This “rap” looks at two main anatomical motions which are complex in terms of their muscular anatomy – shoulder flexion and shoulder abduction, by looking at a well known physiological position of gangster rappers; the Gangster Lean. This position is adopted whilst driving to put one hand on the wheel, and the other around the seat or companion of the driver of a car.
When you hear a rapper say “Gansta Lean”
Anatomically, this is what I think he means,
Flexion at the shoulder to hold the wheel,
Pec major and deltoid, they’re the big deal,
Coracobrachialis and a bit of biceps,
The other arm up has only has three steps,
For shoulder abduction, its supraspinatus,
Only 20 degrees though, for all of you haters,
Deltoid keeps it going til it can’t go no more,
Then scapula rotation like a touchdown score,
(For further anatomical details and explanation..)
The first main player in shoulder flexion is the large anterior muscle which originates from the anterior thoracic wall – Pectoralis Major. This has origins from the sternum, top 6 costal cartilages as far as the aponeurosis of external oblique. This inserts into the humerus on the lateral lip of the intertubercular groove, and as you can imagine from its anterior position, pulls the humerus forward on contraction.
The second main player is Deltoid, and in particular the anterior fibres. This muscle operates in a slightly different way – coming from the lateral clavicle, acromion and superior aspects of the scapula, and attaches on the far side of the humerus at the deltoid tuberosity. Later we will see that contraction of the other portions of this muscle also help abduction.
Hence the “Big Deal” associated with these muscles – they are big, and important in shoulder flexion.
However, other muscles play a role in flexion, and Coracobrachialis is one of them – reading from the name “coraco” from the coracoids process of the scapula, leads to the “brachialis” part – the medial humerus which is its insertion. This is involved in adduction as well as shoulder flexion.
Lastly, and perhaps unfairly belittled by the rap, was Biceps Brachii – spanning two joints, this muscle coming from the coracoid process again, and its long head from the supraglenoid tubercle, inserts way down on the radius. However, given its anterior course, it weakly assists in the shoulder flexion.
What some people call the “three stage rocket”, shoulder abduction can be fairly complex.
It starts with Supraspinatus, originating in the superior of the scapula (supraspinus fossa), and inserting via the capsule into the greater tubercle of the humerous, this initiates abduction to around 20 degrees. You may not think this muscle has many “Haters”, but actually there are two main demographics – firstly those who think that this muscle is misnamed as a rotator cuff muscle because its action is not rotation. Secondly, those suffering Supraspinatus Inpingement or tendonitis, which was first described by Neer in 1972. This comes about because of the narrow space through which the supraspinatus tendon must pass beneath the acromion and coraco-acromial ligament. Big up to my homies with the shoulder ache.
We have already seen Deltoid, and here it features again, until approximately 85 degrees of abduction, when the head of the humerus becomes biomechanically unable to rotate in its position against the humerus.
This requires us to rotate the scapula to achieve any further degree of elevation, as the relationship between the humerus and glenoid is maintained, but the scapula itself causes the arm to raise. This requires a multitude scapula associated muscles for another day and another rap.