Ergonomics/Human Factors

“Occupational Biomechanics”

Hardianto Iridiastadi, Ph.D.

Motivation• Epidemiological support (1984)

– 23 people with musculoskeletal impairments in the US

– Musculoskeletal problems rank second; cost $65 billion/yr

– Problems rank second in physician visit, third in hospitalizations, and third in surgical procedures

– Low back pain (LBP) is one of the most common problems, and is associated with substantial amounts of morbidity, disability, and economic loss

Motivation• Class

– LBP statistics– How it occured– Implications– Other problems?

• See also CTS clip

Definition• What is biomechanics?

– Mechanical behavior during work– Follow laws of physics– Interdisciplinary science

• Physics, biology, engineering• Behavioral science

• Occupational biomechanics– Study physical interactions of workers

and their tools, machines, materials– To enhance performance and minimize

MSDs

Muscle Testing

• Why?

– Understand strength (force or moments)

– Compare with job requirements

• Measurements– Isometric (Static)

– Dynamic

Work-related Musculoskeletal Disorders

(WMSDs)• What it is

– Physical problems, symptoms, issues, etc.– Associated with work

• Risk factors– Deviated postures; Static postures– Repetitive; Forceful tasks– Contact stress; Vibration– Temperature (cold)

WMSDs

• What are they? Injuries that affect the soft tissues of the body (muscles, tendons, nerves, ligaments) that develop gradually over time.

– Other terms• Cumulative trauma disorders (CTDs)• Repetitive strain injuries (RSIs)• Repeated trauma disorders (RTDs)

Costs Associated with WMSDs

• 705,800 cases of work days lost in 1995. Mean job absence per case is 20 days. (Bureau of Labor, 1995)

• $13-$20B in annual costs in lost days and compensation (NIOSH, 1996)

• $7.4B in worker’s compensation costs alone (CTD News, 1996).

Costs Associated with WMSDs

• A recent NIOSH study estimated 22% of all VDT workers had some type of WMSD problem.

• Underestimated• Difficulty in diagnosis these types of injuries• Seen as normal aches and pain• Some states say they aren’t compensable injuries

(can’t prove the work did it)

Types of WMSDs

1. tendon disorders

2. nerve disorders

Types of WMSDs

neurovascular disorders

• Tendinitis is inflammation of the tendon causing pain when the tendon is placed under tension.

• Stenosing tendinitis is inflammation of both the tendon and the synovial tissue that has caused a dramatic increase in the friction between the tendon and the tendon sheath.

Back WMSDs• Abnormal Conditions

– Degenerative disc disease– Herniated disc– Ligament sprain– Muscle strain

• Contributing Factors– Prolonged static load on the upper torso

musculature– Awkward posture: extensive trunk flexion or

extension– Lifting of objects from the floor

Back WMSDs• Prevention and Control

– Proper lifting techniques– Stretching exercises

Shoulder WMSDs• Abnormal Conditions

– Shoulder Tendonitis

• Contributing Factors– Prolonged flexion/abduction of the shoulders– Frequent reaching above the shoulders– Tasks which pull shoulders back and down– Prolonged load on the shoulders– Repetitive throwing of heavy loads

Shoulder WMSDs• Prevention and Control

– Increase recovery time where force requirements are high

– Reduce awkward shoulder postures using fixtures, clamps, etc.

– Reduce frequent reaching above the shoulders– Reduce loads on the shoulders

Tendon Disorders

• Rotator Cuff Injuries – Tendinitis at the head of the humerus.

From Putz-Anderson, V. (1988). Cumulative Trauma Disorders: A Manual for Musculoskeletal Diseases of the Upper Limbs. London, UK: Taylor & Francis., Fig. 6

Finger WMSDs

• Abnormal Conditions– e.g. DeQuervain’s Disease

• Contributing Factors– Vibrating tools– Repetitive Ulnar deviation– Flexion of the wrist with effort– Forceful gripping

Finger WMSDs

• Prevention and Control– Implement job rotation and/or job

enlargement– Use protective materials to absorb

vibrations effects

Wrist CTDs

• Abnormal Conditions– e.g. Carpal Tunnel Syndrome

• Contributing Factors– Repetitive forceful wrist

extension/flexion– High speed finger movements– Ulnar deviation– Rapid wrist rotational movement

Wrist CTDs

• Prevention and Control– Stretching exercises– Implementation of rest periods

Risk Factors• Occupational Factors implicated as

causes of WMSDs of the hand and wrist:– Postural stress

– Forceful exertions

– Repetitive exertions

– Sustained (static) exertions

– Localized mechanical (contact) stresses

– Vibration

– Cold temperature

Wrist Posture

• The wrist has three degrees-of-freedom:– Flexion - Extension– Radial deviation - Ulnar deviation– Pronation - Supination

• Deviations from a neutral posture in any of these three dimensions has been associated with an increased risk of WMSDs

Wrist Posture

From Putz-Anderson, V. (1988). Cumulative Trauma Disorders: A Manual for Musculoskeletal Diseases of the Upper Limbs. London, UK: Taylor & Francis., Fig. 38

Which tool shape is best??

Depends on: location, orientation, task, etc.

Controls for Postural Deviations

From: TJ Armstrong (1986) Hand Clinics 2:553-565

Ulnar deviation of wrist required to hold needle-nose pliers for bench assembly work

Additional Benefits: improved precision (vision)

Example Controls for Postural Deviations

From: TJ Armstrong (1986) Hand Clinics 2:553-565

Ulnar deviation of wrist required to hold knife for boning turkey thighs

Additional Benefits: safety, increased wrist strength in neutral posture

Example Controls for Forceful Exertions

Effort required to support tool weight and keep it from twisting out of the hand

Concern: more effort to ‘engage’ the tool?

Controls for Postural Deviations

Install gas fuel neck (2m from floor)

Operator had neck strain, 8 months of lost work

Installation moved to different place on assembly line

Controls for Postural Deviations

Install brake booster

After installation of engine, radiator, bumper

Installation moved earlier on assembly line

Use of ‘extended’ tool

WMSD Prevention Strategies1. Minimize required hand forces

• design for power grip

• keep handle diameters > 1.5"

• use tool balancing

2. Minimize postural deviations

• keep hand and forearm aligned

• shape the tool to match the task

• change work piece orientation

WMSD Prevention Strategies

3. Minimize Repetition• allow for tissue recovery and circulation• rotate workers among tasks• job enlargement

4. Design Work Practices• train workers to rest (esp. hands)• stretching exercises or warm-up • allow for slow-down with gloves

5. Recognize that these strategies will likely increase productivity in the long run

Sample Intervention: Before

• Risk factors?

• Intervention?

www.osha.gov/ergonomics/ergo1.html

Sample Intervention: After

www.osha.gov/ergonomics/ergo1.html

Task Evaluation and Ergonomic Controls

• D (moments) < C (strength)• Are the demands excessive?

– Is the percentage capable too small?– What is an appropriate percentage?

Task Evaluation and Ergonomic Controls

• Strategies to Improve the Task:– Decrease D

• Forces: masses, accelerations (increase or decrease, depending on the specific task)

• Moment arms:

– Increase C• Design task to avoid loading of relatively weak

joints• Maximize joint strength • Use only strong workers