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I think one of the dangers of study in any field is becoming too narrowly focused on any one idea, and losing the grand integrative scope of things. This is not a critique, but a reminder I guess, something that we should always keep in mind. Neural control of body and brain is a huge part of the picture, but cannot be the whole picture. A central pattern generator may seemingly give us the ability to walk without thinking about it, but if our muscles did not have acetylcholine receptors we would not be able to move at all.

The mechanism of a central pattern generator is great for explaining how we can do things like walk without thinking about it, or (if we were birds) fly without learning how to. The mechanism of a CPG is not responsible for all the aspects of motor control though. Located along our muscles there are nerves with special receptors called stretch receptors, they respond to the stretching of one muscle that occurs when its antagonistic muscle contracts. The impulse elicited is neurally transmitted in a big circle (bypassing "conscious" control) and returns to the muscle in which it originated, and it causes the muscle to contract. This explains why we can spontaneously contract our knee and instead of getting stuck that way, it bounces out again. Because the muscle is contracted with a force proportional to the force with which it was stretched it also accounts for why we dont kick our doctor in the stomache when he does the pataellar tendon reflex test.

I think it would be inefficent of the brain to generate two patterns for each action, one for muscle (a) and one for muscle (b) the antagonistic muscle, or for it to generate a pattern that is twice as long because it specifies all contractions of muscles (a) and (b). It seems most efficent for our brain to take advantage of the role of reflexive motor mechanisms. Also if it does the latter it can utilize the contribution of gravity to the movement in order to expend less energy. So, for our brain to get its job done most efficently, it may utilize mechanisms and forces not under its direct control.