Upper bound is set by the shoulder; lower bound by the cost of bending

What it is

A menu hung above the brow hits the end of shoulder travel first; widgets laid at the feet require a fold at the waist first. Usable height is not arm length. The ceiling is where the shoulder can still lift; the floor is how much stoop cost a person will pay for that one action. Being able to reach and being willing to reach split on the vertical.

Why it happens

Once glenohumeral elevation carries the upper arm past about shoulder height, leftover travel is forced through upward rotation and a shrug. Above the head the elbow must rise and the hand's stable cone narrows fast — not because the arm is short, because the joint is near end range. That is a kinematic cap, a different layer from “holding it up gets tiring.”

Downward, arm length is usually still there. Hip and lumbar flexion shift the headset mass forward and move balance onto the forefoot. One stoop costs more metabolically, and in stability, than one reach at the chest. The lower bound is therefore rarely the envelope of fingers on the floor; it is whether the widget is worth a bend. Objects near the ground are not unreachable; they levy a stoop tax. When the tax is high, people switch to a foot, a ray, or abandonment.

Studying it

Spread the same point or grab along the vertical: below the knee, waist, chest, eye, above the head. Use motion capture or the headset's own trunk estimate to log shoulder flexion or abduction and trunk inclination, not comfort ratings alone.

Independent variables: target height relative to shoulder and eye, whether hip flexion is allowed, headset mass. Dependent variables: completion time, shoulder angle, trunk flexion, abandonment, balance adjustments (whether the feet move).

Report “could finish” separately from “finished with the trunk already past thirty degrees.” The second means the lower bound has already been crossed by stoop tax, even on a successful trial.

Where it stops holding

Sitting raises the lower bound: the seat blocks hip flexion, below-knee targets often become unreachable, and the upper bound drops a little because the pelvis is fixed and the trunk cannot extend. In a space where a squat is possible, the lower bound can be bought with the knees instead of the waist. Restricted shoulder range, or a prior shoulder surgery, brings the upper bound down early — do not lay out to a young-adult shoulder height. A low physical ceiling cuts the virtual envelope before the joint does.

Applying it

• Put repeated controls on the band from about the sternum to a little below the line of sight. Not over the shoulder, not below the waist.
• Reserve above-head and floor for rare actions that can be reached with a ray or by travelling.
• Place by proportion to shoulder or eye, not by centimetres off the floor.
• How to check: the same task at brow, chest, and below the knee. Log trunk inclination and shoulder angle. If the brow hits the shoulder first and the knee hits the waist first, the primary actions are outside what those joints will pay — pull them back onto the chest band.

Related

• Same group: N2.10.1 The usable zone is the intersection of view and reach · N2.10.2 Cost is asymmetric across the body midline · N2.10.4 Reach travels with the person; a world-fixed panel can be walked out of
• Nearby: N2.06 Reach Space · N3.01 Locking Modes
• Search terms: vertical reach envelope · stoop cost · shoulder reach