To lengthen/relax the sternocleidomastoid (SCM) muscle which runs from behind the ears to the central collarbone.
Be standing with feet comfortably apart. Roll/fold a regular bath towel lengthways into a strip roughly 6 wide. Drape the towel strip evenly over the shoulder of the affected side. Grip the end of the strip hanging behind the back in the hand of the affected side, and the end at the front using the other side.
Apply firm downward pressure to the shoulder by pulling simultaneously on each end of the towel. Then slowly and in a controlled manner tilt the head away from the towel toward the opposite shoulder. Then also rotate the head away from the towel. When a stretch is felt in the neck, stop and hold the position for 20 seconds whilst breathing slowly and deeply. Return to starting position.
It is essential to maintain firm downward pressure on the shoulder via the towel to create effective stretch of the SCMmuscle.
Consult your Osteopath first.