The initial impulse phase covers most of the distance quickly

What it is

A single pointing movement — moving a finger to a button, moving a cursor to an icon — doesn't cover its whole path at a constant pace when you look at the full trajectory; it clearly splits into two segments. First comes a high-speed primary movement (the initial impulse), which in a short burst of time covers nearly all the distance from start to target; then speed drops sharply, entering a slower, choppier tail-end process. The initial impulse phase is essentially one ballistic burst of force fired off open-loop — its job is to rapidly shrink the gap from "far away" to "close," not to land precisely on the target.

Why it happens

This fast, coarse impulse comes first because precise aiming isn't actually useful while the target is still far away — even a slight deviation at that point leaves plenty of distance remaining, so there's no reason to slow down and painstakingly check position over a small deviation that far out. The more sensible strategy is to fire off one high-speed ballistic movement, set by the direction and force decided before launch, to clear most of the distance as quickly as possible, saving attention and time budget for the phase where getting close to the target actually starts to matter for precision. This impulse's velocity profile typically forms a single bell shape: accelerating to a peak, then decelerating, with a clear slowdown only once it nears the target — the timing of that peak velocity roughly marks the boundary between the impulse phase and what follows.

Studying it

The basic way to study this phase uses high-speed cameras or motion-capture equipment to record the full displacement-time and velocity-time curves of a pointing movement, identifying the first prominent accelerate-decelerate bell segment and marking it as the primary phase — then measuring what fraction of total distance it covers, when peak velocity occurs, and how much distance error remains once this phase ends. Comparing this phase's relative share across different total distances and target positions confirms whether this fast displacement reliably covers most of the path, or shifts systematically with distance.

Where it stops holding

This phase is identified from a distinguishing feature in the velocity curve (the first peak and the deceleration that follows), and in real data that boundary isn't always clean — for some individuals or under some task conditions, the velocity curve transitions smoothly between the impulse phase and what follows, making it hard to carve out a sharp cutoff; in that case, "covers most of the distance" is better read as a statistical tendency rather than a rule that holds strictly for every single movement. How this phase is set up — how fast, how precise — is also jointly shaped by target size and distance, though the specific tradeoff between them falls outside this phase's own mechanism.

Related

• Same group: A8.08.2 The correction phase closes in on the target with small submovements · A8.08.3 The number of submovements determines total movement time · A8.08.4 Smaller targets require more submovements · A8.08.5 Undershoot and overshoot of the initial impulse carry different costs
• Nearby: A8.06 Open-Loop Control · A8.09 Speed-Accuracy Tradeoff
• Search terms: primary movement · initial impulse · movement kinematics