Enter the Infrasternal Angle (ISA)

infrasternal angle training

What do we have to actually measure the spine? Motion palpation of intervertebral segments? Multiple systematic reviews have shown poor reliability, and no better than chance. How about from a static palpation? Again, very poor reliability. I just recently finished a 4 day unembalmed cadaver dissection, and it was amazing to see the depth we are trying to palpate. Think you’re releasing psoas by pushing through the abdomen? Try again… just massaging the colon.

Pelvis Function

How about appendicular measurements to decide about spinal position? A backdoor approach to surmise spinal position. Here we run into issues. What if the shoulder has laxity or the hip has some sort of congenital malformation? Our limbs have their own problems. We need to find an area of the body that can give us information about spinal position, and has restricted constraints. Specific clinical decisions can then be made on how to approach.

(ISA) Infrasternal Angle

Enter the infrasternal angle. This is an area where we can objectively measure and make quick changes. Now, I’m definitely not the first person to write or talk about ISA. Shirley Sahrmann has been on this for quite some time. Bill Hartman and Zac Cupples have been writing about this measure. Take a look at all of their work.


Before we start talking about ISA, there must be an understanding about healthy breathing mechanics and pressure.

Altering pressure drives how we breathe, move, and interact with our environment. Hiding behind the majority of movement dysfunctions is a pressure problem. The diaphragm is the king inspiratory muscle. When the diaphragm descends (inhalation), the intrapleural pressure decreases and the abdominal pressure rises. With lung volume increasing, a pressure gradient allows air to be driven into the lungs. This asymmetry or gradient of pressure (high to low) is what drives air movement. We can also think of Boyle’s Law. Where volume and pressure are inversely proportional at a constant temperature. On a side note, all movement is driven by asymmetry/gradient and we see this throughout the natural world.

Normal Breathing Patterns

Digging a little deeper into pressure, air will move more easily into the lower lobes versus the apical (upper) portion. Again, this is a pressure issue. Less volume in the apical portion, more volume in the bottom. Easier for air to travel where there is less resistance. Again, air pressure moves from high to low. Why does this matter? Our skeleton will take the shape of the pressures in our body. Ever wonder why a powerlifter looks a lot different than a marathon runner? Of course, the mechanical demands are different, and the resulting change in peripheral tissue occurs. BUT, peripheral tissue requires a pressure gradient to actually contract. The pressure demands are what drives the difference in appearance. Think eccentric activity as a low pressure (increasing surface area) and concentric as high pressure (decreasing surface area).

Pelvis mechanics

Quickly, here are the normal mechanics of the skeleton when a breath is taken in…

  • Ribs 1-7: pump handle (true ribs)
  • Ribs 8-12: bucket handle (false ribs)… Infrasternal Angle
  • Thoracic Spine: Increased kyphosis
  • Lumbar Spine: Decreased lordosis
  • Innominate: Anterior rotation
  • Sacrum: Counter-nutation
  • The pelvic floor descends correspondingly with the descending diaphragm. When the pelvic floor descends, the infrapubic angle (IPA) will decrease. The false ribs should bucket handle, allowing for a descending diaphragm. With inhalation, IPA and ISA angle are opposite (normal). What happens when they are matching? Full respiration cannot occur and movement dysfunctions result.

Measuring The ISA

The ISA is measured using the xyphoid as the vertex. Drop your fingers in between and measure the angle between ribs and fingers. We would want 90 degrees. Anything less than will be considered narrow, and anything wider will be considered wide. Each have their own treatments and recommendations.

Why the ISA?

Keeping it simple, less degrees of freedom, can be measured, and it can be changed. By knowing the ISA, we can make inferences on respiration, diaphragm position, and axial skeleton curves. Also, if we have an impact on diaphragm position, consider all the other systems we may be influencing. Big changes can occur.

Wide ISA’s

If we look at the anatomy, a wide ISA will lose leverage of internal oblique’s and transverse abdominis as exhalation muscles. The false ribs are driven in a position of inhalation and the diaphragm is flattened. In order to reverse this position, we need to drive external oblique’s to straighten the false ribs and allow for IOs and TA to regain the leverage they need to be adequate exhalation muscles.

What else might we see with a wide ISA? The normal mechanics of inhalation with the sacrum are a counter-nutation and ilium anterior rotation. With a wide ISA, we will have a wide Infrapubic angle, and a posterior rotation of the innominate. With a wide IPA there is lack of ability to descend the pelvic floor and the thoracic diaphragm is already in a flattened state. Guts are smashed… constantly. How about the spinal curves? If the sacrum goes into nutation (wide IPA) then we must have changes up the spinal chain. Therefore, an increase in lumbar lordosis, thoracic kyphosis, and cervical lordosis.

How about hip measures? Without compensation, I would expect to lose hip abduction. Hip adductors are positioned concentrically, limiting my ability to abduct.

Here’s where it gets a little complicated, how could adduction be limited? Wide ISA = posterior rotation, but what if my sacrum continues to nutate and drives the entire pelvis into a position of anterior tilt. I still have a relative posterior rotation with respect to sacrum and innominate but entire pelvis is oriented anteriorly. This may limit hip adduction.

How about a narrow ISA?

Consider the dimensions of someone that is narrow. They are going to have more surface area in the A-P direction. As opposed to the wider ISA, which will be wider laterally. When my surface area increases in the A-P direction (narrow ISA), I get a straightening (external rotation) of the false ribs. The normal bucket handle mechanics invert, and the diaphragm gets into extreme flattening. The external obliques pick up leverage, compress the rib cage and pull everything forward. The diaphragm has overly flattened, and now the EO’s are in positon to facilitate these inversion mechanics.

What about the spine? Opposite of my wide peeps. We get a reduction of thoracic kyphosis, decrease in lumbar lordosis, anteriorly rotated inomminates, and a counter-nutated sacrum. Our IPA matches our ISA. I am exhaled at the bucket handle (narrow) and narrow at my IPA. With an exhale, my pelvic floor will stay drooping (narrow IPA) and diaphragm cannot find adequate length/tension position.

For the narrow folks, we need to decrease external oblique leverage and allow the internal obliques and transverse abdominis to IR the ribs. Take a narrow A-P ribcage and flatten out. In order to do this, we use the pecs. When I learned this the first time, it definitely made me double take. Have a quick look at the anatomy and see how this super cool muscle can be used to our advantage.

Here’s a summary…

Wide ISA

  • Wide Medial-Lateral
  • Wide IPA
  • Sacrum nutates
  • Spinal curves increased
  • Posterior rotation of innominates

Narrow ISA

  • Increased in A-P diameter
  • Narrow IPA
  • Sacrum Counter-nutates
  • Spinal curves decreased
  • Anterior rotation of inomminates

Wide Drills:

Arms overhead: create stretch on abs…

Quiet inhale

Forceful exhalation: need external obliques to ER overly bent ribs

Pause: 4-5 seconds at end of exhale, allowing time for diaphragm to re-dome

Narrow Drills:

All 4’s

Diamond hand shape: move ground away… feel pecs

Quiet inhale

Soft exhale: fog the glass

Pause 4-5 seconds at end of exhale, allowing time for diaphragm to re-dome


Most common presentation is narrow on the left and wide on the right. Due to our normal asymmetries, we tend to have an ascended right pelvic floor and wider right infrapubic angle. The left side is the opposite. Descended pelvic floor, narrowed infrapubic angle.

For these folks, go after the narrow first. Try to create the soft/fogging exhale while activating left pec. See what shakes out and then go after the wide side.

The big picture stuff is to give the person the variability required for what they care about. Go after ISA and see what shakes out.

NFL Outside Linebacker Lamarr Houston

Having an objective measure of diaphragm position is a game changer.

Full Video

3 thoughts on “Enter the Infrasternal Angle (ISA)

  1. larry cooper says:

    Mike, thanks for the presentations. I have seen Bill Hartman and others speak on this topic but everyone has a slightly different way of presenting it. I find that helpful in understanding it better. I have a question vis-a-vis the different presentations you presented. I personally have a wide ( based on the testing that I have had done by other PT’s ) ISA. In that scenario, you said that all curves are increased. However, in my case, I have a very flat T spine and straight C spine. Can you explain that presentation to me please??

  2. Mike Kay says:

    Hi Larry. Thanks for the great question. I would guess you have a compensation on top of the wide ISA. So… you are pushing air into anterior chest and extending through mid/upper spine. This will flatten the areas where you’re flat… you have essentially taken t2-7 and bent backwards, prob w military posture in cervical spine. Just strategies to get air in… commonly seen in populations that have lifted heavy weights… ie power lifting posture w bench press. Start w wide ISA resets… make sure you ISA is dynamic and then can work w back to wall and reaching for a chair… quiet pecs and breathe into the posterior mediastinum. Let me know if this makes sense. Thanks again for the question.

  3. Mario Saldivar says:

    Mike, first of all, thanks for posting. Its a great presentation. You mentioned hard exhale vs soft/fogging. When do you use one over the other.

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