The sub occipital muscles consist of the Superior Oblique, Inferior Oblique, Rectus Capitis Posterior Major and Rectus Capitis Posterior Minor. All of these muscles can contribute to headaches when tight or short. A very common headache comes from an entrapment of the greater occipital nerve in the Inferior Oblique.
Now an often overlooked contributor to headache is the Rectus Capitis Posterior Minor (RCPMinor). It has connections to the posterior arch of C1 and travels up to the nuchal line on the occiput. Often overlooked and I believe very important is that it also has connections into the spinal dura.
These connections seem to somehow help monitor the cerebral spinal fluid. We do know that all the sub occipital muscle have loads of muscle spindles in them to provide proprioception to the Central Nervous System.
If the RCPMinor becomes tight the atlantooccipital joint is not as stable. Proprioception will be hindered and perhaps most importantly, cerebral spinal fluid will be altered.
On a personal side note, every person/athlete I have ever treated post concussion have had extremely tight RCPMinor and Major.
If you receive treatment or seek treatment for cervicogenic pain or tension headaches, make sure to ask about the RCPMinor.