R. Port            -based on D. Rosenbaum Human Motor Control, Chapter 1.

A.  Skilled movement vs. unskilled
        reaches: 2 phases - ballistic motion + refined end

B.  Efficiency problem: how to minimize effort to achieve a goal
       -minimizing jerk
       -synergies: special-purpose combination of constraints on muscles

C.  Serial order problem  (Karl Lashley, 1951 `The problem of serial order in behavior', PsyRev)
       1.  response chain model:  a) Each item excites the next item: n->n+1->n+2
              or, for greater speed, each might excite the next but one:
                                     n   ->   n+2
                                         n+1  ->   n+3 ...
        2. interelement inhibition. The first item ihibits the second and the second inhibits
            the 3d, etc. Thus, making the first one fire, releases the inhibition on the 2d, which releases  the 3d, etc.
        3. hierarchical models    eg, comb model, schemas

D.  Degrees of freedom problem : How to control all the thousands of possible variables in a manageable way.
        path planning   (kinematics vs dynamics)  and motor equivalence
         inverse kinematics: find a path given a goal
         joint angle coordinates vs. spatial coordinates
        inverse dynamics problem: find muscle activtn pattern given a path
        cooarticulation problem

E.  Feedback (perceptual-motor integration)
        1.  closed loop control  (eg, a thermostat or the Watt steam governor
          but: a) What is the right correction when there are many possible reasons for an error?  The problem of a multidimensional system
                b) Sherrington's `spinal cat' (which spine severed) could still walk normally (if its body was suspended over a treadmill).
        2.  open loop - actions are performed without feedback.  Especially needed for skilled action

F.  Mass-Spring model:
        a) helps greatly to solve the degrees-of-freedom problem
        b) sets a target, not a movement - so no start/stop control is needed.
        c) thus it is open loop
        d)  rate of motion, peak velocity location, etc require no explicit control

G.  Multiple Coordinated Movements
   1.  Two limbs: Haken-Kelso-Bunz (1985) model for finger wagging
   2. .  Jaw + Lower Lip + Upper Lip
        Folkins-Abbs etc.
        neural model of David Rosenbaum