New aspects of human muscle coordination as revealed by motor-unit studies.

Increasing evidence has become available against the view that individual muscles should be regarded as functional units in the control of force by the motor apparatus. The relative contribution of various motoneurone pools (belonging to a single muscle) to muscle force appears to depend on the direction of torque exerted by the corresponding limb. This clearly indicates the presence of more than one control or activation parameter for the total motoneurone pool of a single muscle. Windhorst et al. (1989) have even proposed a combined sensory and neuromuscular partitioning, such that segmental control mechanisms are based on subdivisions of motoneurone pool--muscle complexes. Size-related recruitment occurs within these subdivisions rather than in the motoneurone pool as a whole. Moreover, motoneurones can operate in various modes (different relative contributions of the recruitment mechanism and the firing frequency mechanism to muscle force) related either to movement or to force. This indicates that muscle coordination is organised differently in force tasks and in movement tasks. These properties of motoneurone activation mentioned above are difficult to incorporate in existing models on sensorimotor organisation. Some of these discrepancies will be reviewed in order to highlight future directions for modelling.