The Scapula, Shoulder Mobility and Physical Therapy

The scapula (AKA the shoulder blade) connects with the humerus and the clavicle forming the shoulder joint.  The scapula has six different motions that consist of protraction, retraction, elevation, depression, upward rotation, and downward rotation.  There are also a number of muscles that help move and stabilize the scapula.  These movements and muscles of the scapula enable the upper extremity to reach full functional movement, as well as help stabilize the joint throughout rhythmic shoulder mobility.  Additionally, the scapula depends on a specific combination of muscles to be able to move as noted below:

Protraction: Serratus anterior and Pectoralis Minor

Retraction: Rhomboid Major, Rhomboid Minor, Middle Trapezius

Elevation: Upper Trapezius, Levator Scapulae

Depression: Lower Trapezius, Latissimus Dorsi, Pectoralis Minor,

Upward Rotation: Upper Trapezius, Lower Trapezius, Serratus Anterior

Downward Rotation: Rhomboid Major, Rhomboid Minor, Pectoralis Minor

Now let’s focus on one singular movement of upward rotation or lifting the arm overhead.  When the scapula is starting to rotate upwards, the Serratus Anterior and the Upper Trapezius first produce movement by pulling in opposite directions on either side of the scapula.  This is termed a force couple: two equal, but oppositely directed forces pulling on either side of an axis.  During the later phase of upward rotation, the Serratus Anterior and Lower Trapezius form a force couple for the shoulder to be able to reach full functional movement. 

 

As noted with upward rotation, there are essentially two force couples acting on the scapula to ensure that a person can reach their arm overhead.  The key aspect of this concept is that the muscles need to exert an equal force in opposite directions on either side of an axis to produce optimal movement.  If one muscle is weak due to a spasm or strain, the movement of the shoulder can be affected.  Therefore, when a patient comes to physical therapy with a strain in their Upper Trapezius, often, asymmetry can be found in their bilateral scapular movement.  This also contributes to how patients with whiplash can start to find difficulty with lifting their arms overhead.  Physical therapy can help to decrease that tension by manual stretching, relaxing and then progressively strengthening the Upper Trapezius so that it can perform optimally with the Serratus Anterior again. 

 

This example just focused on the specific movement of upward rotation of the shoulder, but it can be applied to all movements of the shoulder due to the body mechanics of the joint. 

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