What is the plane of the scapula?
The plane of the scapula can be defined as the normal resting position of the scapula as it lies on the posterior rib cage at an angle of 30-45˚ (Johnson 2005). It can also be defined as the angle assumed by the face of the glenoid fossa (Starkey & Ryan, 2010). In simple terms, this arm position is the angle in which the scapula is laterally facing outwards from an individual’s upper back. The picture below gives an illustration of where exactly the plane of the scapula lies:
Why move within the plane of the scapula?
There are several reasons that explain why it is beneficial to move within this plane. The first, and probably the most important reason, has to do with simple physics. When one is in the plane of the scapula, the mechanical axis of the humerus approximates with the mechanical axis of the scapula (Johnson, 2005). This optimally aligns the deltoid muscles and the supraspinatus muscle for elevation of the arm, thereby avoiding subacromial impingement during shoulder abduction (Paine & Voight, 2013).
Another reason for moving within the scapular plane is that it puts less stress on the shoulder capsule. When the arm is raised overhead in the sagittal or frontal plane, the shoulder capsule is stressed from obligatory humeral rotation. For example in the frontal plane, the anterior capsule structures are tensioned, which may complicate soft tissue healing in patients who have glenohumeral injuries/instability. Whereas within the scapular plane, the inferior part of the capsule is relaxed, since no humeral rotation is required (Paine & Voight, 2013).
A final reason for moving within the plane of the scapula can be explained by the rule of the length-tension relationship. Since the rotator cuff muscles originate from the scapula, the position of the humerus in the scapular plane increases the length-tension relationship of the deltoid and rotator cuff muscles, therefore helping to facilitate optimal muscle force (Paine & Voight, 2013).
Overall, research has suggested that exercises completed in this specific plane can provide great benefits for improving shoulder function. Improvements in function and shoulder biomechanics include relief of the anterior capsular structures of the shoulder, decreased chance of rotator cuff compression, and an overall improvement of the shoulder complex neuromuscular performance. As well as improving the overall function of the shoulder, the improvements also reduce the chance of pain and injury for the shoulder complex, particularly in overhead throwing sports/ movements. (Paine & Voight, 2013)
Pressing exercises in the plane of the scapula vs the frontal plane
An example of a strength exercise being performed in the scapular plane is the 2DB bench press. A study done on the avoidance of shoulder injuries during resistance training showed that performing this exercise, or any overhead press within the scapular plane will help minimize anterior capsular distension (Davies, Durall, & Manske 2001).
Several studies have shown the importance of exercising in the scapular plane versus exercising in the sagittal or frontal planes. For example, a study conducted on the assessment of internal and external rotation in the frontal plane versus the plane of the scapula found that isokinetic strength testing of the shoulder’s internal and external rotators within the scapular plane had several advantages to testing in the frontal plane. Abducting the arm within the frontal plane tends to place the glenohumeral joint into more of a closed packed position. This position may place undesirable stress on the soft tissues of an injured joint, and may even be uncomfortable for healthy subjects (Hellwig & Perrin, 1991). Movement within the scapular plane minimizes tension/rotation of the inferior portion of the capsule and enhances stability of the glenohumeral joint by allowing a maximal amount of congruity between the head of the humerus and the glenoid fossa (Hellwig & Perrin, 1991). Because greater stability and increased comfort are important factors to consider when strength-testing or rehabilitating the shoulder, the scapular plane should be the preferred position.
External rotation in the plane of the scapula
Studies have also shown that external rotation within the plane of the scapula is more beneficial in terms of the amount of torque produced during concentric and eccentric contractions. The study done by Hellwig and Perrin demonstrated that assessing isokinetic strength of the shoulder internal and external rotators in the scapular plane changes the peak torque to body weight ratios for concentric external rotation. They found external rotation peak torque to body weight ratio values to be significantly higher in the plane of the scapula than in the frontal plane (Hellwig & Perrin, 1991). This correlates with the previous finding of increasing the length-tension relationship when one is within the scapular plane, thus increasing the amount of torque and muscle force produced. The table below illustrates the degree of muscle activation during external rotation in several different positions:
The table above demonstrates 3 of the 4 rotator cuff muscles and deltoid muscles that are activated during external rotation and abduction. Here we can see that performing external rotation while standing and in the scapular plane at 45˚ abduction shows the infraspinatus and teres minor being the most active, when compared to the supraspinatus, middle deltoid, and posterior deltoid (Andrews, Escamilla, & Yamashiro, 2009).
This is due to the role that the infraspinatus performs during glenohumeral abduction. At about 70˚, the humeral head becomes depressed by the infraspinatus, teres minor, and subscapularis to allow the humeral head to clear the acromion process. At 115˚, the humeral head is externally rotated by the infraspinatus and teres minor in order to clear the greater tuberosity of the humerus under the acromial arch.
Rhythmic stabilization in the plane of the scapula vs outside of the plane of the scapula: Is it still beneficial?
Rhythmic stabilization movements are short, rapid movements in an acute area performed for a short period of time, i.e., 10 seconds. These movements can be used to enhance shoulder function, as they improve neuromuscular control, proprioception, and dynamic stabilization. Physiologically speaking, these exercise drills facilitate agonist/antagonist muscle co-contractions, which allows for efficient coactivation to occur. This restores the balance in the force couples of the shoulder joint, and thus enhancing joint congruency and joint compression (Townsend et al, 1991). Research has shown that rhythmic stabilization movements performed in the plane of the scapula, and at the end-range external rotation positions can greatly benefit the shoulder complex, especially for the overhead throwing athlete who is either recovering from injury, or who is preventing injury in the first place. In saying this, rhythmic stabilization exercises performed in different planes can be beneficial for the injury prevention of any athlete, as different angles though the various possible ranges of motion can be targeted and improved on (Wilk et al, 2002).
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