The comfort zone is much smaller than reachable space
Aliases: comfort much smaller than reach · max reach is not comfort · near-body work zone
What it is
The shell the arm can touch at full extension, and the volume where the hand will linger with the elbow soft and the upper arm by the torso, are not the same volume. The comfort zone is much smaller than reachable space: the outer layer is the kinematic maximum; the inner one is where the hand will repeatedly stay. Being able to touch is not being able to work there.
The two bounds are often drawn as one. A product parks buttons on the sphere of “just in reach,” thinking it has used the envelope; after a few minutes people pull the elbow back, and those buttons sit outside comfort.
Why it happens
Maximum reach is chain length: shoulder to fingertip, joints at end of travel. Comfort is a different ledger: whether the upper arm must stay abducted, whether the elbow hangs in air, whether the shoulder girdle is holding against gravity. A larger abduction angle lengthens the moment arm; the same hand position then costs an order of magnitude more to maintain. So comfort shrinks to a modest volume in front of the torso — upper arm roughly hanging, forearm near horizontal, hand somewhere ahead of chest-to-waist — while maximum reach is a thin shell far outside that volume.
People abandon the outer shell on their own. The first time they will reach it; the second they try another method; the third they move the object in. Behavioural “reach” converges on the comfort zone quickly. Layout that tiles controls on the maximum shell is tiling daily work onto a boundary where people will not stay.
Studying it
Measure maximum touch and voluntary linger separately. For the first, have people reach a target once at the limit. For the second, have them park both hands where they would work for the next minute, with no further instruction, and wait.
Independent variables: target distance and height from the shoulder, whether a 30-second hold is required, presence of a rest or desk edge. Dependent variables: touch success, spatial cloud of voluntary hand positions, shoulder abduction angle, duration of a held pose.
Overlay the two plots and comfort appears as a much smaller cloud inside the maximum envelope. Do not take a single successful touch as the working range. Human-factors practice already measures a comfortable-reach envelope (upper arm near hanging) apart from a maximum-reach envelope; folding them into one XR “reach” erases that difference.
Where it stops holding
One-shot actions — pulling a distant lever, retrieving something that fell on the boundary — may use full maximum reach; comfort’s constraint does not apply. A desktop, armrest, or belt that takes the elbow expands comfort, because gravity is handed to an external structure. Children’s chains are shorter and the gap between comfort and maximum is smaller; an adult “much smaller” ratio does not scale down as a constant. This leaf compares two nested volumes in one pose. It does not treat left–right asymmetry, nor the intersection of what is seen and what is reached. Participants told to “reach as far as you can” will also inflate the linger cloud; the measured comfort zone will look too optimistic.
Applying it
- Draw two volumes: maximum touch, and where the hand lingers with a soft elbow. Default control placement uses the inner one; the outer is for rare actions.
- Do not park menus, confirms, and tool strips on the sphere that is only hit with a straight arm.
- A single successful touch is a valid acceptance test for a far object. It is not a valid test that “work can happen here.”
- How to check: put the headset on, give no task, say “hands ready to operate.” Photograph the resting hands, then have them hit the controls you thought were in reach. Anything that cannot be reached from that rest without first locking the elbow is outside the comfort zone.