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.
Biomechanics and torque
Biomechanics
The part of physiology about forces and movement: how hard the user can grip, push, pull and twist, and how joints and muscles move while they do it.
Torque
A turning force. Opening a jar, a bottle cap or a tap is a torque task.
What it depends on
Two things: how hard the user can grip, and how far from the centre they apply the force.
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.
Comfort depends on the environment too: A workplace that is too warm raises the heart rate and makes people tire sooner, so performance falls. One that is too cold stiffens the fingers, so grip and dexterity fall. Comfort and fatigue are the physiological factors examiners ask about most.
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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".
Three ways to cut the physical load
Spread the load
Let wheels or bigger muscles take the weight. A wheelbarrow carries the soil on its wheel; a backpack hip belt moves the weight from the shoulders to the hips.
Keep the body upright
A low centre of gravity and a long handle let the user walk upright instead of bending, so the back is not strained.
Roll, do not carry
Rolling replaces lifting, so the muscles do less work and the user tires more slowly and needs fewer trips.
Some tasks need strength
A heavy emergency hatch on a plane or ferry needs strength and the right body movement (biomechanics), which is why only able passengers sit next to it.
Virtual controls
In VR or mixed reality nothing heavy is held, and controls can be scaled to the user's hand and reach: less effort, less fatigue, less risk of injury.
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.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Explain two ways in which biomechanical data could inform the design of the trigger of a garden hose spray gun.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.