Look, your head should be upright, shoulders horizontal - good, very good!
If you have a shoulder rest for your violin make sure it is adjusted to a height where your head is straight and upright when you play. But even more than that, be aware of how much pressure you exert on your chin rest.
Can you turn your move your chin off of the chin rest while you are playing?
Can you play relaxed enough to turn your head?
(an example of bad playing posture)
Instead, play more like this:
Now, to be fair, Splenius Capitis is only one of dozens of muscles in your neck that is used whilst playing the fiddle.
Consider for a moment what a marvelous creation your neck is!
Imagine all the different motions your neck is capable of performing. The head is where our brain is, and it is the brain that controls our fingers, our bow arm and our tapping feet. All nervous system signals are funneled through our neck to the rest of our body. All those nerves, muscles, bone, throat, and blood vessels are packed into a really narrow flexible conduit. So, neck problems can wreak absolute havoc on our playing, or just life in general.
Holding your neck muscles in shortened or extended positions for long periods of time puts a lot of strain on that important conduit called our neck. So, it is very important to avoid problems by keeping a "good posture", and not excessively straining your neck muscles.
That’s right, we are here today to lecture you just like your mother used to (or perhaps still does):
"Sit up straight and don’t slouch!"
So, if at all possible, don’t play like this:
Splenius Capitis is a broad, strap like muscle in the back of the neck.
It pulls on the base of the skull from the vertebrae in the neck and upper thorax.
It is involved in movements such as shaking the head... and holding a fiddle.
Not that you want to play like this all the time, but as a test, try turning your head while playing, if only to take stock of how tense your neck is while you play. Learn to recognize tension. Only by knowing what tension is can we relax those muscles.
To compensate for the pressure that we inevitably put on our necks, it is a good idea to do some head rolls before or after practicing and/or performing (preferably before to stretch the muscles before we make them work).