The rotator cuff muscles assist in centering the humeral head in the shoulder socket during the range of motion of the arm. This graded rotation allows the arm to rise and the humeral head to stay in the socket and not get jammed towards the acromium process. The correct interaction between the deltoids and the rotator cuff muscles centers the head of the humerus bone in the glenoid cavity, the shoulder socket. As the arm rises from 0-150 degrees of flexion, there is a dance of the muscles around the humeral head. The first degrees of flexion of the shoulder, the deltoid and the supraspinatus pull the humeral head up in the glenoid cavity.
As the arm rises, the subscapularis, infraspinatus and the teres minor pull the humeral head into the socket and then down. Optimal centering of the bone in the socket is key with the timing of the movement of the humeral head.
Muscles responsible for the medial rotation of humerus bone are the Latissimus dorsi, Teres minor, Subscapularis and Pectoris major. The lateral rotators of humerus bone are the Infraspinatus and Teres minor. When one set of rotator cuff muscles are tight and/or weak, the humeral head is pulled unequally in the socket. This will affect the scapula-humeral rhythm.
The scapula humeral rhythm is the optimal combination of the rotation of the shoulder blade and the humerus bone in the glenoid fossa, the shoulder socket. The port de bra of ballet is a great example of good use of rotation. These positions of the arms allow the shoulder girdle, scapula and humerus bone in socket to work at the most efficient level.
Imagine the lifting of the arms is of levers responding to the rolling and gliding of the bones of the shoulder complex. When one part of the machine is stuck, it will affect the entire movement.
Scapular Humeral Rhythm To start
Lie on your right side with your right arm placed underneath your head. There should be a plumb line between your head and tail.
Your left hand is resting on your left shoulder. The palm of your left hand is facing towards your left shoulder. Your left humerus bone is in slight internal rotation.
Imagine the tip of the shoulder blade being connected by a string to your left elbow.
Begin to raise your left elbow to the ceiling, feel the shoulder blade gliding wider on the ribcage. Gradually rotate the humerus bone outward so the entire left arm bent is now facing the wall in front of you. Reach the left elbow to the ceiling to help abduct the scapula more.
Glide the scapula in more upper rotation and gently increase the outer rotation of the humerus bone to bring the left elbow over your head. The arm stays in the same plane as the scapula and gradually rotates more outward. For most, this will bring your elbow in front of your ears. (To get the elbow in line with the torso, one has to improve the mobility and the mechanics of the scapula).
As if you were running the film backwards, return the arm back to your hip paying attention to the gradual rotation of the arm and the scapula. When the arm returns to the hip, the arm is slightly inwardly rotated.
Repeat 4 more times
Reverse to the other side and other arm
Observation Were you able to maintain the length of your head-tail?
Full arm Port de bra To start
Lie on your right side with your right arm placed underneath your head.
Your left arm is resting on your left hip. The palm of your left hand is on your left hip. Your left humerus bone is in slight internal rotation.
Abduct the tip of the scapula towards the ceiling to lift the left hand to the ceiling. The scapula is abducting as well as the arm. The arm gradually rotates so the left palm is facing the wall in front of you. Feel the string gently lengthen the connection from your scapula through your elbow to your little pinky finger in the use of space.
Simultaneous glide your shoulder blade in upper rotation as you twirl the humerus bone gently in more outward rotation. Your arm probably is in front of your left ear.
Reverse the movements to bring the left arm back to the left hip. You are tracing back the same pattern as the lift of the arm over head.
Repeat 4 more times
Change to the other side
Observation Were you able to make the movement smooth? Did you maintain the length of your spine throughout the entire movement?
Full arm circles
Reach your left hand in backspace until the left arm is in line with the left shoulder. This will glide your left shoulder blade towards your spine. The arm is gradually rotating outward. As the arm moves in line with the shoulder, the rotation of the palm should be facing towards the ceiling.
Glide the scapula around the ribcage with the humerus bone still gently turning in outward rotation. This will bring your shoulder blade in upper rotation. Your palm will gradually face towards the floor.
Circle the left arm towards the ceiling. Your hand is facing front.
As the arm travels back to your side, the humerus bone gradually rotates back to a slight inward rotation. The palm of your left hand can rest on your left hip.
Repeat three more circles
Reverse to the other side.
Observation Were you able to do a full arm circle from moving the bones of the scapula and the humerus bone? What part of the arm circle lost its connection to the scapula? Did old habits as pulling down the armpit with your Lats or firing the deltoids get in the way of the flow of the exercise?