It is a movement problem where the shoulder blade (scapula) and the upper arm bone (humerus) do not move together in their normal, coordinated pattern, Winging,Early Shrugging,"Catching" or Popping,Painful Arc,Improving Scapular Rotation (The "Force Couple") :Wall Slides (Serratus Anterior Focus),Low Rows (Lower Trapezius Focus),Releasing Tight Muscles (The "Obstacles") :Pec Minor Stretch,Posterior Capsule Stretch ("Sleeper Stretch"),Advanced Stability:Push-up Plus.
