Bizzi & Mussa-Ivaldi (1995) survey experiments
showing that regions in the spinal chord of the frog define
associated force fields in the vicinity of the leg; that it,
microstimulation of that spinal region causes the leg to
exert a consistent force, which depends on the position of
the leg, thus defining a force field over its range of
motion.
They further show that microstimulation of multiple spinal
regions create a force field that is the linear superposition
(sum) of the individual force fields, and that this
superposition determines the location to which the leg moves.
Specifically, a time-varying force field 
results from a linear superposition of time-varying
basis fields 
, each generated by a premotor circuit
in the frog's spinal chord:
[ F(t) = _k c_k _k(t) . ]
As few as four convergent force fields 
are
sufficient to generate a wide variety of resultant fields.