This is why, in order to be truly effective, the Physio has to have a good understanding of the inter-relation of these systems and be able to treat accordingly. This is also why any approach that just focuses only on treating the symptoms is unlikely to be effective in the medium to long term.
In simple terms a muscle imbalance in when muscles (or groups of muscles) attached to either side of a joint (that usually work against one another to control the normal position and movement of the joint) do not have equal strength, length and/or activity. This results in the part of the body effected being pulled out of its normal position.
Typically, treatment involves direct mobilisation of the joints involved, most often the top 3 vertebra of the neck. This is combined with Myofascial (muscles and soft tissue) mobilisation and specific muscle strengthening and specific postural alignment retraining.