This is an excerpt from Conditioning for Dance by Eric Franklin.
Efficient breathing is important in any peak performance activity and needs to be considered part of any conditioning routine for dance. The most important muscle for breathing is the diaphragm, which consists of a large muscular dome with a flat central tendon. It is attached to the lower end of the sternum, to the lowest six ribs, and to the spine via two muscular extensions called crura. The crura are arranged vertically on the front of the spine and help to pull down the diaphragm on inhalation. The diaphragm weaves into the fibers of the transverse abdominal muscles and has various connections to thoracic and abdominal organs such as the liver. The diaphragm has three major openings: one for the esophagus, one for the aorta, and one for the vena cava.
Although we can experience the diaphragm as the most important muscle for breathing, in a sense every muscle in the body supports our inhalations and exhalations. In deep breathing during exercise, the body functions as a giant bellows involving a myriad of muscles in the breathing process. Likewise, every tense muscle, even if it is located in the foot, jaw, or hand, reduces breathing capacity.
A popular method to improve alignment and to create strength in the abdominal muscles is to pull them inward and upward toward the spine. This strategy may make you look thinner for the time you are pulling in, but it also makes breathing more difficult and reduces joint flexibility by increasing the body's general tension level. It also reduces the stability of the lumbar spine. To better understand why pulling in the abdominal muscles is not beneficial to breathing, take a brief look at the evolution of the diaphragm.
Fish don't have a diaphragm. They breathe by letting water pass through the gills and extracting oxygen, which transfers directly into the bloodstream. Land animals cannot function with gills, because air would not pass through the gills with enough pressure to be extracted. A land animal with gills would have to be running at 150 miles per hour all the time to be able to breathe! So the first land-dwelling animals evolved a system to suck air into the newly evolved lungs by expanding the ribs. This system, however, had a drawback: As the ribs expanded, the air was pulled into the space created by the widening ribs, but so were the abdominal organs. So, a connective tissue sheath was developed at the bottom of the lungs. This sheath acted as a barrier to keep the organs out of the space destined for the incoming air.
As animals became more complex, this sheath became a muscle that could push the organs down during inhalation, expanding the lungs to a greater degree. For this pushing to happen efficiently, one problem had to be solved: where to put the displaced organs. In a fish, ribs are all along the spine from head to tail, so there is no place for displaced organs to go. In mammals, the ribs reach down only to the twelfth thoracic vertebrae. The lower ribs were replaced by the abdominal muscles that function as elastic rib replacements in conjunction with diaphragmatic breathing. The vestiges of the actual ribs are the transverse processes of the lumbar and cervical vertebrae, which now serve as important attachments for spinal muscles.
The abdominal muscles owe their existence to breathing, not to pelvic positioning or alignment. They carry much of the weight of the organs in four-legged animals (and crawling babies). If you pull the abdominals inward, you will push the organs against the diaphragm, which is then not able to properly move downward during inhalation. The diaphragm and abdominal muscles are antagonists--when the one lengthens, the other shortens. You may want to sacrifice some breathing for a leaner look and the illusion of strength, but by doing so you create other problems.
The crura of the diaphragm have fibrous connections to the most important hip flexor muscles, the psoas major and the iliacus, known collectively as the iliopsoas. If you pull the abdominal wall inward, you not only have trouble breathing, but you also lose some of the power and flexibility in the hip joint, spine, and shoulder girdle. If you think you need to correct your pelvic alignment by pulling in the abdominal wall, it may be time to instead look at improving organ tone, balancing the joint movement in the pelvis, spine, and legs, and conditioning the iliopsoas and deep lumbar spinal musculature.
On exhalation the abdominals help push the organs back up into the diaphragm, helping the lungs to expel air. The shallow breathing resulting from holding the abdominals reduces their natural conditioning; the deeper you breathe, the more the abdominal muscles condition, 24 hours a day. This superior abdominal exercise also provides you with more energy than a holding strategy. Also, when you hold in the belly, you compensate by breathing with the upper chest, raising your center of gravity and making you less stable for turns and balances. Upper chest breathing tends to make you feel anxious and tense, hence the need to concentrate on abdominal breathing.
The abdominal organs actually aid inspiration by using their weight to pull the diaphragm down. Expiration is aided by the lungs, which retract elastically on exhalation and help to pull the diaphragm upward. Therefore, organ-assisted breathing is impeded by habitually pulling in the belly (Schiebler, Schmidt, and Zilles 1997).
Once you have gained deep pelvic strength and balance, you will have more flexibility and better pelvic alignment, feel less high-strung, and even have flatter-looking abdominal muscles.
This is an excerpt from Conditioning for Dance.