Cervicothoracic junction mobilisation (Cervicothoracic mobilization, closing/opening arms, hands behind head, sitting)

Sit or stand up straight.
Place your hands either side of the base of your skull, just below your ears.
Sitting up tall, move your elbows forward towards each other.
Then, move them away from one another out to the side.
As your elbows move to the side, think about sitting even taller.
Continue this movement forwards and backwards at a steady pace.