Chapter 06: Ergonomics & Human Factors

What is Ergonomics?

Ergonomics is the science of designing products, systems, and environments to fit the people who use them. It combines physiology, psychology, and engineering to create comfortable, safe, and effective user experiences.

Goals:

1. Safety: prevent injury
2. Comfort: reduce fatigue and strain
3. Efficiency: enable effective performance
4. Satisfaction: create a positive experience

"Good design is invisible; people should notice benefits, not the design itself." Ergonomics principle

Anthropometrics

The study of human body measurements and their application to design.

Key Concepts

1. Percentiles Population distribution used for sizing decisions.

Common Standards:

• 5th percentile: small users (typically female)
• 50th percentile: average users
• 95th percentile: large users (typically male)

Design Strategy:

• Accommodate 5th to 95th: fits 90% of users
• Adjustable designs: sliders, telescoping, adaptable
• Multiple sizes: S/M/L like clothing

Why not 0-100%?

• Cost and complexity increase dramatically
• Extreme outliers can use specialized solutions

2. Critical Dimensions

Reach Distances:

Grip Dimensions:

3. Design for Extremes

Maximum Dimensions: Design for largest users.

• Doorways, seats, clothing

Minimum Dimensions: Design for smallest users.

• Reach distances, control access, strength requirements

Adjustable Range: Cover from 5th to 95th percentile.

• Office chairs, car seats, shower heads

Body Mechanics

1. Neutral Postures Body positions that minimize strain.

Standing:

• Spine naturally curved (not bent)
• Shoulders relaxed
• Arms at sides
• Weight evenly distributed

Sitting:

• Feet flat on floor
• Thighs parallel to ground
• Lower back supported
• Arms at 90° angle

Hand/Wrist:

• Wrist straight (not bent up/down)
• Neutral grip (not twisted)
• Minimal force required

2. Force Application

Maximum Force Generation:

Design Implications:

• Heavier = add wheels, handles, or reduce weight
• Tight fasteners = consider tool-free
• High force = use leverage, mechanical advantage

3. Repetitive Strain

Risk Factors:

• High repetition (>10,000 cycles/day)
• Awkward postures
• High force
• Vibration
• Static positions

Solutions:

• Reduce force required
• Allow neutral postures
• Enable position changes
• Add padding/cushioning
• Automate repetitive tasks

Designing for Hands

Grip Types

1. Power Grip Full hand wraps around object, maximum force.

Characteristics:

• All fingers and palm engaged
• Used for high force
• Less precision

Examples:

• Hammer handle
• Suitcase handle
• Bicycle grip

Design Guidelines:

• Diameter: 1.25" - 1.75" (3-4.5cm)
• Length: Minimum 4" (10cm)
• Texture: Prevent slipping
• Slightly oval (not perfectly round)

2. Precision Grip Fingertips and thumb, maximum control.

Characteristics:

• Fingertips only
• Low force
• High precision

Examples:

• Pen
• Surgical instruments
• Small screwdriver

Design Guidelines:

• Diameter: 0.3" - 0.6" (0.8-1.5cm)
• Triangular/hexagonal (prevents rolling)
• Balance near grip point
• Lightweight

3. Pinch Grip Thumb and one or two fingers.

Characteristics:

• Very low force (fatigue quickly)
• Moderate precision

Examples:

• Keys
• Small parts
• Tweezers

Design Guidelines:

• Minimize required force
• Textured surfaces
• Adequate size (not too small)

Handle Design

Optimal Handle Dimensions:

Material Considerations:

Trigger Design

For power tools, spray bottles, etc.

Guidelines:

• Two-finger minimum: index and middle
• Comfortable angle: 15-25° from handle
• Smooth pull: no sticking or catching
• Force required: under 5 lbs
• Travel distance: 0.5-1" typical
• Safety lockout: prevent accidental activation

Designing for the Body

Seating Design

Chair Dimensions (Dining/Work):

Cushioning:

• Density: medium-firm for extended sitting
• Thickness: 2-4" compressed
• Edge: waterfall edge (rounded front) reduces pressure

Table/Desk Height

Standard Desk:

• Height: 28-30" (71-76cm)
• Knee clearance: 24" (61cm) minimum
• Thigh clearance: 20" (51cm) minimum width

Adjustable Desk:

• Sitting range: 24-30" (61-76cm)
• Standing range: 38-48" (97-122cm)

Product Positioning

Principle: Place controls where hands naturally rest.

Zones:

    OVERHEAD REACH
        [rarely]
        
    PRIMARY ZONE
   [frequent use]
     [easiest reach]
     
    SECONDARY ZONE
   [occasional use]
  [requires stretching]

Example (Power Tool):

• Primary: trigger, main grip
• Secondary: speed selector, lock button
• Tertiary: battery release, accessory storage

Cognitive Ergonomics

Beyond physical fit: how products align with mental models.

Affordances

Visual cues about how to use something.

Examples:

• Button: appears press-able (convex, textured)
• Handle: appears grip-able (protrusion, texture)
• Slider: appears drag-able (groove, knob)
• Dial: appears turnable (circular with grip features)

Design Guideline: Make the intended use obvious from the form.

Mapping

Relationship between controls and effects.

Natural Mapping:

• Steering wheel: turn left to go left
• Volume knob: clockwise increases
• Light switch: up for on (in US)

Poor Mapping:

• Stove burners not matching layout of controls
• Multi-gang light switches with unclear assignment

Solution:

• Spatial correspondence (control near affected element)
• Icons/labels when necessary
• Consistent conventions

Feedback

System response to user actions.

Types:

1. Visual: light, screen change, movement
2. Auditory: click, beep, tone
3. Tactile: vibration, resistance, texture change

Guidelines:

• Immediate: no perceived delay (<100ms)
• Proportional: matches action magnitude
• Distinct: different actions have different feedback

Example (Good):

• Mechanical keyboard: audible click + tactile bump + visual character

Example (Bad):

• Touch screen with no feedback: did it register?

Error Prevention

Design to prevent mistakes.

Strategies:

1. Constraints Make wrong actions impossible.

• USB-C cable (reversible, can't insert wrong)
• Outlet with polarized plug (only fits one way)

2. Confirmations Ask before destructive actions.

• "Are you sure you want to delete?"
• Two-step process (unlock then activate)

3. Reversibility Allow undo.

• Undo button
• Removable/replaceable parts

4. Clear Status Show current state.

• Lock indicators
• Power LEDs
• Battery levels

Accessibility (Universal Design)

Design for the widest range of users, including those with disabilities.

Principles of Universal Design

1. Equitable Use Useful and marketable to people with diverse abilities.

Example: Automatic doors (help everyone, essential for wheelchairs)

2. Flexibility in Use Accommodate wide range of preferences and abilities.

Example: Scissors with ambidextrous design

3. Simple and Intuitive Easy to understand regardless of experience or knowledge.

Example: Push-bar door (obvious operation)

4. Perceptible Information Communicate necessary information effectively.

Example: Crosswalk signals with sound (blind users) and light (deaf users)

5. Tolerance for Error Minimize hazards and consequences of accidents.

Example: Power tools with two-handed activation

6. Low Physical Effort Used efficiently and comfortably with minimum fatigue.

Example: Lever door handles (easier than knobs)

7. Size and Space for Approach and Use Appropriate size regardless of user's body size or mobility.

Example: Accessible bathroom stall (spacious for wheelchairs)

Designing for Specific Needs

Visual Impairment:

• High contrast
• Tactile indicators (bumps, texture)
• Audio feedback
• Large text/icons

Hearing Impairment:

• Visual feedback
• Closed captions (digital)
• Vibration alerts

Mobility Impairment:

• Large buttons (1" / 2.5cm minimum)
• Low force required
• Accessible placement
• Stable (doesn't tip or slide)

Cognitive Impairment:

• Simple operation
• Clear labeling
• Consistent behavior
• Error tolerance

Age-Related Changes:

• Presbyopia: need larger text
• Reduced dexterity: larger controls
• Strength loss: lower forces
• Memory: clear visual cues

Safety Considerations

Risk Assessment

Hierarchy of Controls:

1. Elimination: remove hazard entirely (best)
2. Substitution: replace with safer alternative
3. Engineering: design safety into product
4. Administrative: warnings and instructions
5. PPE: personal protective equipment (worst)

Example (Table Saw):

1. ❌ Can't eliminate blade
2. ❌ Can't substitute (need sharp blade)
3. ✅ Blade guard, riving knife, brake system
4. ✅ Warning labels, user manual
5. ✅ Safety glasses, push stick

Common Hazards

Sharp Edges/Points:

• Round corners (radius >0.5mm)
• Chamfer edges
• Guard with covers

Pinch Points:

• Minimum 0.25" (6mm) gaps to prevent finger trapping
• Clearance markings
• Auto-stop sensors

Tip/Stability:

• Low center of gravity
• Wide base
• Anti-slip feet
• Weight distribution

Electrical:

• Proper insulation
• Ground fault protection (GFCI)
• Strain relief on cords
• Clear voltage labeling

Thermal:

• Insulation on hot surfaces
• Warning labels
• Cool-to-touch exterior
• Auto-shutoff

Testing & Validation

Usability Testing

Process:

1. Recruit: 5-8 representative users
2. Prepare: define tasks, create prototype
3. Test: observe users completing tasks
4. Analyze: identify patterns in struggles
5. Iterate: fix issues, retest

Example Tasks (Coffee Maker):

1. "Fill water reservoir"
2. "Add coffee grounds"
3. "Brew a pot"
4. "Clean after use"

Metrics:

• Task completion rate
• Time to complete
• Number of errors
• Satisfaction rating (1-10)

Physical Testing

Comfort Test:

• Use product for extended period
• Note discomfort, fatigue, pain
• Multiple users (range of sizes)

Force Test:

• Measure force required for operations
• Compare to design guidelines
• Test with weakest users

Durability Test:

• Simulate extended use
• Drop tests
• Environmental exposure

Case Study: Redesigning a Can Opener

Original Issues:

• High force required (arthritis pain)
• Awkward angle (wrist strain)
• Sharp edges on cut lid
• Difficult for left-handed users

Ergonomic Analysis:

Redesign Solutions:

1. Mechanical Advantage

   • Longer lever handles (reduce force 40%)
   • Gear ratio 3:1

2. Grip Design

   • Larger diameter (1.5")
   • Soft rubber coating
   • Oval cross-section

3. Cut Method

   • Cut from side (not top)
   • Leave blunt edge

4. Ambidextrous

   • Symmetric design
   • No handedness preference

Result:

• Force reduced to 9 lbs ✅
• Wrist at 10° deviation ✅
• No sharp edges ✅
• Equal for both hands ✅

Key Takeaways

1. Design for real humans, not idealized users: consider the full range of sizes and abilities
2. Neutral postures reduce strain: keep the body in natural positions
3. Test with actual users early and often: assumptions about comfort are often wrong
4. Accessibility helps everyone: universal design benefits all users
5. Feedback is essential: users need to know their actions registered
6. Safety first: engineer out hazards, don't just warn about them
7. Measure, don't guess: use anthropometric data, not intuition

What's Next

In Chapter 07: Color, Material & Finish, you'll learn how surface treatments and color choices affect user perception and emotional response.

Exercise: Evaluate a tool you use frequently:

1. Sketch how you hold it
2. Measure handle dimensions
3. Identify any discomfort after extended use
4. Compare to ergonomic guidelines
5. Sketch improvements based on findings
6. Test with cardboard mockup