The function of this muscle is to rotate the head to the opposite side or obliquely rotate the head. It also flexes the neck. When both sides of the muscle act together, it flexes the neck and extends the head. When one side acts alone, it causes the head to rotate to the opposite side and flexes laterally to the same side (ipsilaterally). It also acts as an accessory muscle of respiration, along with the scalene muscles of the neck.
Cause that perpetuate trigger points :
Activities Swimmers crawl stroke, prolonged looking up (Painting a ceiling) , horse riding/ handling, a tight neck collar or tie. high pillow, reading in bed with a lamp off to the side, breathing Improperly,
Other Muscles: Tight Pectoralis Muscle , Trapezuis, Masseter, Scalene , Splenius Capitis, Splenius Cericis, Cervical Multifidi, Semispinalis Cervicis, Semispinalis Capitis, Suboccipitals, Longissimus Capitis , Lavatorscapula, Sternalis , Temporalis , Orbicularis Oculi , Zygomaticus, Platsyma , Buccinator, Frontalis , Occipitalis
Suffering from any of the following whiplash,chronic cough, cold , flu , sinusitis , a dental abscess hangover headache or leakage of cerebrospinal fluid following a spinal tap.
Structural issues such as forward -head posture, injury that restricts upper-body movement forcing the neck to over compensate to keep balance such as severe scoliosis, anatomical leg-length inequality or unequal hemipelvis size.
The SCM has two parts to the muscle with there own referral patterns. The Sternal causes pain in the eyes and sinus , While the Calvicular causes pain in the forehead and ear.

The X's represent the Trigger Points apply pressure gently until your on the edge of "good" pain. The red shaded area is the referred pain caused by the Trigger Point and the darker red means more people experienced pain in that area.
SELF MASSAGE TECHNIQUES
Position: Lying down on your back / or sitting down with back support.
Tilt your head bring the ear closer to the shoulder and then slightly rotate your chin up
Start at the lower X right hand pinches & pulls right side for up-to 60 seconds
Left hand grabs the Right Middle X pinches & pulls outward for up-to 60 seconds
Right hand works the Top X behind the ear this is the usually the tightest for most people and also the most critical to work on.
STRETCHES
Side-Bending Neck Stretch
Position: Lying down on your back with the hand of the side you are stretching under your butt. Your opposite hand over the top of your head.
Pull your head gently toward your shoulder, then release and take a deep breath.
Repeat with your head turned slightly toward the left and again toward the right.
Stretch the opposite side following steps 1-3
Repeat steps 1-4 three times
Scalene Stretch
Position: Sitting up
Rotate your head all the way to one side and bring your chin down. Hold for 5 seconds
Return to the forward position and take a deep breath.
Repeat on the opposite side
Repeat 4 times in each direction
AREAS SCM TRIGGER POINTS REFER PAIN TO :
Back of Head Pain
Cheek Pain (like Sinusitis)
Dizziness When Turning Head or Changing Field of View
Double/Blurry/Jumpy Print Vision
Dry Cough
Ear Pain Earaches/Tinnitus (Ringing)/Itch
Feeling Continued Movement in Car After Stopping
Feeling Tilted When Cornering in Car
Front of Chest Pain
Headaches or Migraines
Post Nasal Drip
Runny Nose
Sore throat
Tearing/Reddening of Eye, Drooping of Eyelid
Temple and Eyebrow Pain
Temporal Headache (Temples)
Temporomandibular Joint (TMJ) Pain
Throat & Front of Neck Pain
Traveling Nocturnal Sinus Stuffiness
Vertex Pain
Visual Perception Problems