By the end of this topic, you should be able to...
explain limiting aspects of user capabilities, including users’ visual accuracy, colour perception, strengths, fatigue, muscle control and hearing thresholds.
Guiding Question
How do ergonomic considerations influence the design of a product?
Did you know?
The 1979 Three Mile Island nuclear incident remains a landmark study in physiological factor failure. Operators were confronted with: Warning lights using red/green colour coding — ignoring that statistically ~8% of male operators may have red-green colour deficiency Controls requiring sustained grip force during high-stress periods — triggering rapid fatigue Auditory alarms operating simultaneously at varying frequencies — exceeding hearing threshold discrimination capacity Fine instrumentation dials demanding visual accuracy beyond practical working distance The design failed to account for the limiting nature of human physiology under operational conditions. This is precisely why physiology factors — human factor data related to physical characteristics used to optimise the user's safety, health, comfort and performance — are central to ergonomics: the application of scientific information concerning the relationship between human beings and the design of products, systems and environments.
Limiting Aspects of User Capabilities
Visual Accuracy
Visual acuity is the eye's ability to resolve fine spatial detail. It is measured using the Snellen scale (20/20 vision being the standard benchmark).
Why it limits design:
Limiting Condition | Physiological Cause | Design Consequence |
|---|---|---|
Ageing (presbyopia) | Loss of lens flexibility reduces near-focus ability | Text must be ≥3mm at 500mm reading distance |
Low light environments | Rod cells activate over cone cells — reduced resolution | High contrast ratios required |
Fatigue | Ciliary muscle strain reduces sustained focus accuracy | Screen refresh rates, rest intervals |
Distance from object | Angular subtense of detail falls below resolution threshold | Dashboard instruments, signage sizing |
From an industrial design perspective, static data — human body measurements when the subject is in a fixed position — informs the seated eye height and viewing angle, while the acceptable resolution limit governs minimum feature sizes on displays.
Design application:
The minimum readable font size for a standing user at 700mm is approximately 3.5mm letter height. Failing to apply this limit excludes users at the lower percentile range of visual acuity.
Colour Perception
Colour vision deficiency (CVD) is a physiological constraint that significantly limits user capability when interacting with colour-coded systems.
Key data:
Approximately 8% of males and 0.5% of females have some form of CVD
Red-green deficiency (deuteranopia/protanopia) is most prevalent
Blue-yellow deficiency (tritanopia) is rarer but affects a defined demographic
Why it limits design:
The human retina contains three types of cone photoreceptors (S, M, L). A reduction or absence of one cone type creates confusion between specific wavelength pairs. This is not a matter of preference — it is a hard physiological boundary.
Design consequence:
Any safety-critical system that relies solely on colour to communicate information (e.g., red = stop, green = go) excludes a statistically significant proportion of users. Redundant coding — shape, pattern, label, position — must supplement colour.
Cross-reference — Glossary: This is a psychology factor and physiology factor intersection. While colour perception is physiological (cone cell response), the interpretation of colour meaning (red = danger) is psychological.
Strength
Physical strength is one of the most variably distributed anthropometric characteristics across a population. Anthropometrics — the aspect of ergonomics that deals with body measurements — includes force data as well as dimensional data.
Strength data is always expressed using percentile ranges:
Percentile: a term that describes how a data point compares to all data in that set, divided into 100 equal parts.
Strength Metric | 5th %ile Female | 50th %ile Mixed | 95th %ile Male |
|---|---|---|---|
Grip strength | ~18 kg | ~40 kg | ~72 kg |
Push force (one hand) | ~9 N | ~45 N | ~90 N |
Torque (jar lid opening) | ~1.2 Nm | ~2.8 Nm | ~5.5 Nm |
Why it limits design:
A product designed to the 95th percentile of male grip strength requires a force that excludes the majority of the female population and elderly users. The limiting factor in strength-critical design is always the 5th percentile of the weakest relevant demographic.
Biomechanics — research and analysis of the mechanics of muscles, joints, tendons — informs the maximum safe operational forces to prevent musculoskeletal injury.
Design application: Child-resistant medicine caps are tested against the grip and rotational strength of children (5th–50th percentile child) while remaining operable by adults. Adjustability — the ability of a product to be changed in size — can extend the range of strength applicability (e.g., adjustable spring-loaded mechanisms).
Fatigue
Fatigue is the progressive decline in physiological and cognitive performance resulting from sustained physical or mental effort. It is one of the most critical limiting factors in workspace and product design.
Two principal types:
A. Muscular Fatigue
Occurs when a muscle is required to sustain a contraction. Research establishes:
Contractions above 15–20% of Maximum Voluntary Contraction (MVC) cannot be sustained indefinitely
Contractions above 50% MVC can only be held for seconds
B. Repetitive Strain (Cumulative Fatigue)
Repeated sub-maximum efforts accumulate micro-trauma in tendons and muscles. This manifests as:
Repetitive Strain Injury (RSI)
Carpal Tunnel Syndrome (sustained keyboard/mouse use)
Lower back fatigue (sustained seated posture)
Design consequence:
The workspace envelope — a 3D space with defined permissible boundaries of movement and operation — must be designed so that frequently used controls fall within the primary reach zone (elbow-height, within forearm radius) to minimise muscular effort and delay fatigue onset.
Reach — the range that a person can stretch to touch or grasp an object from a specified position — directly determines whether a user must extend beyond their comfortable zone, accelerating fatigue.
Muscle Control
Muscle control encompasses both gross motor control (large-limb movements) and fine motor control (precision finger/hand movements). The limiting nature of muscle control is population-dependent and condition-dependent.
Populations with reduced fine motor control:
Children under 7 years (developing nervous system myelination)
Elderly users (reduced proprioception and hand tremor — essential tremor prevalence rises with age)
Users with neurological conditions (Parkinson's disease, cerebral palsy)
Users in cold environments (vasoconstriction reduces finger dexterity)
Design consequence (Dynamic Data): Dynamic data — human body measurements taken when the subject is in motion — is critical here. A surgeon's hand tremor amplitude (±0.5mm) defines minimum scalpel handle diameter for stable grip. A touchscreen button must be ≥9mm × 9mm to accommodate 95th percentile fingertip contact area.
Hearing Thresholds
The human auditory system operates within defined physiological limits that vary significantly across the population.
Standard audible range: 20 Hz – 20,000 Hz
Key limiting factors:
Factor | Effect on Hearing Threshold | Design Implication |
Age (Presbycusis) | Progressive loss of high-frequency sensitivity (>4kHz) after age 40 | Alarm frequencies should target 1–3 kHz |
Noise-induced hearing loss | Sustained exposure above 85 dB causes permanent threshold shift | Workplace product noise levels regulated |
Masking | Ambient noise raises the threshold for detecting a signal | Warning signals must exceed ambient by ≥15 dB |
Directional localisation | Accuracy degrades below 500 Hz and above 8,000 Hz | Emergency signals use broadband frequencies |
The equal-loudness contour (Fletcher–Munson curves) demonstrates that human hearing is most sensitive between 2–5 kHz — the frequency range of the human voice.
Designing auditory warnings in this range ensures the widest user coverage.
Design consequence:
A smoke alarm emitting at 3,150 Hz reaches the maximum population. However, research (Harman et al., 2006) demonstrated that sleeping adults — particularly those with age-related hearing loss — failed to wake to standard high-frequency alarms. A low-frequency (520 Hz) square-wave alarm proved significantly more effective, particularly for elderly users and those with high-frequency hearing loss.
Case Studies
iOS Accessibility Features
Apple's iOS colour filter system directly addresses colour perception limitations. The system converts the entire display to a colour space accessible to users with deuteranopia, protanopia, and tritanopia. This is a software-level response to a hardware-level physiological limitation (cone cell deficiency).
Formula 1 Cockpit Design
F1 steering wheels are engineered around muscle control and fatigue data.
Dynamic data of driver hand movements at 200+ mph informs:
Button tactile resistance calibrated to prevent accidental activation (fatigue-tremor threshold)
Critical controls positioned within a 60mm reach of thumbs (primary workspace envelope)
Paddle shift mechanisms requiring <15N actuation force to remain operable under G-load fatigue
Quick Summary
Physiological Factor | Key Measurement | Data Type |
|---|---|---|
Visual Accuracy | Snellen Acuity / Angular Resolution | Static |
Colour Perception | Cone cell sensitivity range | Physiological constant |
Strength | Maximum Voluntary Force | Static/Dynamic |
Fatigue | % Maximum Voluntary Contraction | Dynamic |
Muscle Control | Fitts' Law — target size & distance | Dynamic |
Hearing Threshold | dB SPL / Frequency (Hz) | Static |
Key Vocabulary
Term | Definition | Relevance to A1.1.6 |
Physiology factors | Human factor data related to physical characteristics used to optimise safety, health, comfort and performance | The umbrella term for all six factors above |
Ergonomics | Application of scientific information concerning the relationship between humans and products/systems/environments | The discipline within which physiological factors are applied |
Anthropometrics | The aspect of ergonomics dealing with body measurements | Provides the percentile data for strength and reach |
Biomechanics | Research and analysis of the mechanics of muscles, joints, tendons | Underpins strength and fatigue analysis |
Percentile / Percentile range | A data point's position within a population divided into 100 equal parts | Used to define design limits (5th–95th %ile) |
Workspace envelope | A 3D space with defined permissible boundaries of movement | Defines reach zones to minimise fatigue |
Reach | The range a person can stretch to touch or grasp an object | Determines control placement relative to fatigue onset |
Dynamic data | Body measurements taken in motion | Used for muscle control and fatigue analysis |
Static data | Body measurements in a fixed position | Used for visual distance, seated dimensions |
Adjustability | Ability to change size to increase percentile range | Strategy to accommodate varying strength/reach |
Practice Questions
Question 1 (4 marks)
Explain how colour perception limitations must be considered in the design of a public transport signage system.
Question 2 (6 marks)
Using the concept of percentile range, explain how muscle fatigue should influence the design of a surgical instrument intended for use during a 4-hour procedure.
Question 3 (3 marks)
Explain why hearing threshold data is particularly important when designing emergency warning systems for use in elderly care facilities.
Question 4 (5 marks)
Compare the use of static data and dynamic data when designing to account for the physiological limiting factors of muscle control and visual accuracy in a vehicle instrument panel.
Sources
IB Design Technology Guide (First Assessment 2027)
Pheasant, S. & Haslegrave, C. — Bodyspace: Anthropometry, Ergonomics and the Design of Work (3rd ed.)
Grandjean, E. & Kroemer, K. — Fitting the Task to the Human
ISO 9241 — Ergonomics of Human-System Interaction
Fletcher & Munson (1933) — Loudness, its definition, measurement and calculation
Harman et al. (2006) — Low Frequency Alarms and Awakenings
Linking Questions
How are user-centred research methods used to collect human factor data? (A2.1)
Which aspects of ergonomics are appropriate for user-centred design (UCD) practice? (B1.1)
How does ergonomics affect modelling and prototyping of potential design solutions? (B2.2)
How important is ergonomics to inform effective inclusive design? (C1.2)