By the end of this topic, you should be able to...
explain the reasons why designers choose adjustability and/or range of sizes for a product, and identify products that use one or both strategies.
Guiding Question
How do ergonomic considerations influence the design of a product?
Did you know?
In 1999, the Swedish furniture retailer IKEA introduced the JULES office chair — a fixed-height, non-adjustable swivel chair aimed at the budget domestic market. It was cheap, cheerful, and sold in enormous volumes. It was also biomechanically inappropriate for approximately 80% of its users. A fixed seat height of 450mm — close to the 50th percentile adult popliteal height — was simultaneously too high for shorter users (feet not reaching the floor, causing circulation restriction and lumbar strain) and too low for taller users (knees above hip level, creating anterior pelvic tilt and lower back compression) (Pheasant & Haslegrave, 2006). The chair did not fail structurally. It did not violate any safety standard. But it failed ergonomically for the majority of the population it was sold to — because the designer made no adjustability provision and no range of sizes was offered. This is the fundamental problem that adjustability and range of sizes exist to solve. Both strategies are responses to the same reality: the human population is dimensionally diverse, and no single fixed dimension can optimally serve that diversity.
Anthropometric Diversity
Anthropometrics — the aspect of ergonomics that deals with body measurements (IB DT Glossary, 2024) — reveals that human bodies vary considerably across every measurable dimension. For a given demographic (gender, race, age), body measurements follow a normal distribution, with the 50th percentile representing the median value. But the spread between the 5th and 95th percentile across any single dimension is significant — often 200mm or more for linear measurements such as stature or arm length (Pheasant & Haslegrave, 2006).
A product designed to a single fixed dimension will:
Optimally serve only the narrow band of users whose body dimension happens to match that fixed value
Adequately serve users whose dimensions fall close to that fixed value
Inadequately serve — or actively harm — users whose dimensions fall far from that fixed value
The percentile range (upper and lower limits) defines the proportion of a population whose dimensions fall at or below a given value (IB DT Glossary, 2024). The designer's task is to decide what proportion of the population the product should serve — and then provide the mechanism (adjustability or range of sizes) to achieve that coverage.
Adjustability
Adjustability is the ability of a product to be changed in size, commonly used to increase the range of percentiles for which a product is appropriate (IB DT Glossary, 2024).
An adjustable product contains a built-in mechanism that allows the user — or an operator — to modify one or more critical dimensions to match their individual body measurements or preferences.
The adjustment may be:
Continuous — infinitely variable within a range (e.g., a gas-lift office chair seat height)
Incremental — adjustable in discrete steps (e.g., a bicycle saddle post with pin-and-hole positions)
Tool-free — operable by the user without specialist equipment (e.g., a car seat sliding rail)
Tool-assisted — requiring equipment to set (e.g., a workshop vice jaw width)
Why Designers Choose Adjustability
There are seven primary reasons a designer will specify adjustability rather than a fixed dimension or a range of sizes:
1: The Product Must Serve a Wide Percentile Range With Optimal Fit for Each User
Some products require a precise match between the product dimension and the individual user's body measurement — not merely an approximate fit. Where the population spans a wide anthropometric range, and where each user within that range requires an individually optimised fit for the product to function safely or effectively, adjustability is the only viable strategy.
A fixed size — even the 50th percentile — will produce a suboptimal fit for the majority of users. A range of discrete sizes approximates individual optimisation but cannot achieve it precisely between size boundaries.
Example: An adjustable office chair seat height must match each individual user's popliteal height (the measurement from floor to the back of the knee in a seated position). The correct seat height allows both feet to rest flat on the floor while the thigh is horizontal and the knee is at approximately 90°.
The popliteal height of adult workers in a typical UK office spans from approximately 355mm (5th percentile female) to 480mm (95th percentile male) — a 125mm range (Pheasant & Haslegrave, 2006). No single fixed height achieves correct posture across this range. Adjustability allows every user within the range to achieve the correct ergonomic position.
2: The Product Is Shared Between Multiple Different Users
When a product is used sequentially by multiple different people — each with different body dimensions — it must be capable of being reset to a different configuration for each user. A fixed dimension optimised for one user will be incorrect for all subsequent users. Only adjustability enables each user to reconfigure the product to their own dimensional requirements.
Example: A car seat in a shared or rental vehicle is used by drivers of widely varying stature, leg length, and torso length. The seat must be adjustable in fore-aft position (to match leg length to pedal distance), height (to achieve correct sightline over the dashboard), and recline angle (to support the correct torso-to-thigh angle) so that each driver can achieve a safe and ergonomically correct driving position. A fixed position safe for one driver may render the controls unreachable or the sightline inadequate for another.
3: Incorrect Fit Creates a Safety or Health Risk (Musculoskeletal Disorders)
Where a mismatch between product dimension and user body measurement does not merely create discomfort but actively causes physiological harm over time — particularly musculoskeletal disorders (MSDs) — adjustability is a safety imperative.
Physiology factors concern human factor data related to physical characteristics used to optimise the user's safety, health, comfort and performance (IB DT Glossary, 2024).
MSDs — including repetitive strain injury, lower back pain, carpal tunnel syndrome, and neck strain — are among the most prevalent occupational health conditions globally (HSE, 2023). They are strongly associated with sustained incorrect posture, which is caused by a mismatch between workstation dimensions and the user's body measurements.
Example: A fixed-height keyboard tray set too high forces the user into elevated shoulder posture — the trapezius and deltoid muscles sustain static load to hold the arms raised. Over hours of daily computer use, this creates chronic shoulder and neck pain. An adjustable keyboard tray that descends to elbow height for each individual user eliminates this static load. The adjustability is not a convenience feature — it is a health protection measure.
4: Performance Optimisation Requires Individual Fit
In high-performance contexts — competitive sport, precision work, military or aviation equipment — the difference between an optimal fit and a merely adequate fit measurably affects performance, fatigue, and error rate. Adjustability allows each individual user to tune the product to their specific body dimensions for optimal performance.
Example: A competitive road bicycle requires adjustment of: saddle height (to maximise leg extension at the bottom of the pedal stroke without full knee lock), saddle fore-aft position (to position the knee correctly over the pedal axle), handlebar height (to achieve the desired aerodynamic position without placing excess load on the hands and wrists), and stem length (to achieve appropriate reach without shoulder overextension). Each of these dimensions is determined by the rider's individual leg length, torso length, and arm length. No range of fixed frame sizes, without adjustable components, can achieve correct biomechanical position for all riders across all these dimensions simultaneously.
5: A Range of Sizes Would Require Too Many Discrete Variants
In some product categories, the number of discrete sizes required to adequately cover the 5th–95th percentile range would be economically or logistically impractical. Where a continuous or near-continuous adjustment mechanism is available and cost-effective, it provides superior population coverage with a single product variant.
Example: A hospital patient bed must accommodate patients ranging from paediatric (as small as 5th percentile child dimensions) to bariatric adult patients. The critical dimensions — bed height (for staff safe handling), backrest angle, leg elevation, and side rail height — must all be adjustable. Manufacturing a range of discrete-height fixed beds sufficient to cover this population would require multiple product variants, significant storage complexity, and still produce less precise individual fit than a continuously adjustable bed. A hydraulic or electric multi-position adjustable bed serves the full population with a single product variant.
6: The Workspace Envelope Requires Configuration
In complex workstations, the workspace envelope — the 3D space within which the user must operate — must be configured to match the specific user's reach envelope (the 3D space within which the user can comfortably and safely operate without overreach or underreach). Since reach envelopes differ between users of different body dimensions, the workstation must be adjustable to match each user's specific envelope.
Example: A laparoscopic surgical workstation (robotic surgery console) must be configured so that the surgeon's hand controls are positioned within their comfortable reach envelope — neither requiring overreach, which creates shoulder and wrist strain, nor requiring the arms to be excessively contracted, which reduces control precision. The console is adjustable in multiple dimensions to match each surgeon's seated height, arm length, and hand position preferences. In a safety-critical and precision-critical surgical environment, this adjustability directly affects both the surgeon's physical wellbeing and patient safety outcomes.
7: Regulatory and Standards Requirements
Many product categories are subject to ergonomic standards or legal requirements that mandate a specified range of adjustability. In these cases, adjustability is not a design choice — it is a compliance requirement.
Example: EU Directive 90/270/EEC on display screen equipment requires that workstation chairs used with display screen equipment must be adjustable in seat height, and where adjustable seat height alone cannot ensure proper posture, seat tilt and backrest height must also be adjustable (EU, 1990). UK equivalent: Health and Safety (Display Screen Equipment) Regulations 1992. A fixed-height chair does not comply with these regulations in a workplace setting regardless of its ergonomic quality at its fixed height.
Range of Sizes
A range of sizes is a selection of sizes a product is made in that caters for the majority of a market (IB DT Glossary, 2024). A range of sizes strategy accepts that a single product variant cannot serve the full population and instead creates multiple discrete variants — each optimised for a different portion of the anthropometric distribution — that collectively cover the intended population range.
Reasons Why Designers Choose a Range of Sizes
There are six primary reasons a designer will specify a range of sizes rather than adjustability:
1: Adjustability Would Compromise Structural Integrity or Performance
In products where material continuity, stiffness, or structural integrity is essential to function and safety, the introduction of adjustable joints, mechanisms, or attachment points creates weak points, flex, or potential failure modes that compromise the product's primary function.
Example: A cycling helmet must absorb and distribute impact energy in a collision. This requires the EPS (expanded polystyrene) foam liner to be a single continuous moulding that conforms precisely to the head. An adjustable foam liner — with movable sections or gaps — cannot provide consistent, predictable energy absorption. A range of shell sizes (XS/S/M/L/XL), each covering a head circumference range, provides appropriate fit without compromising impact protection integrity.
2: The Product Is Worn or Carried on the Body — Fit Is Predetermined
Many products must fit the body correctly before use begins and cannot be adjusted during use. The user must select the correct size in advance. In these cases, a range of sizes is the appropriate strategy — adjustability at point of purchase is irrelevant if the product cannot be adjusted during use.
Example: Footwear cannot be meaningfully adjusted during use to accommodate different foot lengths or widths. A range of sizes — expressed in standardised numerical systems (UK, EU, US) corresponding to measured foot length in 6.67mm increments — allows each user to select the discrete variant closest to their foot measurement. Adjustability within a shoe is limited to lacing width (which addresses girth but not length) — the primary sizing strategy is a discrete range.
3: Adjustment Mechanisms Would Unacceptably Increase Cost, Weight, or Complexity
In mass-market consumer products, the cost of engineering an adjustment mechanism — and the tooling, assembly, and quality control costs associated with that mechanism — may be disproportionate to the benefit, particularly when a range of sizes can provide adequate fit at lower cost and complexity.
Example: Work gloves in PPE (personal protective equipment) applications are manufactured in sizes XS/S/M/L/XL based on hand breadth and hand length measurements. An adjustable glove mechanism robust enough to withstand the mechanical, thermal, or chemical exposure of industrial PPE use would be costly, potentially fragile, and would introduce failure modes in a safety-critical product. A range of five sizes covers the majority of the working adult population adequately at low cost.
4: Aesthetic or Formal Integrity Would Be Compromised by Visible Mechanisms
In product categories where appearance is a primary purchase driver — fashion, luxury goods, consumer electronics — visible adjustment mechanisms can conflict with the intended aesthetic. A range of sizes preserves formal integrity while achieving appropriate fit.
Example: Wristwatches and jewellery are products where aesthetic continuity is paramount. A watch with an external adjustment mechanism for case diameter or strap width would be aesthetically unacceptable in most market segments. Instead, watches are produced in a range of case diameters (typically 36mm, 40mm, 42mm, 44mm) targeting different wrist circumferences, with strap adjustment provided through a standardised pin-and-buckle system that is integrated into the strap design rather than appearing as a mechanism on the case.
5: The Product Serves a Population With Discrete, Well-Defined Size Categories
Some user populations have documented and well-characterised size categories that align naturally with discrete product variants. Where the anthropometric data supports clearly defined size boundaries — and where users are accustomed to selecting from those categories — a range of sizes is both the conventional and practically effective strategy.
Example: Children's furniture — chairs, desks, and storage — is designed around age-banded anthropometric categories (typically 3–5 years, 6–8 years, 9–11 years) based on age-stratified static data for seated height, popliteal height, and shoulder height. The rapid growth of children within each age band is accommodated within each size variant's dimensional tolerance. Adjustability would add cost and mechanical complexity to products used in environments — primary schools, nurseries — where robustness and low maintenance are priorities.
6: Multiple Independent Dimensions Make Comprehensive Adjustability Impractical
Some products require correct fit across multiple independent dimensions simultaneously. Where those dimensions are not correlated — meaning a user who is large on one dimension is not necessarily large on another — the number of adjustment mechanisms required to address all combinations would make the product impractically complex or costly. A range of sizes that addresses the primary dimensions through variant selection, with limited adjustability for secondary dimensions, is more practical.
Example: Clothing involves at minimum three independent dimensions: torso length (height-correlated), chest/bust circumference, and waist circumference. In men's formal shirts, these are addressed through: a range of collar sizes (neck circumference), a range of sleeve lengths (arm length), and a range of chest sizes — with some adjustability provided by garment cut (slim fit, regular fit, relaxed fit). Full adjustability of all dimensions within a single garment is not achievable. The conventional clothing sizing system provides a range of variants across the primary dimensions with tailoring as the individual optimisation solution.
Products Using Adjustability
Product | Adjustable Dimension(s) | Why Adjustability Rather Than Range of Sizes |
|---|---|---|
Office chair | Seat height, armrest height, backrest angle, lumbar support position | Shared product; individual ergonomic optimisation required; MSDs result from incorrect fit; regulations mandate adjustability |
Car driving seat | Fore-aft position, height, backrest recline, headrest height | Shared product; multiple drivers; safety-critical pedal reach and sightline require precise individual fit |
Bicycle — saddle and handlebar | Saddle height, saddle fore-aft, handlebar height and reach | Performance optimisation requires individual fit across multiple uncorrelated dimensions |
Adjustable walking stick/crutch | Handle height | Medical aid for users across the full stature range; individual optimisation required for correct biomechanical load transfer |
Camera tripod | Leg extension height, head tilt, head pan | Shared equipment used across varied terrain and camera heights; continuous adjustment required |
Hospital bed | Bed height, backrest angle, leg elevation | Patient safety handling requirements mandate height adjustment; therapeutic requirements mandate positional adjustment |
Sit-stand workstation desk | Work surface height | Alternating seated and standing use requires height range from seated elbow height to standing elbow height — a 400mm+ range |
Rifle stock (military) | Stock length-of-pull | Safety-critical weapon control requires trigger reach appropriate to individual shooter's arm length; shared between soldiers of varied stature |
Prescription eyeglasses | Nose pad position, temple arm length | Individualised fit to facial geometry required for optical alignment and comfort |
Scaffolding/adjustable leg | Working platform height | Site-specific terrain variation requires continuous height adjustment |
Products Using a Range of Sizes
Product | Sizes Offered | Why Range of Sizes Rather Than Adjustability |
Footwear | Numerical size ranges (UK 3–13, EU 36–47) in full or half sizes | Foot length cannot be adjusted during use; structural integrity requires single-moulded last |
Cycling helmet | XS/S/M/L (by head circumference) ± internal retention ring | Impact-critical EPS liner must be continuous moulding; cannot be adjustable without compromising protection |
Clothing | Numerical or S/M/L/XL banding across multiple dimension variants | Too many independent dimensions for comprehensive adjustability; garment construction constraints |
Work gloves (PPE) | XS/S/M/L/XL by hand breadth | Safety-critical PPE; adjustment mechanism would introduce failure modes in chemically or mechanically hostile environments |
Children's classroom furniture | Age/height-banded sets (e.g. BS EN 1729 Size 2/3/4/5/6) | Robust, low-maintenance environment; cost constraint; discrete age bands align with anthropometric growth data |
Bicycle frames | Frame sizes by seat tube length (e.g., 50cm/54cm/58cm) | Structural frame triangles cannot be made adjustable without compromising rigidity and fatigue life |
Surgical gloves | Half-size increments (5.5/6/6.5/7/7.5/8/8.5/9) | Sterile field — adjustment mechanisms impossible; precise tactile fit critical for surgical dexterity |
Hard hats (PPE) | Typically one or two shell sizes + internal headband adjustment | Shell protects by distributing impact; shell must be structurally continuous; internal band provides secondary fit adjustment |
Wedding/engagement rings | Discrete ring sizes (UK: A–Z+, US: 3–13) | Precious metal construction; aesthetic integrity; sizing via professional measurement and made-to-size manufacture |
Ski/snowboard boots | Shell size range + inner liner moulding | Shell structural stiffness for lateral control cannot be compromised by adjustment mechanisms; liner provides secondary fit |
Products Using Both Adjustability and a Range of Sizes
The most sophisticated ergonomic products employ both strategies simultaneously — offering a range of sizes to address the primary dimensional variation, combined with adjustability to allow individual optimisation within each size variant.
Product | Range of Sizes Strategy | Adjustability Strategy |
Ski boots | Shell sizes by foot length (e.g., mondo size 22–33) | Buckle tension, power strap, custom heat-moulded liner, canting adjustment |
Climbing harness | S/M/L/XL leg loops and waist belt variants | Buckle adjustment of waist belt and leg loops for precise individual fit under load |
Running shoes | Numerical sizes by foot length; width variants (narrow/standard/wide) | Lacing system adjusts forefoot girth and tongue pressure |
Automobile (as product system) | Range of vehicle body sizes (city car → full-size SUV) targeting different stature ranges | Adjustable seat, steering column, mirrors, and headrest within each vehicle |
Military body armour | S/M/L/XL shell sizes | Shoulder strap length, side panel width, cummerbund length adjustable within each shell size |
Medical examination table | Adult and paediatric variants | Height adjustment, backrest angle, leg section angle — all continuously adjustable |
Office chair (professional grade) | Seat width and depth variants (standard and large) | Seat height, armrest, lumbar support, tilt tension, headrest — all adjustable |
Case Study
IKEA JÄRVFJÄLLET Chair
In 2018, IKEA released the JÄRVFJÄLLET chair — an ergonomic office chair with seat height adjustment (37–49cm), adjustable lumbar support, and adjustable armrests, retailing at approximately £215 (IKEA, 2018). This represented IKEA's direct response to the ergonomic failure of fixed-height budget seating (exemplified by the JULES chair).
Why adjustability was commercially justified at this price point
The sit-stand office market had grown significantly through the 2010s, driven by occupational health research linking fixed-height furniture to MSDs. IKEA's market research identified that consumers were increasingly aware of the health consequences of non-adjustable seating.
The cost of gas-lift seat height adjustment — a mature, commodity mechanism — had reduced to the point where it added only £8–15 to manufactured cost at volume (industry estimate). The commercial case for adjustability was made not only on ergonomic grounds but on market capture grounds: a chair that serves the 5th–95th percentile range captures a significantly larger market than a chair that serves only users close to a fixed 50th percentile dimension.
The JÄRVFJÄLLET does not offer size variants — it uses a single seat pan size with a moderate adjustment range. This is a commercially practical compromise: a single SKU (stock-keeping unit) is easier and cheaper to manufacture, warehouse, and ship than three size variants. It accepts that users at the extreme ends of the seat pan dimension distribution are not optimally served, in exchange for simplicity and cost efficiency.
This case illustrates that adjustability and range of sizes decisions are never purely ergonomic — they are always negotiated against commercial, manufacturing, and logistical constraints. The designer's task is to maximise population coverage within those constraints.
Quick Summary
Strategy | What It Does | Ideal When | Limitation |
|---|---|---|---|
Adjustability | Allows user to modify a dimension to their individual measurement | Shared products; safety-critical fit; performance-critical fit; MSDs risk; regulatory requirement | Adds cost, weight, complexity; may compromise structural integrity; adjustment mechanism can fail |
Range of sizes | Multiple discrete variants each targeting a portion of the population | Structural integrity required; worn products; cost constraint; aesthetic integrity; product not adjustable during use | Cannot achieve individual precision fit between size boundaries; requires user to select correct size; inventory complexity |
Both combined | Range of sizes addresses primary structural dimension; adjustability addresses secondary dimensions within each size variant | High-performance products; products with multiple independent dimensions; professional/medical/safety equipment | Maximum cost and complexity; requires user education on both size selection and adjustment |
Key Vocabulary
Term | Definition |
|---|---|
Adjustability | The ability of a product to be changed in size, commonly used to increase the range of percentiles for which a product is appropriate |
Range of sizes | A selection of sizes a product is made in that caters for the majority of a market |
Anthropometrics | The aspect of ergonomics that deals with body measurements |
Percentile | A term that describes how a data point compares to all data in that set, divided into 100 equal parts |
Percentile range (upper and lower limits) | That proportion of a population with a dimension at or less than a given value; the 50th percentile is the median for a given demographic |
Physiology factors | Human factor data related to physical characteristics used to optimise the user's safety, health, comfort and performance |
Static data | Human body measurements when the subject is still |
Dynamic data | Human body measurements taken when the subject is in motion |
Clearance | The physical space between two objects |
Reach | The range that a person can stretch to touch or grasp an object from a specified position |
Biomechanics | Research and analysis of the mechanics of the human body (muscles, joints, tendons) |
Workspace envelope | A 3D space that is typically physical and/or virtual with defined permissible boundaries of movement and operation |
Ergonomics | The application of scientific information concerning the relationship between human beings and the design of products, systems and environments |
Practice Questions
Question 1
Explain two reasons why a designer would choose adjustability rather than a range of sizes for an office chair used in an open-plan workplace. [4]
Question 2
Explain why a cycling helmet manufacturer uses a range of sizes rather than an adjustable mechanism to achieve correct fit. [3]
Sources
IB Design Technology Guide (First Assessment 2027)
Pheasant, Stephen, and Christine Haslegrave. Bodyspace: Anthropometry, Ergonomics and the Design of Work. 3rd ed., CRC Press / Taylor & Francis, 2006.
Stumpf, Bill, and Don Chadwick. Aeron Design Story. Herman Miller Inc., 1994. www.hermanmiller.com/research/research-summaries/aeron-chair-design-story/
Health and Safety Executive (HSE). Work-Related Musculoskeletal Disorders: Statistics in Great Britain 2023. HSE, 2023, www.hse.gov.uk/statistics/causdis/musculoskeletal.
European Union. Council Directive 90/270/EEC on the Minimum Safety and Health Requirements for Work with Display Screen Equipment. Official Journal of the European Communities, 1990.
IKEA. JÄRVFJÄLLET Swivel Chair Product Specification. IKEA, 2018, www.ikea.com.
Kroemer, Karl, and Elbert Grandjean. Fitting the Task to the Human: A Textbook of Occupational Ergonomics. 5th ed., CRC Press, 1997. (Chapter 7 provides a comprehensive treatment of workstation adjustability and sizing strategies.)
Pheasant & Haslegrave (2006) — as above. (Chapters 4–6 provide the anthropometric data tables most directly applicable to adjustability range specification.)
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)