Anatomy can seem daunting at times, especially in regards to the shoulder. The “shoulder complex” is quite complicated at first glance. However, observing patterns and then grouping similarities together makes the "complex" much more simple to understand.
For example, let’s take a look at the number 30. This is a common number that repeatedly shows up in anatomy. Below, you will find some examples of how this applies specifically to shoulder anatomy and kinesiology.
The scapular plane = 30 deg
Humeral head retroversion = 30 deg
AC joint angle = 30 deg
SC joint rotates upward = 30 deg
AC joint rotates upward = 30 deg
Clavicular protraction = 15-30 deg
Clavicular retraction= 15-30 deg
AC joint sagittal plane adjustments = 10-30 deg
Posterior rotation of the clavicle unloads the coracoclavicular ligament, permits scapula to rotate upward its final 30 deg
The glenohumeral joint (aka shoulder joint) consists of the glenoid which is part of the scapula and the humerus. 17 named muscles attach to the scapula and 13 named muscles attach to the humerus. 17+13 = ? You guessed it... 30! Yes there is overlap between muscles of the scapula and humerus but talk about a coincidence.
Are these numbers fixed? Of course not! The human body is highly variable and these numbers are not absolute. However, grouping the general commonalities saves bandwidth in your brain to memorize the subtleties that are less apparent.