Rethinking the origin and insertion points

Pectoralis major (sternal head)



Think about this muscle with me for a second. What are the actions of this muscle?

• Internal rotation
  
• Horizontal adduction
  
• Flexion
  
• Extension
  
• Adduction
  

All of these are occurring at the glenohumeral (shoulder) joint.

But wait a minute. Let's think about positioning. You may notice two antagonistic movements on that list, namely flexion/extension. No srsly dats freal. You may be thinking something slightly resembling the following picture:



But either movement is entirely possible given the right conditions, which in this case would mean positioning of the arm. If your arm is overhead, your pec can pull it down (extension), or when your arm is at your side your pec can pull it up (flexion).

That is why we need to rethink the origin and insertion sites as ATTACHMENT sites.

The actions I have listed above are what you learn in school. I took them right out of my notes from the anatomy class I took.

But let's take it a step beyond what you learn in school...

Reconsider positioning. Instead of the arm overhead, what if the arm was FIXED? Completely still. Locked down. What then would move during the shortening of the muscle?

Answer: The rib cage.

Some may say this isn't possible, but if you've ever felt a rib cage, it is VERY movable.

The arm is fixed, so it pulls the sternum closer to that arm. This affects the position of the rib cage, and ultimately can affect many of the things connected to it.

And [sarcasm alert] surprisingly, it comes down the breathing. The pec can open up one side of the rib cage to allow more air to fill the lung. As you can imagine, some pretty nasty postural adaptations can come from this.

Ultimately, I would like you to not get locked into the notion that the insertion point has to be the one to move. The "origin" and "insertion" are both simply attachment sites for a muscle.