The fitness industry is famous for having a “flavor of the week” or a hot-button topic that people gravitate towards because it might just be the next best thing.
Reflexive Performance Rest (RPR), Functional Range Conditioning (FRC), Postural Restoration Institute (PRI), and Dynamic Neuromuscular Stabilization (DNS) are all fun three-letter systems that have become exceptionally popular in the last five years.
The latest and greatest appears to be the idea of the respiratory biases and how it relates to the Infrasternal Angle.
TL;DR: The Infrasternal Angle will give us a starting indicaition in terms of where you likely are restricted in terms of breathing and what your skeleton joint position biases are.
What is the Infrasternal Angle?
The Infrasternal Angle (ISA) is a representation of an individual’s respiratory strategy.
There are only two: inhalation and exhalation. Expansion and compression.
The ISA reflects the strategy the body is using to most easily direct air in and out via the path of least resistance. The infrasternal ribs are the most pliable (“changeable”) in the entire axial skeleton, so they are easily reformed because they don’t attach on the sternum.
It helps bias us toward certain skeletal positions and joint actions.
There are two starting points for an ISA: A wide or narrow orientation.
Wide Infrasternal Angle
A wide infrasternal angle (usually over ~110 degrees), is reflective on an individual who has a compressed axial skeleton. Their posterior (back) ribcage cannot easily expand with air as it normally should because it is is overly compressed.
This means:
- They’ve created a strategy to expand their ribcage in the front (ISA) in order to take in air. This person is likely a “belly breather” with a diaphragm that is descended and unable to function well for inhalation.
- Because their “pump handle” ribs can’t expand, their lower “bucket handle” ribs are up and air goes forward, causing those ribs to be elevated.
As a result, their ribcage is biased towards a state of compression, and their pelvis toward a state of sacral nutation with the pelvic innominates in extension, adduction, and internal rotation.
When we exhale, this is a visual of what happens in our pelvis. Wide ISAs (compressed individuals) are biased here.
The wide ISA individual typically has exaggerated spinal curves due to the excessive lumbar arch (extension) via the forward orientation of the sacrum which drives the lower spine forward. This is your typical lower/upper crossed syndrome individual.
Narrow Infrasternal Angle
A narrow ISA (usually under ~110 degrees but usually much less) is an individual who:
- Has overly-inflated “pump handle” ribs anteriorly in the front (fun fact – their diaphragm is especially descended. When the diaphragm is extremely flattened/descended, the line of pull of the muscle on the ribcage changes and it sucks the abdomen inward and upward which helps pull together the ISA (De Troyer, 1997).
- Have a ribcage toward a state of expansion posteriorly, and their pelvis toward a state of sacral counter-nutation with the pelvic innominates in flexion, abduction, and external rotation.
When we inhale, this is a visual of what happens in our pelvis. Narrow ISAs (expanded individuals) are biased here.
These individuals tend to have a flatter spine due to the reduced lumbar flexion via the orientation of the sacrum driving the lower spine back.
This is a spectrum. Imagine a range of a huge male to a thin female. Generally, bigger individuals tend to become more compressed (exhaled ribcages that can’t get air in them). Think Powerlifters.
Thinner people tend to be more expanded (inhaled ribcages that can’t get air out of them). Think marathon runners.
So why does this matter?
It tells us where you can’t get air – A restriction of air usually means a restriction of range of motion. We can see this in my breathing post.
If we know what your ISA is, we have a general idea of where you could be limited. And what we can give you in order to restore function and posture.
If you’re a wide ISA, you need more upper ribcage expansion and lower ribcage compression. We can achieve this via something like a Wall Supported Downward Reach.
If you’re a narrow ISA, you need more upper ribcage compression and lower ribcage expansion. We can achieve this via something like All four breathing.
This isn’t new, it’s just reframing factual anatomy concepts we (the field of Kinesiology) already knew.
In reality, people tend to compensate and are rarely just simply a narrow or wide ISA. If we can’t access expansion or compression mechanics, we tend to compensate into positions that help us better achieve those mechanics we lack. That is beyond the scope of this post, but hopefully this gives you some insight into how individual differences play a role in posture.