Sometimes at social gatherings, people will ask me to fix their posture, AKA. "Tell Us what we're doing wrong, right now!" This is what I tried to tell them, albeit only somewhat successfully.
As musicians, movers, and teachers, most of us have been taught "good posture" markers, like sit/strand up straight, shoulders back, head upright, suck in the belly. Ironically, many of these cues will not yield good posture at all, but will hide the body's deficiencies through overcorrection. For most of us, this was continue out into our daily activities, including yoga, weight lifting, running, etc. and yields rib thrust or rib flare. What is this, you ask? Take a look at this image below of Katy Bowman, biomechanics movement goddess.
Take a look at the three difference variations of posture (there are infinite variations- these are just a few). Which one looks "the best" to you? Number three?- in which the body is upright and the shoulders back? Or Number Two, in which the spine looks slightly rounded or kyphotic? Also look at the vertical plumb line between the three images- only number two has the head stacked over the heels. In both image one and three, the head is trailing forward of the heels. Which image do you most resemble? The "best aligned" posture of these three is not number three, but number two, in which the body's natural spinal curves are preserved, and the head, ribs, and pelvis are all in a unified line. Number three reveals the rib thrust, which can be defined as the ribs moving forward of the pelvis which then does gnarly things to the lumbar (low back) spinal vertebrae. This is separating an aesthetic marker (shoulders back, chest forward) from a true body alignment marker (ribs and head over hips and heels).
Time out: What are forces? Has it been awhile since you studied physics? (For me, over a decade, so force = mass x acceleration) Fear not- gravity (a force) is still working with you. There are other forces (non Star Wars related) working on you as well in movement, including tension, compression, shear, and torsion. Go read about them here!
So even if this whole force thing isn't making sense, you can see that the ribs are shifting forward of the pelvis, right? What that does is compress the lower vertebrae, as well as make for some unhappy soft tissues, a dysfunctional core (can lead to diastasis recti, especially when pregnancy and post natal issues are coming up ), and a possibly dysfunctional pelvic floor (incontinence, pain, etc.) amongst other things. (See this woman on the right? Does she have "good posture" or is she just masking her body's dysfunction by pushing her ribs forward and throwing her shoulders back?
Most of us have some degree of rib thrust, but it will manifest more often when wearing high heels, because the body is trying to mask the inherent distortion that high heels creates for the spine. (Gentlemen, most likely your dress shoes have a heel, so you are not immune to this conversation)
Let's review- in the rib thrust, the bottom most ribs are shifting forward of the pelvis, which draws the scapula back and creates the illusion of good posture (upright body, broad collarbones). It is in fact masking the issues at hand, which are lack of mobility in the thoracic spine, and possibly hyper-kyphosis, as seen in the image on the right. So what can we do about the over-slump? Start to work on thoracic mobility, shoulder mobility (with stable ribs), and keeping the bottom-most ribs down.
So first, do some exploration, and see where you fall in the spectrum- where are your ribs hanging out?
Here's some more reading about rib thrusting!