The lungs are passive extensible organs located within the thoracic cage, the movement of air through them is possible due to the respiratory muscles which drive environmental air in and out the airways.
Indeed, the breathing mechanism can be imagined of as a pump in which the contraction of muscles brings the expansion and compression of the thorax. Thus, changes in the volume of this cavity produce changes in the pressure within it, and as a consequence, this creates the gradient that moves air in and out the cavities. At rest the average adult takes 10 to 15 breaths per minutes, with a volume of about 0.5 litres, producing a minute ventilation of 7.5 l /min.
Essentially, all muscles that attach to the rib cage have the potential to generate a breathing action (figure 1), but we can easily divide these muscles as inspiratory muscles which expand the thoracic cavity causing inhalation and expiratory that compress the thoracic cavity causing exhalation (McConnell 2011).
The principal muscle of inspiration is the diaphragm, a domed sheet muscle that separates the thoracic and abdominal cavities. It attaches to the lower ribs and the lumbar vertebrae of the spine. When it contracts, the dome sheet moves downward into the abdominal cavity like a piston. This movement increases the volume of the thoracic cavity, creating a negative pressure. Its contraction also induces the lower ribs to move upward and forward, which also increases thoracic volume.
But the diaphragm is also part of the Deep Front Line (DFL) which starts from the underside of the foot, passing up behind the bones of the lower leg and behind the knee to the inside of the thigh. From here the major track passes in front of the hip joint, pelvis, and lumbar spine, while an alternate track passes up the back of the thigh to the pelvic floor and re-joins at the lumbar spine. From the psoas-diaphragm interface, the DFL continues up through the rib cage around and the thoracic viscera, ending on the underside of the viscerocranium (figure 2) (Myers 2001).
As stated by Myers, the DFL plays a significant role in the body’s support especially in lifting the inner arch, stabilising each segment of the legs, supporting the lumbar spine from the front, maintaining the chest while allowing the expansion and relaxation of breathing, balancing the neck and the head.
Other primary inspiratory muscles are the external intercostal muscles located in the area between adjacent ribs. Their contraction moves the ribs upward and outward (similar to the raising of a bucket handle) and also serves to stabilise the rib cage, to make it more rigid, as well as to help in small rotation of the trunk.
Finally, the scalene and sternocleidomastoid muscles are attached to the top of the sternum, to the upper two ribs, and clavicle, to the cervical vertebrae and mastoid process. When these muscles contract, they lift the top of the chest contributing to the inhalation.
The principal muscles of expiration are the rectus abdominis, the transversus abdominis and the internal and external oblique muscles. When these muscles contract, they pull the lower rib margins downward, and compress the abdominal compartment, raising its internal pressure.
But, is worth to mention that resting exhalation is a passive process brought about by the recoil of the lungs and rib cage at the end of inspiration. Thus these muscles only come into play as breathing muscles during exercise or in forced breathing manoeuvres (including coughing and vomiting).
- Cover: Thorax and Abdomen of Leonardo da Vinci 1507
- McConnell, A., 2011. Breathe strong, perform better.: Champaign, IL: Human Kinetics, c2011.
- Myers, T. W. L. M. T., 2001. Anatomy Trains: myofascial meridians for manual and movement therapists / Thomas W. Myers; forewords by Leon Chaitow, Deane Juha.