Movement complexity increases the preparation time needed to program it

What it is

Between receiving a "go" signal and actually starting to move, there's a delay — and it isn't fixed. It grows with how many parts the upcoming movement has and how much timing and force control each part needs. A single simple tap and a continuous stroke that turns, speeds up, and slows down can both receive the go signal at the same instant, yet the latter needs more preparation time before it actually starts. This is the complexity effect: what grows isn't the execution of the movement itself, but the time spent packaging its parameters into a complete instruction.

Why it happens

Preparation time grows with complexity because the number of parameters that must be set all at once grows. A movement with one direction, one fixed force level, and a single start-stop needs little packaging; once a movement becomes a multi-segment sequence with direction changes and force-level switches, the packaging step has to fix the relative timing and magnitude of more variables simultaneously — and all of it has to be settled before the movement actually starts, since nothing gets adjusted on the fly once it launches. That packaging work itself takes time, and it scales with the number of components that must be coordinated rather than with the movement's own travel distance or speed — which is why a simple and a complex movement covering a similar path length can still show very different waiting times before launch.

Studying it

The classic paradigm controls how many components the upcoming movement sequence contains — the number of independent segments, or how many times direction or force must switch — while holding the stimulus constant, and measures only the reaction time from signal onset to the limb's first movement. If reaction time rises monotonically with the number of components while the execution time after movement onset stays relatively stable, the delay sits in the programming stage before movement, not in a slower execution phase. Such studies typically log reaction time and movement time separately so the two different kinds of delay aren't collapsed into one number.

Where it stops holding

The complexity effect describes sequences that haven't been drilled and have to be packaged on the spot; once a complex sequence has been rehearsed enough to be treated as a single integrated unit, packaging cost drops sharply and the effect weakens or disappears — what used to be "complex" is now handled at preparation time as one simple unit. The size of the effect also depends on how independent the components are: if the segments are highly similar and can share the same parameter set, the extra preparation time from added complexity is much smaller than when the segments are entirely distinct.

Applying it

• For operations demanding "as fast as possible" — an emergency stop, a hazard confirmation — design the triggering action itself as a single, simple movement (one tap rather than a multi-finger gesture or a multi-segment swipe), cutting the pre-launch packaging burden instead of letting a complex trigger itself slow the response.
• For actions that are already performed frequently (well-practiced shortcut gestures), it's fine to let them keep a more complex composite form, since heavy rehearsal drives packaging cost down; don't misapply this to mean every action should be flattened to a single step.
• How to check: with the same participants, measure the time from cue onset to the finger leaving rest for a single-step trigger versus a multi-step composite trigger. If the multi-step trigger is clearly slower and the difference sits in the time before movement rather than during it, the complexity effect is genuinely present in that scenario.

Related

• Same group: A8.05.1 An action is programmed in full before it is launched · A8.05.3 A launched action has a non-cancelable window · A8.05.4 Cancelling an already-programmed action itself takes time and carries a cost · A8.05.5 Changing the target inside that window guarantees a mis-hit
• Nearby: A8.02 Stimulus-Response Compatibility · A8.23 Stages of Motor Learning
• Search terms: movement complexity effect · response programming · sequence complexity