Pranayama is the fourth limb of Pantajali's Eight Limbs of Yoga. Pranayama refers to yogic techniques for breath control that enable the free flow of prana, the life force, accessed through the breath. Anatomically, breathing is accomplished through the contraction and relaxation of muscles, including the intercostal muscles between the ribs, but mainly due to the action of the diaphragm. The diaphragm is a dome-shaped muscle that separates the chest or thoracic cavity from the belly or abdominal cavity. Inhalation occurs when the muscle fibers of the diaphragm contract, exhalation as they release. With inhalation, the diaphragm moves downward thereby producing a suction-type influx of air into the lungs. By maximizing the space in the abdominal cavity, the diaphragm can move downward to a greater extent, consequently increasing the volume of air we take in as we inhale. In yoga, increasing the space in the abdomen is accomplished through the elongation of the spine. Any rounding of the spine results in a loss of space in the belly as organs are "crunched" together and less space is available for the diaphragm to move downward. Many practitioners whom practice pranayama in sukhasasana or easy pose or cross-legged round their backs, their knees up in the air. Elongation of the spine can be achieved by bringing the hips above the knees. Sitting on a cushion accomplishes this, and for some, a stable tripod can be formed with the sitz bones and knees as the three grounding points. Sitting up on two blocks with the knees on the mat accomplishes the same thing and is wonderful for having that nice long spine and opening up the belly for more room for that diaphragm. Sitting in a chair allows for a nice long spine, one of the many benefits of chair yoga. Patanjali said that a sitting position should be steady and comfortable. Introversion realized through pranayama is enhanced by less awareness of the physical body.