It’s true that Serratus Posterior Inferior is not the musclebrity that Serratus Anterior is. If you read up on yoga anatomy, or take yoga from a teacher who does, you’re most likely enamored with your Serratus Anterior (a crucial scapular stabilizer in almost every inversion and arm balance). That being said, I wouldn’t necessarily recommend writing Serratus Posterior Inferior off the A-list of muscles you should know about. Especially if you’re interested in practicing a breath control technique called the Yogic Complete Breath.
As practiced in the YTU method, a Yogic Complete inhalation takes place in two phases to maximize lung capacity. Phase one of this yoga breathing technique secures the rib cage (Enter Serratus Posterior Inferior. But more on that later!) so just the lower belly swells until the lungs are about 75% full. In phase two, the yogi permits the rib cage to lift upward filling the lungs the remaining 25% of the way. Try it! Bring your spine into a neutral position. Fully exhale. On inhalation, only let your belly puff out. ¾ of the way through let your ribs puff up as well. Exhale passively. Repeat several times and observe how this pranayama boosts lung capacity.
How does this work? In phase one of the Yogic Complete inhalation, the diaphragm (a domed muscle that horizontally divides your trunk into its thoracic and abdominal cavities) flattens downward toward its inferior attachments on the lowest 6 ribs and lumbar vertebrae, packing your abdominal organs deeper down and clearing space for your lungs to swell. Your belly bulge is evidence of organ displacement as you take an abdominal inhalation. Phase two completes the process with a thoracic inhalation, during which the ribs puff upward and outward further augmenting space for the lungs to engorge. [This rib expansion is also assisted by the diaphragm’s attachments to the ribs.] It’s phase one that we’re concerned about with regards to the importance of Serratus Posterior Inferior and its role in bracing the rib cage to leverage a deeper descent of the diaphragm.
Like all muscles, the attachment sites of Serratus Posterior Inferior determine its function. Its serrated strips connect from the spinous processes (the jagged topography of your spine felt through the skin of your back) of vertebrae T11-L2, and reach upward and outward to ribs #9-#12. Seen from a posterior view of the skeleton, both the right and left Serratus Posterior Inferior muscles form a shallow V that overlay the lowest floating ribs. SPI’s function is to anchor ribs #9-#12 downward toward its attachment on the spinous processes below so that the ribs don’t elevate during the first phase of the Yogic Complete Inhalation. But why would we want to secure the ribcage during phase one?
Imagine you have wedged yourself between a heavy dresser and a wall with the hopes of shoving the dresser out into the middle of the room with your legs. With your back against the wall, you harness the complete force of your legs, including the rebounding force, which, because of the wall behind you gets redirected back into the dresser. To put it another way, imagine how much harder it would be to move the dresser if while pushing with your legs, both the dresser and the wall moved. The dresser would only move ½ the distance! In this metaphor, the diaphragm is the pusher wedged between the rib cage and the abdominal organs. Its job is to move the abdominal organs down and out of the way to make space for the lungs above to inflate. Just like the role of the wall, in stage one of a Yogic Complete inhale, we want the ribcage to remain stable so that the diaphragm’s descent downward isn’t diminished by the ribcages ascent upward. Serratus Posterior Inferior (along with help from its synergist friends) cancels out the ribcages inclination to elevate. This maximizes the influx of breath during stage-one of the Yogic Compete inhale, making stage two, or the thoracic inhalation, icing on the oxygenation cake.
For this reason, Serratus Posterior Inferior deserves some recognition and respect. Please, next time you hear somebody fawning all over Serratus Anterior, take a moment to remember and appreciate Serratus Posterior Inferior too. It IS inferior with regards to its position on the body map, but certainly NOT with regards to its role in enhancing the pranayama technique, Yogic Complete Breath, or in Serratus Posterior exercises. Your diaphragm descends deeper and your body’s cells receive more oxygen because of it.
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