Cost is asymmetric across the body midline

What it is

The same panel parked a little left or a little right of straight ahead is not a mirror for a right-handed person. Reaching across the midline costs horizontal adduction at the shoulder and a following twist of the trunk, and the non-dominant hand crossing is dearer still. Time, error, and awkwardness on the two sides are not one bill.

Why it happens

The workspace splits at the trunk midline into ipsilateral and contralateral reaches. Same-side reaches run mainly on shoulder flexion and abduction, forearm in front of the eyes. Crossing adds horizontal adduction, often with trunk rotation; endpoint precision drops and the preparatory posture lengthens. The accurate envelope is already larger on the dominant side, so putting a frequent control on the non-dominant contralateral side is the worse hand on the harder path.

Headsets amplify it. Turning to look contralateral can walk the ipsilateral hand out of the forward tracking cone; not turning leaves the target in the periphery, where depth and hand alignment both degrade. People buy those centimetres with the trunk, so a layout that looks left–right symmetric is one shoulder and one waist in the kinematics. Left-handers are not a mirrored right-hander: they are fewer, and many still switch to the right to cross. A symmetric layout does not average across the two groups.

Studying it

Place equal targets at centre, 30–40° ipsilateral, and 30–40° contralateral. Point or grab with each hand. Log trunk yaw and shoulder horizontal adduction; completion time alone is not the effect.

Independent variables: side relative to midline, which hand, whether trunk rotation is allowed. Dependent variables: movement time, endpoint error, trunk rotation, shoulder horizontal adduction, discomfort.

Split “switch hands” from “force the cross.” Mean time mixes people who switched successfully with people who wrenched the shoulder.

Where it stops holding

Bimanual layouts that give each hand a side turn crossing into coordination rather than deleting it. A chair back that blocks trunk rotation makes the contralateral cost higher than standing. Large, low-precision targets are merely slower across the midline, not failures. Tasks that pass a tool from hand to hand treat the midline as a path, not a fence. Some people are close to ambidextrous and wash out the group asymmetry — stratify on a handedness inventory before talking left versus right.

Applying it

• Put frequent, precise direct actions in the dominant ipsilateral sector, not across the midline. Contralateral is for rare or read-only.
• Design a mirrored pair of panels for the dominant side first, then decide whether the other side loses functions or offers a hand-switch.
• Let people drag a whole interface horizontally across the midline; do not weld controls to the world's left.
• How to check: the same task once left of midline and once right. Compare time plus shoulder and trunk angles. If the far side is slower or elicits a hand-switch, the layout is not symmetric — pull the primary actions back into the dominant sector.

Related

• Same group: N2.10.1 The usable zone is the intersection of view and reach · N2.10.3 Upper bound is set by the shoulder; lower bound by the cost of bending · 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: midline crossing · contralateral reach · handedness