values are nearly equal!
Factors affecting strength: Age
Muscle strength normally greatest in the late twenties to early thirties, and typically starts to decline in the 40s.
