Reaction time and movement time are two independent stages that must be measured separately

What it is

Reaction time (RT) and movement time (MT) are two sharply distinct stages within a single action that need to be measured separately: RT runs from stimulus onset to the moment the limb starts moving (not when it finishes); MT runs from that moment to the completion of the action (pressing a button, the cursor reaching the target). Lumping both into one undifferentiated "response speed" hides whether the delay is happening at the decision stage or the execution stage.

Why it happens

RT mainly reflects the central processing time needed to identify the stimulus and select a response, so its length is set by how discriminable the stimulus is, how many alternatives there are, and how complex the task rule is. MT mainly reflects the time the limb itself needs to complete a displacement, so its length is set by movement distance, target size, and required precision — a kinematic constraint rather than a decisional one. The two stages share almost no limiting factors: speeding up a decision does not make an arm move faster, and shortening a movement distance does not make someone recognize a stimulus sooner. That is exactly why the two must be measured separately rather than treated as interchangeable.

Studying it

The standard approach records three timestamps: stimulus onset, movement onset (the limb leaving its start position or the muscle beginning to fire, often captured with a microswitch or EMG), and movement completion (reaching the target, completing the press). RT is the second timestamp minus the first; MT is the third minus the second. Recording only the total interval from stimulus to completion — common in simple keypress-accuracy experiments — conflates the two stages and makes it impossible to diagnose where a delay actually originates.

Where it stops holding

Separating RT from MT depends on being able to reliably pin down the instant the action "starts." In continuous, smooth actions (dragging, handwriting), the movement onset is often fuzzy and hard to define as cleanly as a discrete keypress, so the RT/MT split becomes subjective and hard to reproduce unless a separate onset criterion is defined (e.g., the moment velocity crosses some threshold).

Applying it

When a product gets feedback that "an interaction feels slow," first confirm whether the slowdown sits in the decision stage or the execution stage before deciding what to fix: replay the screen recording frame by frame, mark the frame where the user's gaze or finger starts moving and the frame where the action completes, and compute the two durations separately. If most of the time is spent before movement begins, the culprit is elements that are hard to recognize or too many options — a decision problem, to be fixed by simplifying information or trimming choices. If the time after movement onset is what's long, the culprit is a distant control, a target that's too small, or an awkward movement path — an execution problem, to be fixed by adjusting layout and control size. How to check: retest the same participants on both designs and compare how the RT segment and the MT segment each change, confirming the fix moved the intended stage without accidentally slowing the other one.

Related

• Same group: A8.01.1 Reaction time breaks into sensory conduction, central processing, motor conduction, and muscle activation · A8.01.3 Simple reaction time: one stimulus, one response · A8.01.4 Choice reaction time: RT grows with the number of alternatives · A8.01.5 Discrimination reaction time: the stimulus category must be judged before deciding whether to respond · A8.01.6 There is a floor on reaction time; anything faster is anticipation, not a genuine reaction · A8.01.7 Individual differences in reaction time exceed what most interface optimizations can gain
• Nearby: A8.08 The two-phase structure of target acquisition · A8.09 Speed-accuracy trade-off
• Search terms: movement time · reaction time · RT-MT dissociation