Why is it easy to control your arms ?

The complexity of our muscular systems may be regarded, not as a complication for the brain, but as a source of variety providing enough easily controlled movement recipes to do most of the things we ordinarily need to do. This simplifies the control task, in that if there are enough ways of moving, a recipe involving just a few of them can usually be found that will approximate any desired movement with little supervision. In particular, the presence of"redundant" degrees of freedom allows us to use ballistic (free swinging) movements, so that physics, rather than computation, accounts for much of trajectory. Computations are required to set up the constraints defining initializing a low-dimensional subsystem in such a way that a satisfactory movement exists. One theme of current research is that these recipes may generated by specifying the parameters of oscillators and spring-like components. We should expect actions to be represented as patchworks of recipes, each working best for some subset of variants of the action.