We tend to think that the pivot point for our arms originates at our shoulders at the sides of our bodies. Unlike our legs, which connect directly to the spine through the pelvis, our arms connect indirectly to our core and spine through the front of the rib cage. The compressional architecture of the Shoulder and Arm translates loading through the Clavicle and Manubrium to the 1st and 2nd ribs and then to the Spine. This “Shoulder Girdle” mechanism connects in compression with the CORE of our body only at the articulation of the Clavicle and Manubrium [#1 in the image at right. Click on image for a full-size view]. The Manubrium is Latin for Handle or Haft, which is a good description of its role as the Handle for the Shoulders, Arms, and Hands. The tensional connection to the Locomotive Core is through a broad web of soft tissue connecting to the shoulder, extending from our waist to the back and sides of our heads.
The key concept here is that the Arm pivots at the Sternum at the front of the rib cage. The connection of the Clavicle to the Shoulder at the Shoulder Blade is a second pivot that, by conjoining with the Sternum/Clavicle articulation, allows the Arm to cross the midline of the body. There are many layers of soft tissue connecting the Core and Shoulder Girdle in tension, enabling a free range of motion in a semi-spherical, three-dimensional space. Because the range of motion of each Arm is independent, the spherical volume for both arms combined is ~3/4 of the total spherical reach of the arms.
Our usual perspective, based on the arm visibly connecting with the body at the shoulder, is that we have a lower and upper limb. When considered from its anatomical mechanics, we have a lower, middle, and upper arm, which ends at the Sternum. It is challenging to habituate to manipulating the arm from the pivot at the Sternum. With some practice, one comes to perceive the Manipulation Core of our bodies as a discrete functional unit, not dissimilar from the feel of using the tools that we use.
The Rotator Cuff of the Shoulder, where the Arm connects, rests in it’s neutral position on top of and to the side of the rib cage, stabilized by the Shoulder Blade at the back, and the Clavicle at the front. The arm can extend forward of this neutral position by allowing the Shoulder Blade to slide over the Ribs.
The Manubrium is the topmost bone in our Sternum. This bone plays a significant role in our posture and movement dynamics:
1) It is the interface between our LOCOMOTIVE CORE and our MANIPULATION CORE.
2) The ribs that attach to the Body of the Sternum below the Manubrium form the walls of our rib cage, while the 1st and 2nd ribs attaching to the Manubrium form the roof.
3) The triangular shape of the Manubrium facilitates translating compressional loading from one arm to the other arm, across to ribs 1 and 2, or down into the rest of the rib cage.
4) The articulation between the Manubrium and the Body of the Sternum form a “hinge”, enabling flexion of the Spine when we breathe and move.
When the Manubrium is “locked” in the “hinge-open” position as shown in the diagram it cannot flex as we move thereby depressing the rib cage and increasing the angle of the vertebrae at the top of the back relative to the gravity line of our body. The angle of this vertebrae establishes the angle at which the neck vertebrae must “stack” to support the head, bringing the head forward of the gravity line and requiring increased tension in the neck and shoulders to support the weight of the head. With the exercise described in the Core Buoyancy section full flexibility of the Sternum/Manubrium hinge can be restored.
The centerlines of the load distribution of our body mass cross over at the Manubrium. This can be seen in the graphic on the page on Stance.
It is quite common for someone who feels the need to protect their heart to roll their shoulders forward. When the Core is properly aligned our Heart Space is exposed