In other words, no single study can sum up the system. You need to look at
different levels.
That depends on how you define the "system". But I assume your reference to
"different levels" allows for analyses that include more than just isolated
individual components.
And let me throw in that the neck muscles (including the shoulder muscles)
really have to get incorporated into it.
Absolutely. In fact, you make my point about the animal being a system of
inter-related parts--jaws, neck, shoulders, arms, legs, hips, etc. All the
more reason for at least considering methods used in complex systems theory
as a way to investigate these relationships.