SIT UP STRAIGHT!!!?
People are often asked to sit up straight. This rarely has permanent effect because by the time someone needs to be reminded to do so, their body has adapted to be more comfortable in the slouching position. When they attempt to sit up "straight" they actually tighten the already over-shortened frontal muscles and tendons and this causes restrictions in the ease of breathing volume; tightening these muscles even slightly to make oneself more erect causes tightness in the entire upper body and reduces the ease of deeper breathing. We intuitively do not like this and soon adjust back to where it was easier to breathe. That is why most people that are advised to sit up straight, remain erect for only a few minutes before reverting to the former slouch where breathing is a little easier. Their breathing is still held back from being fully deep, easy and balanced. For them it becomes easier and more what they have become accustomed to. Often what we perceive as satisfactory is a lack of adequate understanding.  Extreme examples of this are called delusions.

Sitting for long periods of time are good opportunities to cause shortened frontal muscles. Lots of computer time, desk jockying or vehicle driving help create the posture that makes for semi-permanent shallow breathing. Many easy chairs and "airlessplane" seats invite suppressed breathing.

To repeat, you cannot HOLD yourself upright to achieve optimal breathing. By the time you might benefit from that, the muscles in the front of your body have become over-shortened from slouching and over-lengthened in the back.  Making the body more erect actually shortens those already too short muscles in the front. This causes a lessening of ease in breathing volume. and reduces the ease of deeper breathing; we intuitively do not like that. That is why most people remain erect only for a few minutes before reverting to their usual slouch.

SUBTLE SIGNALS
Poor posture causes or is a result of tensions in various parts of the body. If your knees or pelvis are locked up they block off energy flowing up and down your body, including your spine.  This reduces the depth of your easy breathing. Try standing perfectly balanced in both legs for 5 natural effortless breaths each and adjust your weight to more weight on one leg (wait 5 breaths), then both legs (wait 5 breaths) then the other leg (wait for 5 breaths) and experience the breathing going less deep and easy as you transfer your weight into one leg or the other. The point is that even this seemingly insignificant "posture" restricts your ease and depth of breathing and eventually your energy.  

ATTITUDE- EMOTIONAL ANATOMY
What ever your posture at any given moment in time, you can bet it is influencing your breathing and attitude. It is often easy to spot someone with a distorted or negative attitude by their slumped shouldered, bent over, weak kneed hang dog posture. Posture often shapes and forms attitude. Attitude can help create good or bad posture. 

BENT OVER POSTURE
Poor posture restricts the rise and fall of the breathing diaphragm. We also know from chiropractors and the study of the sushumna in pranayama that an overly bent or imbalanced spine restricts vital life force. Nerve force energy restrictions in the spine mean less life force into all the major organs fed by the spinal nerves and celiac ganglia, including your heart, lungs and brain. So now we have lessened nerve force in addition to reduced ease and depth of natural oxygenation and nervous system balance stemming from restricted or unbalanced breathing.

Have you ever noticed that people that sit up straight seem to be more alert? ........... They probably are.

ROUNDED SHOULDERS
Rounded shoulders restrict the expansion of the rib cage, diaphragm rise and lung volume.  Forcing the shoulders back and the the body to remain erect improves the situation somewhat but again, also tightens up those same problem rib, torso and neck muscles and restricts the deepest easiest breathing.  Great techniques and exercises to offset all that is in the #176 Fundamentals video  

Observe the pictures below for extreme examples of negative tensions, posturing and emotions.

Above left is a fully inhaled lung. The chest is flat or slightly rounded outwardly. Observe that there is usually more lung tissue in the back of the torso than there is in the front.  But due to poor posture it gets less easily accessed.
Note the right pic of exhaled, inward curve of the chest with the lungs greatly emptied (residual volume). When we exhale, the lungs and chest naturally collapse and less air is inside us.  The lungs have no muscle and are almost completely passive so when we go to breathe in again the ribs must allow for expansion of the lungs to the fullest. This allows the diaphragm to draw in the deepest fullest breath.  But the diaphragm must be of sufficient size (not strength) to draw in the maximum air.

TOO OFTEN LOOKING DOWNWARD
When you look downward you only see what is on the ground and your body adapts to being bent over.  This also limits natural vision exercising which needs varieties of depth and color. When you limit what you see you limit your moment to moment experience. This looking downward may be helpful for "deep thought" but it has a negative postural and attitudinal outcomes.

Note above the rigid hyper tense upper body and its suppression of the rib expansion. Try tightening every muscle in your body and then take a breath. Was it harder to breathe? Even neck or ankle muscles that are too tight will lessen one's ability to breathe easy. 

Taking a breath with collapsed or over-tight chest muscles is like trying to blow up a six liter balloon inside a two liter bottle.

CLOTHNGd
Restrictive clothing also impedes breathing and invites slouching. Brassiers, belts, tight pants and shorts inhibit breathing volume as the lung housing/rib cage and diaphragm excursion is restricted. 

An overly muscled upper body cannot easily expand. Stomach, chest and shoulder muscles are like the body of a man needing to carry or pull heavy loads and slouch slightly all the while. When the muscles set in cement he never gets to set the load down. His ribcage ALWAYS stays stiff and inflexible.  A heart attack, lung disease or malady caused from shortness of oxygen is waiting to happen. May be a key reason why many soon die after retirement.

In her book "Posture, Getting it Straight"  Janice Novak states that good posture can add up to ten years to your life. I suggest it is even more than that. Why do you suppose that is? 

AWARENESS
One needs to rapidly open the breathing up to make it larger and and in balance and with that you become more apt to notice changes in it from moment to moment. This optimal awareness comes from Optimal Breathing Development.

REMEMBER
You cannot HOLD yourself upright to achieve optimal breathing because  the muscles in the front of your body have become over-shortened front to back, side to side and in rotation from slouching. They have also been over-lengthened in the back from the same slouching. This slouching also develops weakened abdominal or over-tight back muscles.  You need to develop internal coordination and posture, from the inside out. The easiest most effective and lasting way to do that is to develop that which you can be most aware of on a moment to moment basis............ your breathing. 

WHEEL CHAIRS
Cause most to slouch and round the shoulders causing shallow, unbalanced breathing.

DEVELOPING OPTIMAL POSTURE. The more you develop your breathing the less likely you will be comfortable slouching.
Fundamentals of Optimal Breathing
or Attend The Optimal Breathing School