Intention tremor grows stronger as the hand nears the target

What it is

Intention tremor (also called terminal exaggeration of kinetic tremor) is a pathological tremor pattern in which the hand's oscillation grows larger, not smaller, the closer it gets to the endpoint of a targeted movement — the most violent shaking shows up exactly during the terminal phase that most needs a precise landing. It's common in people with cerebellar dysfunction (cerebellar ataxia, multiple sclerosis affecting cerebellar pathways). This is the direct opposite of physiological tremor in the general population: physiological tremor is a roughly constant background oscillation present at rest or during a held posture, and does not systematically grow just because the hand is approaching some target. Intention tremor, by contrast, is specific to the act of moving — and specifically to the terminal phase of that movement. These are not two severities of the same tremor; they are two entirely different temporal patterns.

Why it happens

As a pointing movement nears its target, it depends on a continuous error-correction process: the system keeps comparing current position against the target and drives it to zero with small corrective sub-movements. Normally, the cerebellum runs this correction loop by predicting the trajectory and pre-emptively damping upcoming deviations. When cerebellar function is impaired, that predictive damping fails — each corrective sub-movement tends to overshoot, the overshoot triggers a new correction, and this correct-overshoot cycle becomes more pronounced the closer the hand gets to the target (which is also when precision demands are highest). The result is oscillation amplitude that gets amplified specifically in the terminal phase, rather than being present uniformly from the start of the movement.

Studying it

Clinical and kinematic research typically uses reach-to-target tasks with motion capture recording the hand's trajectory, segmenting it in time to compare oscillation amplitude and frequency (intention tremor is usually around 3–5 Hz) between the early and late phases of the movement, with healthy controls or patients with a non-cerebellar tremor etiology as comparison groups.

Methodological note: intention tremor, postural tremor (as in essential tremor, relatively constant while holding a posture), and resting tremor (as in Parkinson's disease, prominent at rest but reduced during voluntary movement) are three tremor patterns with entirely different temporal profiles. "Does the hand shake" alone cannot distinguish them — the movement phase during which the tremor appears has to be examined, and conflating them sends compensation strategies in exactly the wrong direction.

Where it stops holding

The "gets worse as it approaches the target" pattern is a distinguishing feature of intention tremor specifically, not a property of pathological tremor in general: Parkinsonian resting tremor actually diminishes during voluntary pointing; essential tremor's amplitude during a held posture stays relatively stable and doesn't systematically grow as a target is approached. Treating every tremor etiology as "worse the closer you get" will make compensation strategies ineffective — or actively unhelpful — for people with resting or postural tremor instead.

Applying it

• If an interaction's success hinges on a precise landing during the terminal phase of a movement (a drag endpoint requiring exact alignment, an operation needing the hand to settle within a small range), recognize that this is exactly the phase where someone with intention tremor struggles most — don't design "the final step" as the sole pass/fail gate.
• An interface has no reliable way, and no need, to determine which tremor type a given user has, but it should avoid structures that demand "the end of the movement must be more precise than the movement itself," since that specifically amplifies the impact of intention tremor's worst time window.
• The specific filtering and target-enlargement compensation techniques are a separate layer of response and are not developed here.

Related

• Same group: A8.16.2 Pathological tremor amplitude far exceeds ordinary tolerance design · A8.16.3 Compensation requires both input filtering and target enlargement · A8.16.4 This population has a higher mis-tap rate, and undo beats confirmation
• Nearby: A8.15 Physiological tremor · A8.08 The two-phase structure of target acquisition
• Search terms: intention tremor · cerebellar ataxia · kinetic tremor · terminal tremor exaggeration