Granted, this information is much simplified, as is not for your exam purposes: your ortho lecturers might not approve! But don't be shy to open your atlas, as from my own experience musculoskeletal anatomy is surely among the more neglected aspects of anatomy. As if anatomy isn't enough neglected!
The key to the shoulder joint complex is understanding it as 2 separate joints - the glenohumeral joint and the scapulothoracic joint. Different muscles act at each.
The main common problems with the shoulder joint are impingement, bicep SLAP injuries and traumatic injuries (AC joint, GH joint dislocations). We will look at the former here.
The main cause of impingement is a reduction in the subacromial space, leading to injury of the supraspinatus. There are a few factors which determine the risk of impingement:
1) acromial arch subtype (genetic/acquired)
2) slouched shoulders (acquired)
3) downward rotated scapula (acquired)
4) glenohumeral internal rotation (acquired)
5) weak rotator cuffs (acquired)
6) hyperkyphotic thoracic spine (acquired)
7) weak scapular stabilizers (acquired)
See where I'm going? With that, it's easy to identify people at risk:
Computer users - slouched shoulders, internally rotated humerus, hyperkyphotic thoracic spine
Strength athletes who don't train their mirror "muscles" (too much bench pressing, not enough rowing) - internally rotated humerus, inactive serratus anterior
Swimmers - internally rotated humeri
Let's examine how impingement occurs or is prevented.
When talking about impingement, most talk about the rotator cuff. Why is it so important? Let's look at a simple example.
The deltoid acts as a shoulder abductor, similar to the supraspinatus. But look at the point of pulling. The insertion of the supraspinatus, and the rest of the rotator cuff is much closer to the glenohumeral joint than the deltoid. Thus when the deltoid pulls the humerus upwards, the humeral head is also pulled up, reducing the subacromial space. The rotator cuff acts to depress the humeral head into the glenoid fossa, preventing this space reduction.
Internally rotated humeri expose more of the supraspinatus to the subacromial space. An easy way to induce impingement is to perform shoulder abduction with the arms fully internally rotated, ie thumbs pointing down. What are your internal rotators? They are the pec major, lat dorsi and subscapularis (which is a part of the rotator cuff muscles). Have these dominant against your main external rotators (infrascapularis, teres minor), and the static posture will be internal rotation.
A slouched shoulder reduces the subacromial space. This is prevented by having proper posture, in which the shoulders are retracted. A slouched shoulder also indicates a shortened pec minor, which pulls the scapula (and thus the acromion) downwards & anterior "tilts" it. This reduces the space.
Another thing to understand is the concept of upward and downward rotation of the scapula. When performing overhead movements, the scapula must rotate upward to allow further flexion/abduction. If it can't rotate, the subacromial space becomes smaller.
The scapula is controlled by a myriad number of muscles. These are the upward rotators:
Upper trapezius
Lower trapezius
Serratus anterior
These are the downward rotators:
Levator scapulae
Rhomboids
Pec minor
The upward rotators must be active and strong enough to pull the scapula into upward rotation. The downward rotators must be flexible enough to allow it.
When transmitting force, the base must be stable. Can a cannon shoot from a canoe? Or do you want a ship?
The upper limb is unique in that force is transmitted not through bony structures, but through 100% soft tissue, except at the AC joint. The scapulothoracic "joint" has no true bony articulations. The scapulae are attached to the "core" by means of the traps, rhomboids, levator scapulae, serratus anterior and pec minor; all muscles. Out of these structures, the traps and serratus anterior are responsible to maintain scapular stability during movement. Weak stabilizers predispose one self to injury during heavy upper limb movement.
Proper scapular movement also depends on the upper back posture. There should be only normal thoracic kyphosis. Try slouching your upper back forward. You won't be able to flex the shoulders overhead. Good thoracic spine flexibility is a key component to safe overhead movements.
So what do we need for a healthy shoulders to prevent or reduce the risk of impingement?
1) Good rotator cuff strength
2) Retracted scapulae (good posture). Some S&C coaches prefer retracted and depressed scapulae (relaxed rhomboids, levator scapulae)
3) Proper internal rotation-external rotation balance. This includes having good flexibility for external rotation
4) Strong, dynamic scapular stabilizers
5) Good flexibility of the thoracic spine
6) Scapular upward rotation
For this blog post, let's identify shoulders at risk. Another post can be done to examine various methods of reducing the risk/rehabbing the shoulder.
The Internally Rotated Shoulder
Stand up, and grab a pencil in each hand, let the arms hang loose. In what direction do the hands point? If it's straight forward, you're good. If it's 45 degrees inwards, you're OK. If the pencils point towards like each other, you have gorilla arms and are at risk, just like Justine Timberlake.
The Slouched Shoulder
Stand sideways to the mirror and look at the shoulders in relation to the body. The best posture is one in which you see more of the anterior deltoids than the rear deltoids. The shoulders should occupy the posterior 1/2 of your AP dimension. The more forward the shoulders, ie the more of the rear deltoids are seen, less of the anterior delts, the shoulders are more forward as regards to the AP dimension, the worse. You're looking for the "showing off look" or "busty model look" or a "military chest out look". Whichever motivates you more.
The Downward Rotated Shoulder
1) Have a friend examine from the back. Perform full flexion of the shoulders. The scapulae should rotate.
2) If they don't rotate, have them perform a shoulder shrug while the shoulders are in full flexion (ie directed upwards). This "exercise" is known as overhead shrugs/shrug-ups. Again, they should rotate.
3) As the shoulders are being rotated, look for limitations. The person will either compensate by lumbar hyperlordosis (especially with hyperkyphotic thoracic spine) or with "forcing" the humerus to become perpendicular to the floor despite lacking the flexibility for it, which might lead to pain.
Active Impingement Test
Put your hands on the opposite shoulder. LIft the elbows past the head, without lifting the hand. Look for pain.
There are some other tests, but these tests are for other problems.
We will talk about fixing the above problems in the next post. If you need pics/more descriptions I'll link them for reference/elaborate further.
