Textbook of Usability

Motor control refers to the neural and muscular systems that translate intention into physical movement. Human motor performance has two distinct components relevant to interface design.

Open-loop control governs rapid, pre-planned movements executed without real-time correction — a quick flick gesture, a ballistic keystroke. The movement is programmed in advance and launched; the motor system does not adjust mid-flight.

Closed-loop control governs slower, more precise movements where the motor system continuously monitors progress through visual and proprioceptive feedback, making corrections during execution. Guiding a cursor to a small target is closed-loop: the hand gets close quickly (ballistic phase), then slows and corrects to hit the exact target (corrective phase).

This two-phase structure underlies Fitts's Law: small targets are disproportionately slow because they extend the corrective phase. Big targets allow users to stay in open-loop and move fast.

Other motor control considerations:

• Reaction time (150–300 ms for simple reactions) precedes every movement
• Bimanual interaction (Guiard, 1987): the non-dominant hand sets the frame of reference; the dominant hand performs fine manipulation
• Muscle fatigue: sustained postures (gorilla arm, mouse shoulder) cause repetitive strain
• Input device differences: mouse, touch, and keyboard have distinct motor characteristics

Motor control is the physical endpoint of every interaction, and its limits set absolute boundaries on how fast and accurately users can operate an interface.