Tremor amplitude grows with arm extension distance

What it is

For the same person in the same fatigue and emotional state, pointing at a distant target with the arm fully extended produces noticeably more tremor amplitude at the endpoint (fingertip or handheld device) than pointing with the arm close to the body and the elbow bent. Extension itself doesn't create a new source of oscillation — the same source is amplified to a different degree depending on posture.

Why it happens

Most of tremor's source activity — muscular and neural — happens near the proximal joints (shoulder, elbow) and produces a small angular oscillation. With the arm fully extended, the lever arm from shoulder to fingertip is close to its maximum length, so the same angular oscillation, multiplied by a longer lever arm, converts into a larger linear displacement at the endpoint — a purely geometric consequence, independent of whether the tremor at the shoulder or elbow itself has changed in strength. On top of that, a fully extended posture itself requires more sustained isometric muscle effort against gravitational torque, and that postural load independently increases the proximal tremor's own amplitude — the two effects stack, amplifying endpoint jitter further still.

Studying it

A typical setup has participants hold a pointing posture still at different arm-extension angles (close to the body, half-extended, fully extended), using an accelerometer or motion capture to record displacement amplitude at the fingertip or device tip, comparing readings across angles. This kind of study often shares equipment with fatigue research on unsupported operation.

Methodological note: recording endpoint displacement alone cannot separate "the proximal tremor itself got bigger" from "the same proximal tremor got amplified by a longer lever arm" — both the proximal joint's own angular oscillation and the endpoint displacement need to be recorded to tell the two apart.

Where it stops holding

This relationship is clearest when a task genuinely requires actively extending the arm to reach a distant target; if the operation lets the body move closer to shorten the reach, or if the endpoint itself has extra support (say, the extended arm is holding a heavier, damped device), the observed amplification will be smaller than the pure geometric calculation would suggest.

Applying it

• Interactions requiring a large arm extension to reach a target (large displays, the far zone of an in-air gesture) should use a wider tolerance than close-range targets, because the endpoint jitter is genuinely larger there — not because the user is being less careful.
• If a hover-confirmation or aim-based interaction supports both near and far triggering distances, don't cover both with the same fixed tolerance radius — scale it up with the degree of arm extension involved.
• To verify: have users hold still while hovering over both a near and a far target, compare the spread of landing points at each distance, and check whether the current tolerance was only ever calibrated against the near-distance case.

Related

• Same group: A8.15.1 An unremovable baseline jitter with a characteristic frequency exists even at rest · A8.15.2 Fatigue, cold, stress, and caffeine all amplify tremor · A8.15.4 Hover and dwell interactions must set a tolerance radius · A8.15.5 Input smoothing introduces latency, conflicting with directness
• Nearby: A8.14 Hand support and precision · A8.19 Fatigue from sustained arm elevation
• Search terms: tremor amplitude · lever arm effect · arm extension · mid-air pointing accuracy