This post, minus the last paragraph, is a short excerpt from my book An Introduction to the Spine.
There is only one muscle connecting the arms to the spine, the latissimus dorsi –also known as the lats. The latissimus dorsi are massive muscles that connect all the way down to your pelvis. So while just this one muscle connects the arm to the spine and pelvis, it is enormous (latissimus means broad). Anatomically there is an interesting twist to the latissimus dorsi- the insertion is literally twisted when the arm is down by your side. It untwists when the arm lifts which increases both its ability to stretch and generate force. It can pull the body up for climbing or depress the arm against resistance. Another way of looking at it is the latissimus dorsi were our swinging from the trees muscle.
Let’s go back to evolution and the shoulder. When the first creatures slithered out of the primordial muck to move along the earth, arms developed and the full weight of the body sat on the shoulder and “arm”. As creatures evolved and moved up into the trees to swing from branches our entire body was hanging from the arm. So we have moved from compression to suspension and finally freedom as now the arm hangs from the shoulder, free to move in any and every direction.
In our minds two main things are restricting this freedom— a tucked pelvis, and the age old instruction “take your shoulders back”. This is the essence of what is known as military posture as well as Mom’s classic lament, and unfortunately from my perspective an essential instruction of most yoga teachers. All three of these disrespect the amazing freedom the shoulder allows us.
The arm is meant to hang from the shoulder. This is key to upright posture and especially key to optimal functioning. If your pelvis is tucked under the shoulder joint is restricted. Feel free to try it. Tilt the top of your pelvis forward a bit and see if you feel the hang of the arm. Tuck under and feel it disappear. The same restriction occurs when we jam the arms back to “stand up straight” for better posture. Good posture is determined by a free swinging arm. You can’t have one without the other.
The latissimus dorsi along with the psoas major and piriformis, are the only muscles connecting the extremities to the spine. As we will see in subsequent posts, the latissimus is critical to backbending in yoga as it has a role in extending the spine that is often lost due to improper alignment.