Week 2: Occupational Biomechanics Flashcards
What is the goal of occupational Biomechanics
Design tasks that do not exceed the capacity of the musculoskeletal system
- Improve performance
- Reduced Risk of Injury
Components that can be changed to achieve main goals of occupational Biomechanics
- Tool Design
- Workplace Design
- Job Design
- Worker/task matching
- Material Handling
What is ergonomics
- Scientific discipline concerned with the understanding of interactions among humans and other elements of a system
- applies theory, principles, data, and methods to design in order to optimize human well-being and overall system performance
- Promotes holistic approach in which considerations of physical, cognitive, social, organizational, environmental and other relevant factors are taken into account
What is the difference between occupational biomechanics and Ergonomics
- Both recognize a multi-disciplined approach to understanding the worker: work interface
- Ergonomics can be broader including environment, cognitive issues, social, organizational elements
History of occupational biomechanics in the 1700s
- Berbardino Ramazzini founder of occupational medicine
- Published first comprehensive work on occupational disease
History of occupational biomechanics Pre-1900s:
- Most manufacturing and farming was done as a craft industry in which the worker was a craftsman and produced the entire product from beginning to end
History of occupational biomechanics in the industrial revolution
- The invention of electricity and the assembly line was a dramatic change in the way people were selected for employment, the types of tasks that were performed and the relationship between the worker and the product
- Production increased dramatically by having many workers performing partial steps and working in shifts around the clock
- Worker was fit to the task
- Removed if injury or could not perform
- Conditions were not controlled
History of occupational biomechanics in the second half of the 20th century
- Government started compensating injured workers and charging corporations premiums
- Became economically important to create a safer workplace
- American with Disabilities Act require employers to make reasonable accommodation to tasks to not discriminate against those with disabilities
- Fitting worker to the task switched to fitting the task to the worker; better design of occupational tasks and assembly line
Def: Occupational Biomechanics
- The examination of human disorders and performance limitations produced or aggravated by the mismatching of human physical capacities and the performance requirements in industry
- The study of the physical interaction of workers with their tools machines, and material so as to enhance the worker’s performance while minimizing the risk of musculoskeletal disorders
Work-related MSK disorders
- Disorders of the muscles, tendons, discs, ligaments, and nerves, caused by occupational tasks
- other names include: strains, sprains, ergonomic disorders/injuries, occupational overuse syndrome, repetitive motion disorders, repetitive strain injuries, cumulative trauma disorders
- Most frequently injured body part is back, second neck, commonly caused due to chronic overexertion
Causes of work-related MSK disorders
- Force
- Awkward posture
- Time, repetition, duration
- Mechanical stress
- vibration
- Environment
Force as a cause for MSK disorder
- External forces: typically applied at hands or caused by gravity
- Internal forces: Muscles and passive tissue (outcome depends on tissue loaded)
- Acute injuries = high forces which exceed tissue tolerance
- Chronic injuries = lower forces combined with repetition
what can awkward posture lead to?
- Isometric loading
- Overloaded muscle and tendons
- Pinching or impingement of tissues
- Increase moment arm of load (increase moments of force = increase internal forces)
- Asymmetrical tissue loading
Pinch grip
- Uses the smaller, weaker muscles in the fingers to apply the force
- With pinch grip, one or more fingers oppose the thumb to grasp an object
Power Grip
- Uses larger, stronger muscles in the forearm to perform the task
- With a power grip, the hand is wrapped around the object with the thumb overlapping the fingers
Repetition as a risk factor for MSK disorders
- May lead to progressive decrease in tissue tolerance level
- May not allow sufficient time for recovery
Duration as a risk factor for MSK disorder
- Accumulation of fatigue and tissue damage
- May lead to decreased coordination and tissue stability
Rest schedule as a risk factor for MSK disorders
- Rest leads to recovery from fatigue and tissue damage
Def: Repetition
The time quantification of a similar exertion performed during a task
Risk of injury proportionality
ROI= force x repetition x duration / tissue tolerance
High Force Injury
- Specific instance of high force above tissue tolerance
- Acute in nature
Low, repetitive force injury
- Repeated low applied force across long time period
- slow decline in tissue tolerance until it is below force level
Low, constant force injury
- Low force applied constantly across a period of time
- Tissue tolerance decreases over time until it is less than applied force
Mechanical Stress
Pressure to the skin and soft tissues from direct contact with parts, tools, fixtures, etc.
- Sustained, prolonged use of hand tool
Whole body vibration
- Exposure of the whole body to vibration has some support as a risk for injury
- Prevalence of reported back pain approximately 10 percent higher in tractor drivers than in workers not exposed to vibration and prevalence of back pain of back pain increased with vibration dose
- Operators of earth-moving machines with at least 10 years of exposure to whole body vibration showed lumbar spine morphological changes earlier and more frequently than non-exposed people
Localized vibration
- Vibration applied to the hand can cause vascular insufficiency of the hands/fingers
- Can interfere with sensory receptor feedback leading to increased hand grip force to hold the tool
Environments
HEAT
Acute outcomes
- Sweating, dehydration, increased fatigue
Chronic Outcomes
- Heat stress/stroke
- Decreased physical/mental performance
COLD
Acute outcomes:
- loss of tactile sensitivity - increased grip efforts due to misjudgment
Chronic Outcomes:
- Decreased physical/mental performance
Sagittal Plane Modeling
Most analyses use a sagittal plane model assuming static equilibrium to calculate joint moments and L5-S1 spinal compression
Static vs Dynamic Equilibrium
- While lifting is dynamic, low accelerations during heavy lifting make static equilibrium sufficiently accurate
- Static analysis is simpler and aligns well with dynamic results for high-load tasks
Data collection advantages
- Static analysis requires only a scaled image of the posture and anthropometric data
- Large databases of risk factors are derived from these methods, making it a standard approach
Free Body diagram application
A still and scaled image of a work in posture is used to generate a free body diagram for biomechanical assessment
Stoop vs Squat Lift
- Stoop lift requires greater ES force due to greater moment and increasing greater compression
- However, if load cannot be placed between knees during squat, it is placed beyond the knees increasing the moment arm of the weight compared to the stoop lift leading to increased compression
Action Limit
Tasks above 3400N are 3x more likely to result in low back pain
Maximum Permissible Lift
Tasks above 6400N are 8x more likely to cause back pain
Spinal Compression Trends
Compression increases with load weight and distance from the body
Who provides safety recommendations
The National Institute for Occupational Safety and Health
Tissue Tolerance Controversy
Cadaveric and pig studies show tissue failures at lower forces than those calculated using sagittal plane models, meaning compression forces in real-life lifting may be higher than calculated values
1. The erector spinae may be closer to 10cm than 5cm, reducing calculated forces by 50%
2. Living tissues under hydrostatic pressure can withstand greater stress
3. Over-estimation of forces ensures safety by avoiding false sense of security - recommendations based on 5 cm model
Bartelink’s 1957 Theory on intrabdominal pressure
- Increased intra-abdominal pressure reduces lumbar spine compression via the balloon mechanism
- Abdominal cavity acts as a closed chamber when bearing down
Mechanism behind Bartelink’s 1957 Theory
- Contraction of deep muscles or using a belt forces abdominal contents upward, reducing load on the lumbar spine
- Traction form the diaphragm pulls on L4/L5, theoretically reducing compression forces
Kapanji’s Estimate of intraabdominal pressure
Abdominal support reduces:
- L5/S1 disc compression by 30%
- Erector spinae muscle force by 55%
Recent evidence on intrabdominal pressure
- Increase IAP does not reduce compression on the spine - may increase it
- No significant reduction in force required by lower back muscles
- IAP stiffens the trunk, preventing buckling and strain
- may reduce shear lads on the spine
- Belts provide some protection by limiting ROM during bending or twisting, though less effective than once believed
Risks for wearing Back Belts
- Muscle Weakness: Reduced activity of supportive spinal muscles can lead to weakness and a higher risk of injury after belt use stops
- Increased Cardiovascular Strain: Back belts may raise blood pressure and heart rate, posing risk for individuals with cardiovascular issues
- False Sense of Security: Workers may lift heavier objects, increasing the risk of injury
Recommendations for Back Belt Use
- Use belts only temporarily
- Focus on proper lifting form, posture, and trunk conditioning exercises to strengthen supportive muscles
Workplace ergonomics and general health tips for back health
- Perform ergonomic assessments to reduce spine overloading
- Train workers on lifting mechanics and early recognition of back discomfort to prevent sever injury
- Encourage regular fitness programs and weight management avoid smoking
Overuse injury
- From performing tasks that are highly repetitive
- Poor posture is neglected as the forces seem so small and insignificant that care is not taken about task performance
- light work can lead to injury over time by not realizing the task has negative effects on muscles and nerves
Carpel tunnel syndrome
A nerve disorder in the hand due to chronic pressure on the median nerve as it passes through the carpal tunnel in the wrist
Symptoms of CTS
- pain in thumb and fist 3 fingers
- numbness and tingling in these areas
CTS Detection and testing
CONDUCTION VELOCITY TESTING:
- nerve entrapment is diagnosed by measure the slowing of conduction velocity along the nerve
- Electrodes are placed at two points along the nerve to record action potentials
- Time and distance between recordings are sued to calculate conduction velocity
PHALEN’S TEST
- Hold wrists at 90 degrees with back of wrist facing each other for 30 seconds and assess for any tingling
Risks factors for CTS
- Arthritis, diabetes
- Fractures
- Repetitive, prolonged activity
- Excessive force
- Posture
- Palmer compression
- pinching is worse than gripping
- tools that vibrate
- cold working environment
- Lack of skill
Prevention of Overuse Injury
- Gloves that increase friction and decrease the required grip force and that reduce vibration amplitude and increase temperature
- Reduce repetition (job rotation)
- Tool design (avoid posture of wrist extension, ulnar or radial deviation)
