Thumb reach is an arc swept around the carpometacarpal joint, placed on-screen by grip

What it is

Thumb reach is not a fixed circle on screen but an arc swept around the carpometacarpal joint. How a device is supported, which side grips it, and whether it is one-handed place that arc differently on the display; a comfortable button in one posture may require regrip or two hands in another.

Why it happens

Thumb length, joint range, wrist rotation, and device position relative to palm jointly form reach, and the region is an arc rather than a circle precisely because two independent groups of joint motion stack on top of each other. One group is the thumb's own joints flexing and extending, which only moves the fingertip within a fairly small radius. The other is wrist rotation at the carpometacarpal joint, which relocates the entire arc's own origin to a different spot on the screen. Whatever the first group alone can cover is usually called the comfort zone — no regrip needed, stability barely affected. The moment a target sits outside that zone, the second group, wrist rotation, has to be added, sometimes along with loosening the grip, to make up the distance. Reachability does not disappear at that point, but stability, occlusion angle, and false-touch risk all worsen at the same time. This is why "comfortably reachable" and "reachable with strain" are not shades of depth along the same arc — they correspond to two qualitatively different motor strategies: using one joint group, or having to bring in both.

Studying it

Across one- and two-handed use, handedness, sitting, and walking, record reach time, regrip, fatigue, error, and abandonment. Separate comfortable, reachable-but-straining, and unreachable areas rather than asking people only to stretch once to maximum. Concretely, record whether an observable change in wrist angle occurs when a target is reached — targets reached with essentially no wrist rotation belong to the comfort zone, and ones requiring a visible wrist turn belong to the straining zone; this joint-motion-based split explains poor experience with some seemingly-not-that-far targets far better than raw distance alone.

Where it stops holding

The arc is not a universal heatmap: hand size, handedness, device size, cases, and strategies vary. It also does not require every function in the comfort zone; low-frequency work with a second hand can sit outside, but frequent key actions should not require strain.

Applying it

• Place frequent primary actions in common one-handed comfort arcs and allow placement customization.
• Give distant functions bottom access, gestures, keyboard, or two-handed alternatives rather than shrinking controls.
• Test action zones across grips and device sizes; do not treat one heatmap as universal.

Related

• Same group: C2.11.2 Screen diagonal regions are the hardest to reach · C2.11.3 When screens get taller, action zones should move down rather than content merely stretching · C2.11.4 Reach zones vary with hand size, handedness, and grip
• Nearby: C2.02 Minimum touch target size · C2.05 Finger occlusion
• Search terms: thumb reach · grip · reachability