Spinal motoneuron recruitment in man: rank deordering with direction but not with speed of voluntary movement.

Single motor units in human interosseous muscle are recruited in order from small to large in slow or brisk voluntary abduction of the index finger. When the same muscle acts as a synergist as opposed to a prime mover, about 8 percent of the unit pairs consistently reversed their recruitment order. Motor commands appear to be patterned in terms of movements rather than muscles and to involve different connectivities to the motoneuron pool of a muscle executing movements in different directions.