  Understanding how movement is controlled by the central nervous system
remains a major challenge, with ongoing debate about basic features underlying
this control. In this review, we introduce a new conceptual framework for the
distribution of common input to spinal motor neurons. Specifically, this
framework is based on the following assumptions: 1) motor neurons are grouped
into functional groups (clusters) based on the common inputs they receive; 2)
clusters may significantly differ from the classical definition of motor neuron
pools, such that they may span across muscles and/or involve only a portion of
a muscle; 3) clusters represent functional modules used by the central nervous
system to reduce the dimensionality of the control; and 4) selective volitional
control of single motor neurons within a cluster receiving common inputs cannot
be achieved. We discuss this framework and its underlying theoretical and
experimental evidence.
