Set-up: Begin by lying on your side, crossing one leg over the other and bending the upper leg to 90 degrees. Now extend both arms at your side straight away from your body.
Movement: Start the movement by looking back over your shoulder and rotating your top arm towards your backside. Gently attempt to bring that arm to the floor, hold momentarily and return to the start position. Perform desired number of repetitions and repeat on the other side.
Repetitions: Perform all prescribed reps on one side, then on the other. This counts as one set.
Benefits: This exercise is meant to develop mobility in your thoracic spine, which will help ensure stability and health in both the lumbar spine and the shoulder girdle.
Instructions and effects for the Side Lying External Thoracic Rotation exercise 
Movement in Sagittal Plane