Multi-muscle control of head movements in monkeys: the referent configuration hypothesis.

It is suggested that the nervous system may specify a referent configuration (R) of the body determined by the set of the threshold joint angles at which all skeletal muscles may be silent. At the same time, electromyographic (EMG) activity and forces are generated to resist deflections of the body from this configuration. The R configuration may thus be considered an internal geometric image with which the actual body configuration (Q) is compared. Thereby the difference between the R and Q is a major factor determining the recruitment and gradation of the activity of each skeletal muscle. Control systems may produce movements by changing the R configuration according to task demands. The referent hypothesis predicts that when the R and Q configurations match each other, a global minimum in the EMG activity of all muscles involved should occur, an event most likely observed in movements with reversal in direction. To test the validity of the R hypothesis for head movements, three-dimensional kinematics and EMG activity of 14 functionally diverse neck muscles were analysed in monkeys during head rotations to and from fruit targets placed beyond the oculomotor range. Despite the functional and anatomical diversity of the neck muscles, the activity of all muscles was minimised at a reversal point of the movement trajectory, as predicted by the R hypothesis. This study thus illustrates the notion that a change in the internal geometric image of a biomechanical system may underlie movement production.