The big idea: Physiology is the study of the body's systems and biomechanics — their responses, limitations and capabilities.
For a designer it answers one question: can the user actually do what this product asks, repeatedly, without harm?
Anthropometrics tells you how big the user is. Physiology tells you what they can do — how hard they can squeeze, how small a detail they can resolve, how faint a sound they can hear.
Five limits, with a sustainable zone and an absolute ceiling. Step through to see what happens when a design sits near the ceiling.
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Six capabilities to know
Visual accuracy
The smallest detail a user can resolve. Sets text size, icon size and viewing distance — and it falls with age, which is why small print excludes the users most likely to need the information.
Colour perception
Roughly 1 in 12 men has reduced red–green discrimination. A status light that is red for fault and green for ready tells them nothing — so colour must never be the only channel.
Strength
The force a user can apply — grip, push, pull, turn. Sized from the weaker user, which is why jar lids and medicine caps are a recurring design failure.
Fatigue
Capability falls over a shift. A demand that was comfortable at 9am can be beyond the user by mid-afternoon, so a designer targets the sustainable zone, not the ceiling.
Muscle control
How precisely a movement can be made. Sets how small and how close together controls can be, and how much travel a dial needs to be set accurately.
Hearing thresholds
The quietest sound a user can detect — and it has to be detected above the background noise of the place the product is used, not in a quiet lab.
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A demand can sit in one of three places, and only one of them is a good design.
Inside the sustainable zone
- The user can do it all day
- No fatigue accumulates
- This is the target
Near the ceiling
- Works once, for a rested user
- Passes a quick showroom test
- Fails in real use as fatigue lowers the ceiling
Beyond the ceiling
- The user simply cannot
- Text below resolvable size; a colour-only code
- Usually fixed cheaply — bigger type, a second channel
The one that passes testing and fails in life: A jar lid needing almost all of a typical grip opens fine when a rested tester tries it once. It is the fatigue case — and it is why "we tested it" is not the same as "it works".
How this is tested — explaining how a physiological limit constrains a design decision. It comes up two ways:
Paper 1 — multiple choice
- Identify the limit a described failure is caused by.
- Pick the change that brings a demand inside a limit.
Paper 2 — analysing a product
- Explain limiting aspects of user capability for a named product.
- Suggest changes that bring a product within its users' capabilities.
The trap: Naming the limit and stopping. "Some users are colour blind" is half a mark; "so the fault state must also differ in shape or position" is the rest of it.
A home boiler shows its status with a single small light: green for running, red for fault. Explain two physiological limitations this design ignores, and give one change for each.
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