The structure of every single tissue within the human body is a result of the evolution of tasks required to be completed by those structures. In strength and conditioning we call this the S.A.I.D. principle:
Specific Adaptation to the Imposed Demand (S.A.I.D.) - the adaptation is a direct result of the demands that are placed on the system.
This is easiest to understand when talking acutely about strength and conditioning training. When we stress muscles with progressively heavier weights, our central nervous system and muscles become stronger. When we progressively increase running volume, our heart and lungs become more efficient and we get fitter.
The same has occurred over time with our evolution from an anatomical structure standpoint. Our muscles, bones, organs and connective tissues are designed as a result of the progressive demands that have been placed on humans over time. It is the same exact S.A.I.D. principle phenomenon occurring in the acute view of strength and conditioning training, except these adaptations have occurred over a far more ‘chronic’ timeline - millions of years of evolution as opposed to strength training in a week to week or month to month representation.
This means that the way our bones, muscles and organs are formed is a direct result of the progressive tasks of these bones, muscles and organs over time. So today, I want to quickly touch on 3 pieces of anatomy that’s anatomical function is a result of their anatomical structure.
1) The Diaphragm
The diaphragm is a flat muscle that sits at the bottom of the rib cage. In it’s resting state the diaphragm is shaped like a dome. When the diaphragm contracts it flattens out. This ‘flattening’ action creates more available space above the diaphragm in the rib cage/ thoracic cavity and less available space in the abdominal cavity underneath it. This drop in pressure within the thoracic cavity allows air to flow into the lungs. The diaphragm is responsible for getting in into the lungs.
When the diaphragm relaxes and reassumes its resting dome shape within the rib cage, the pressure change reverses and the air is drawn out of the lungs.
The anatomical function of the diaphragm is completely representative of the anatomical structure.
2) The Thoracic Spine, Rib Cage and Scapular
The rib cage is built for protection of the vital organs inside. Each individual rib connects to the thoracic vertebrae at the costo-vertebral joints. As a result of the rib cage housing the vital organs we get a structure that is ‘round’ to offer physical protection and also to distribute forces more evenly, like all round structures in nature. This means that the thoracic spine reflects this shape, it is curved and mimics the curved nature of the ribs.
Sitting atop the curved rib cage is the scapular, a flat bone with a slight concave shape. The scapular has 17 muscular attachments and only 1 true stable ‘boney’ attachment, the acromio-clavicular joint. It also has ligamentous attachments at the gleno-humeral joint, but it’s far less stable than the AC joint. By nature of having 17 muscular attachments and only 1 true stable boney attachment, the scapular is designed to move and move freely, in every direction possible. It’s concave shape allows the scapular to move freely over and around the convex shape of the rib cage.
The anatomical structure of the Thoracic Spine/ Rib Cage and the Scapular is completely representative of the anatomical function.
3) The Rotator Cuff
The rotator cuff is a group of 4 muscles that sit atop the scapular and attach proximal on the humerus. These muscles are quite small in comparison to some of the prime movers of the upper body like the pec major, latissimus dorsi and deltoids. So it is quite easy to see why these muscles are not considered prime movers of the upper extremity. The rotator cuff does however have great control of the arthrokinematics (sliding, gliding rocking and rolling of the ‘ball’ on the ‘socket’) of the gleno-humeral joint throughout larger, more global movements patterns of osteokinematics. This is a result of the structure of their insertion points on the humerus:
Supraspinatus - wraps superiorly over the humeral head.
Infraspinatus - wraps posteriorly around the humeral head.
Teres Minor - wraps posteriorly around the humeral head inferior to the infraspinatus.
Subscapularis - wraps anterior around the humeral head.
These 4 small muscles almost completely encase the humeral head from every direction. They attach proximally, close to the joints center, meaning they have great influence on the accessory movements occurring alongside the more global movements. The rotator cuff is designed to control the stability of the gleno-humeral joint actively (this idea is discussed in last weeks blog post - What Makes a Joint Stable?). The anatomical structure of the Rotator Cuff is completely representative of the anatomical function.
There are an abundance of examples within the human body of how anatomical structure dictates anatomical function as that is a result of the demands placed on the human body over the course of evolution.