A waist anchor is more stable than a shoulder anchor

What it is

The same riding panel, pinned near the clavicle versus pinned near the belt, is a different target once the hand goes to click it. A waist anchor is more stable than a shoulder anchor: the shoulder girdle is the proximal end of the reaching chain, so a reach moves the anchor first; the waist sits near the centre of mass and almost stays put, so the target is still where the hand remembered it.

Stable here is the anchor’s own motion during input, not whether the waist is more comfortable than the shoulder. Comfort is another ledger. This comparison is only: after the hand leaves, is the target still at the coordinate it had at departure.

Why it happens

Scapula and clavicle travel with the arm. Reach forward, abduct, cross the body, and the girdle protracts or elevates, carrying a shoulder-pinned panel by centimetres to more than a decimetre. The hand is flying to “the point I just saw”; the point has moved house. Waist and hip barely join that chain during a reach. Displacement from breathing and gait is also smaller than the girdle’s, and closer to periodic, which the hand can predict. A waist-anchored target is then like a device clipped to a belt: the hand reaches a location that is still there. A shoulder anchor is more like a note stuck at the root of the arm you are currently swinging.

The shoulder also splits left and right: the dominant girdle moves harder during work, the other side is quieter. The waist is one belt, both sides sharing a slower rigid body. Parking an entry on the shoulder hands stability to whichever girdle is currently working.

Studying it

Anchor the same entry at belt midline, ipsilateral shoulder, contralateral shoulder, have people do large reaches (ahead, to the side, slightly up), then come back and hit the entry.

Independent variables: anchor site, reach amplitude, whether they also walk. Dependent variables: displacement of the entry from its departure coordinate when they return to click, hit time, empty clicks.

The entry’s own displacement is the primary measure. A shoulder anchor should relocate with the reach; a waist anchor should nearly sit still. Walking tests gait: the waist will twist a little with the pelvis, usually still less than the girdle in one retrieve. If the headset’s own torso estimate is used, state where the origin actually is — many stacks put a “body” origin somewhere under the head, which is neither waist nor shoulder, and cannot stand in for this pair.

Where it stops holding

Sitting with the back locked to a chair back partly limits the girdle; a shoulder anchor then looks stabler and the gap shrinks. A rigid backpack that pins the straps does the same. A waist whose origin drifts because of a wide belt, tool hangers, or pregnancy is not automatically stabler than the shoulder. Tasks that are mostly looking down with almost no elevation move the girdle little, and the advantage fades. If torso tracking has no independent hip node and only pushes a fixed offset down from the head, the “waist” is fake, stability is head-related, and “better than shoulder” does not hold.

Applying it

• Riding entries that must be hit in the gaps of reaching work belong on the belt or hip, not on the clavicle or acromion.
• Do not implement a “body” origin as a ghost point tens of centimetres under the head and call it a waist. The waist should follow the pelvis or the belt.
• A shortcut on one shoulder is for rare actions whose hand is already near that shoulder, not for a primary entry.
• How to check: reach a distant object, then immediately hit the riding entry. Overlay two frames: entry position at peak reach, and entry position before the hand left. A waist anchor should nearly coincide; a shoulder anchor that has already walked out of the hit zone was placed on the kinematic chain.

Related

• Same group: N2.07.1 The body supplies a location reference that does not need vision · N2.07.2 Body-anchored interfaces travel with the person · N2.07.3 Choice of reference frame decides what happens after a turn · N2.07.4 The head is the worst body site for an anchor · N2.07.6 A wrist anchor brings the interface into view at the cost of raising the arm · N2.07.7 Anchor distance must scale with body size, not a fixed length
• Nearby: N3.01 Locking Modes · N3.08 World-locked, Body-locked, and Head-locked
• Search terms: waist vs shoulder anchor · hip-anchored UI · shoulder girdle motion