A support surface constrains the available range of motion

What it is

A fixed support point improves precision for nearby movements, but it also carves out a reach envelope centered on that point, with a radius set by the remaining joints beyond it — targets outside that radius can't be reached by those joints alone, and the whole hand has to leave the surface to get there. This is an inherent trade-off of the support effect, not a flaw in a particular device's design.

Why it happens

Once the wrist or forearm is anchored on a surface, the only joints left free to swing are the ones beyond the anchor point — fingers, or small wrist rotation relative to the palm — and each has a fixed anatomical range of motion. Once a target lies beyond that range, those joints alone can no longer reach it: the user either lifts the whole arm and repositions the anchor elsewhere, or slides the anchor point itself (dragging the wrist across the desk) to extend coverage — but both momentarily give up the stability the support was providing, reverting to something closer to unsupported operation.

Where it stops holding

This constraint holds within a single, continuous operation that never leaves the support surface; if the task allows the user to lift and re-anchor repeatedly (like a mouse's clutch-and-reposition motion), the reach envelope can be stitched together across segments to cover a larger area, at the cost of a brief recalibration each time the anchor is re-set. The envelope's size also depends on which joint serves as the anchor — anchoring at the wrist covers more area than anchoring at the elbow, because more free joints remain distal to it.

Applying it

• Keep any interface region requiring fine operation within the reach radius from a typical anchor point (wrist on the desk, the edge of a laptop palm rest) to common target locations; operations beyond that radius should be expected to require the user to lift and reposition, not assume continuous coverage from a fixed anchor.
• If a stylus or tablet application's canvas exceeds the hand's natural reach envelope, provide panning/zooming rather than requiring the user to leave the support surface to draw toward the edges — otherwise line precision degrades sharply near the canvas boundary.
• To verify: log whether the anchor point (palm heel or wrist) loses contact with the device during large-canvas tasks, and whether those breaks cluster near the canvas edges — clustering indicates the canvas size already exceeds the reach envelope this support method provides.

Related

• Same group: A8.14.1 Wrist or pinky support markedly improves pointing precision · A8.14.2 Unsupported operation reduces precision and fatigues faster · A8.14.4 Mobile contexts usually lack a stable support surface
• Nearby: A8.12 Grip types · A8.08 The two-phase structure of target acquisition
• Search terms: reach envelope · support surface · pen tablet · workspace boundary