The Lats + Teres Major

In part two of my fascial layers blog, I wrote about aponeuroses being thick sheets of connective tissue that serve as attachment sites for some muscles.  One of those muscles that has a point of origin on the thoracolumbar aponeurosis is the very large, thin and superficial back and arm muscle known as the latissimus dorsi, or the lats.  This muscle is particularly interesting for a few reasons.  One, while most of its fibers are found covering the majority of the lower and middle back, it’s actually an arm mover.  The lats connect the pelvis to the upper arm bone to create medial rotation, adduction and extension at the glenohumeral joint, which is the ball and socket joint more commonly known as the shoulder joint.  The actions of the latissimus dorsi are easy to remember if you can remember that it’s the handcuff muscle.  If you were being arrested and handcuffed (and I hope you never are), your arms would be medially rotated, a little bit extended and also adducting across the midline behind you.  

The main actions of the lats are medial rotation, adduction and extension of the shoulder.  If you stand or sit upright with your arms relaxed heavily by your sides and turn the palm back behind you or spin your thumbs back behind you, that’s medial rotation of the shoulder.  If you reach both arms back behind you, that’s extension of the shoulder and if you then clasp your hands, bringing the arms together behind the back, that’s adduction.  (Side note: ABduction is when you take away from the midline. To abduct is to take away so it you take the arm or leg out to the side away from the body, that is abduction of the shoulder or hip.  ADduction is the opposite, it’s adding to the midline.  When the arms are out to the side and you bring them back down, that’s called adduction because you’re adding the arms back to the midline).  In addition to being able to move the arm, the lats also assist in lateral flexion of the trunk and can help extend the spine as well as tilt the pelvis anteriorly and laterally.  

HOW DOES A BACK MUSCLE MOVE THE ARM?

The lats have points of origin on the posterior iliac crest (top of the back of your hip bones), the thoracolumbar aponeurosis, the last few ribs (varies in cadavers), the spinous processes of last six thoracic vertebrae and in some people they cross the inferior angle of the scapula as well.  That is a huge origin and a lot of words.  All you really need to know is that it’s a large and superficial back muscle that originates on the pelvis, ribs, spine, fascia and sometimes the scapula so it can affect all of those things.  

The second interesting thing about the latissimus dorsi is that from its origin, it thickens around the armpit and wraps underneath to insert on the front of the upper arm bone in the space between the greater and lesser tubercles of the humerus called the intertubercular groove.  The lats form the posterior wall of the axilla, which is the fancy word for armpit.  You can feel for these more lateral fibers by taking your right hand to find the back flap of your left armpit and gently pressing your elbow in to your side to engage the fibers.  

A third interesting thing is that the lats have a helper, a smaller muscle that does exactly the same thing to help get the job done.  This muscle is called teres major, not to be confused with teres minor which is just above it and rotates the shoulder in the opposite direction.  Teres major originates on the inferior angle of the scapula (as the lats sometimes do too) and the lower third of the lateral border of the scapula.  Just like the lats, teres major wraps under the armpit to also insert on the front of the arm just distal to the intertubercular groove at the crest of the greater tubercle.  It doesn’t affect the trunk or the pelvis because it doesn’t cross them, but it does medially rotate, extend and adduct the shoulder.  Both the latissimus dorsi and the teres major are handcuff muscles.  

WHEN ARE THE LATS + TERES MAJOR CONTRACTING?

When you grab onto a pull up bar and pull yourself up, the lats and teres major are contracting (or shortening) which means the origin is working to bring the insertion toward it to complete one, some or all of the actions.  When the hands are clasped behind the back, the lats and teres major are also contracting as the shoulder is medially rotates, extended and adducted.  People with really developed lats may have a hard time clasping their hands behind their backs because of high motor tone and lots of dense muscle fibers.  

WHEN ARE THE LATS + TERES MAJOR LENGTHENING? 

Child’s pose with the arms stretched out in front is a nice stretch for the lats and teres major.  You can walk the arms and upper body off to one side to deepen the stretch on the other and vice versa.  Any time you are reaching the arms overhead, the lats are stretching.  

Fascial Layers, Part 2 + Anatomy of a Nerve

For the most part, fascia can be classified as either superficial or deep, with the superficial layer being just beneath the skin and the deep layer being, well, everything else. As far as the deep layer goes, the “everything else” can be classified as either meningeal fascia or visceral fascia. 

DEEP FASCIA

Joint capsules, ligaments, tendons and the three layers that weave their way through each muscle (epimysium, perimysium and endomysium) make up the deep fascia. Aponeuroses are included in this classification as well. An aponeurosis is a broad, thick sheet of connective tissue that serves as an attachment site for muscles. A big one is the diamond-shaped thoracolumbar aponeurosis (where the latissimus dorsi originates) that spans across the lower back. I explained in Part 1 that fascia is rich in lots of sensory receptors. I’ll go into detail about each of those in a later post, but one of the sensory receptors found in fascia is called a nociceptor, otherwise known as a pain receptor. I once came across a study showing that people with lower back pain had a thickening of the thoracolumbar fascia compared to those in the trial who didn’t suffer from any LBP. This makes sense because with pain receptors being spread all throughout the fascial layers, a thickening of the fascia would equal an increase in pain receptors.

VISCERAL FASCIA

The visceral fascia is the layer that surrounds the heart (specifically called the pericardium) and lungs (pleura) as well as the abdominal organs. It also suspends the organs within their respective cavities (thoracic or abdominal) by way of ligaments that are meant to hold the organs against the body wall as well as allow for necessary physiological movement like breathing, the heart beating and peristalsis. 

MENINGEAL FASCIA

This is the layer that surrounds the brain and the nervous system. To better understand this, we’ll look at the anatomy of a nerve. But first...

WHAT IS A NERVE?

A nerve is a structured pathway that allows for the transmission of impulses to and from the brain and nervous system. Nerves either have one type of neuron, in which case they are classified as sensory or motor nerves, or like most nerves they have both motor and sensory neurons and are called mixed nerves. Nerves are structurally very similar to skeletal muscle in that each nerve has three separate layers of fascia, just like each muscle. 

Let’s look at the structure of a nerve from superficial to deep. The outer fascial covering of a nerve is called the epineurium (translates to on the nerve). Inside of that, nerve fibers (also called axons) are bundled together the same way muscle fibers are bundled, the layer that surrounds each bundle of axons is called the perineurium (around the nerve). Each individual axon that makes up the bundle is also surrounded by its own layer of fascia, this is called the endoneurium (within the nerve). 

Quick Trapezius Overview

The trapezius muscle is huge and has fibers that run in multiple directions which gives it the ability to do lots of different things. As a whole, the trapezius originates at the middle portion of the base of the skull (external occipital protuberance and the medial portion of the superior nuchal line or you can just say middle of base of skull), it latches on to the big thick neck ligament and then grabs on to the spinous processes of C7-T12. It inserts itself on the lateral third of the clavicle, the acromion and the spine of the scapula. 

Probably everyone knows where their upper traps are because nearly everyone complains about how tight they are. What tends to happen is the upper traps get overdeveloped while the middle and lower traps get weak. Learning how to use the middle and lower traps to retract and depress the scapulae rather than trying to force them down with the the upper traps (which don’t have the pulling capacity to do that at all) can be helpful in reducing shoulder and neck tension/pain and headaches. 

The upper fibers of the trapezius muscle are very involved in head and neck movement. The traps laterally flex the head and neck, rotate (turn) the head to the opposite side (so if you look to the right, your left upper trap is contracting) and when both the left and right upper trapezius muscles fire together, they extend the head and neck. In addition to that they also elevate the scapula and they upwardly rotate it as well. 

Moving down to the middle fibers... it makes sense that they retract and stabilize the scapula because the origin is on the spine. If the middle fibers of the trapezius which are perpendicular to the spine contract, the origin will pull the insertion toward it which draws the shoulder blades in toward the spine. Think about someone placing their fingers between your shoulder blades and then you gently pinching them by drawing the shoulder blades together. This activates the middle fibers of trapezius.

If you look at the fibers of the lower trapezius you can see that they run in a more downward direction. This gives them the leverage they need to depress the scapula or draw the shoulder blades down the back. 

Try this: Place your right hand on your left upper trap and see how it feels. Then relax your arms and shake them off a little bit. From there, gently gather the shoulder blades together, and then imagine sliding just the bottom tips of the shoulder blades down and into your back pockets. Maintain that and take the right hand to the left upper trap again and see how it feels softer with the support coming from the middle and lower fibers below.