A kinematic theory of rapid human movements. Part II. Movement time and control

This paper describes how a synergy made up of a pair of agonist and antagonist systems involved in the production of a rapid movement can control movement time. A quadratic law is derived to predict the movement time as a function of the various parameters describing the neuromuscular synergy. Conditions for a simplified description of the process, using a power law, are also presented. It is predicted that movement time can be controlled at the input level by the ratio of the agonist to antagonist commands or at the system level by modifying the total log-time delay or the log-response time of the agonist or antagonist neuromuscular networks. Adapting this approach to the specific case of movements executed under different spatial accuracy demands, it is found that movement time is linked to the inverse of the relative spatial error by similar laws. The whole approach is used to explain within a single framework all the observations that have been reported concerning speed/accuracy trade-offs. Strategies for controlling movement amplitude and duration are analyzed, and other predictions dealing with EMG, acceleration patterns, load effects and changes in the asymmetry of the velocity profile are also discussed.