Models and Theories in Human-Computer Interaction/Predictive (models of performance): Fitts

Fitts’ Law in rapid and reciprocal aiming movements? (Wei-Ting Yen) edit

Fitts’ law states the log-linear relationship between the amplitude of the movement (A), the target width (W), and the mean movement time (MT). Fitts believed that a person has to adapt to a difficult combination of A and W by increasing the MT or by becoming less accurate, because the amount of information a human neuro-motor system can process is limited. However, it has been found that this statement represents a far too limited brain concept. The functional limitations are not solely defined by transmission rates within the brain itself. It appears that the human neuro-motor system abruptly engages a different control mechanism when task difficulty increases.

Many research studies have been conducted to criticize the theory foundations of Fitts’ Law. For example, Huys et al. (2009) demonstrated that Fitts’ Law is discontinuous in reciprocal aiming movements owing to the transition in operative motor control mechanisms with increasing task difficulty. Smits-Engelsman (2002) examined whether Fitts’ Law still applies if people use different muscle use strategies during reciprocal aiming tasks. They found that there were fundamental characteristics differences between the discrete and cyclic aiming movements. Accordingly, the performance of cyclic movements was twice as high as of discrete movements with the same task difficulty. Therefore, it can be concluded that a more modern view of brain capacity would imply the relationship between central brain processes and more peripheral dynamic contributions to motor outputs from the strategic use of limb muscles.


  • Huys, R., Fernandez, L., Bootsma, R.J., & Jirsa, V.R., (2010). Fitts’ law is not continuous in reciprocal aiming. Proceedings of the Royal Society B: Biological Sciences, 277, pp.1179-84
  • Smits-Engelsman, B.C.M., Van Galen, G.P., & Duysens, J., (2002). The breakdown of Fitts’ law in rapid, reciprocal aiming movements. Experimental Brain Research, 145, pp.222–230