The term intra-abdominal pressure refers to the pressure that forms inside the abdominal region when breathing is stopped. The abdominal region is that region
anatomical bounded above by the diaphragm and below by the pelvis, the bones of the pelvis, in particular the upper strait.
Under the abdominal region is the pelvic region, a portion between the upper and lower strait of the pelvis; the thorax is located above. The abdominal region is the largest cavity in the human body and does not contain direct communication with what is outside the body. The fact that the abdomen is not in direct relationship with the outside allows to keep the most important viscera safe and also allows an easier formation of a pressure difference.
Intra-abdominal pressure and physiology
In order to be in a condition of increased pressure it is necessary that the volume is reduced due to the contraction of some tissues. We obviously refer to the muscles, in particular the diaphragm and the various abdominal muscles. Nature never creates anything without reason, this also applies to the phenomenon of intra-abdominal pressure. The effects of this are:
- Greater efficiency in efforts
- Contribute to breathing
- Maintain postural attitude
Let's start with the simplest: breathing. The exchanges of air with the outside allow us to stay alive and make almost everything in our body work properly. The respiratory dynamics are as follows: the diaphragm contracts and the volume of the thoracic cavity decreases, at the same time the rib cage expands, further increasing this negative pressure.
Since the lungs are in communication with the outside, an increase in their volume causes the air to enter the respiratory tract and fill the thoracic volume. The lungs expand together with the rib cage because they adhere to it thanks to a particular lining: the pulmonary pleura.
The pulmonary pleura is a kind of double membrane with a cavity in which there is a negative pressure. Since this pressure is negative, it is as if it were sucking the outer wall of the lungs close to the inner chest wall.
The role of the abdomen is quite evident: this cavity is in direct contact with the expanding thoracic cavity and, since it has no communication with the outside, the abdomen must "change shape" to accompany the movements of the diaphragm. For this reason, when you inhale the abdomen tends to dilate.
In order for the abdomen to expand, the muscles that can contract it must relax: the abdominals and lumbar muscles (and other muscles). If the abdominals contract and at the same time inhale and then retain the inhaled air, the intra-abdominal pressure increases.
When you exhale, the diaphragm relaxes and returns to its dome shape which "squeezes" the chest cavity together with the contraction of other muscles in the chest area. At the same time, the movement of the diaphragm is accompanied by a contraction of the abdominal muscles.
If these muscles contract then the abdominal volume is pushed upwards and the return of the diaphragm is therefore facilitated. In addition, the abdominals lower the last ribs reducing the chest volume. If there were no intra-abdominal pressure, breathing would be much less efficient.
Furthermore, by exploiting intra-abdominal pressure, the diaphragm and abdominal muscles contribute to maintaining a correct posture of our body. The diaphragm in fact contributes to the increase in intra-abdominal pressure, and this exerts a thrust force on the entire volume of the abdominal cavity.
This means that the intra-abdominal pressure generates forces that push the spine upwards, partially counterbalancing the force of gravity which instead tends to compress it. Proper breathing, and therefore a physiological abdominal pressure, contribute to the decompression of the spinal column.
In order for these forces to be created in the abdominal cavity, these forces must not be dissipated and therefore the abdominal wall musculature must be strong. Generally the back of the abdomen is strong enough, while the front and sides are not as strong. For this reason, the abdominal muscles are essential for correct posture and good spinal health.
To move a weight it is necessary to leverage on something stable. For this reason we are unable to walk on water or swim in the air. We are able to move because our weight force is counterbalanced by an equal and opposite thrust from the ground. Fluid materials do not have this behavior, solids do.
The human body is a particular material: it can be assimilated to a solid for some components while others have a more elastic behavior. Still others, however, have a behavior similar to a fluid. The entire abdominal cavity is lined with several muscle layers.
When a muscle contracts, it allows to transmit the forces it is subjected to and becomes rigid. The contraction of the abdominal muscles is both tonic and phasic. Tonic means that it is a lasting effect
in time. Phasic refers to a contraction of limited duration and, in general, of moderate intensity.
There are reflex gestures for which automatically when an effort is made, the intra-abdominal pressure tends to increase with a phasic contraction of these muscles. This reflex occurs automatically because the body knows how to be stronger.
This principle is the same that leads us to perform certain exercises incorrectly: the body automatically tends to put itself in the most advantageous position by recruiting the strongest muscles or that it is more used to using. Labyrinth-cervical reflexes have also been observed, i.e. they respond to stimuli from the vestibular system and cervical muscles, involved in balance.
Following a stimulus in a certain direction, an automatic contraction of the abdominal muscles occurs, both to prevent movement and to make the action of the muscles involved in movement more efficient.
This increased efficiency is the result of the rigid behavior of the abdominal cavity. Sufficient intra-abdominal pressure allows forces to be transmitted from the bottom up and also provides a solid foundation on which to perform a movement.
All exercises involving axial loading are affected by intra-abdominal pressure. The most common are squats and deadlifts but there is also talk of military press and flat bench press up to barbell curls. At even more evident levels we can consider the gymnasts while performing the various skills to the rings: if their abdomen were not a rigid section there would be no way that the body would remain perfectly aligned.
For this reason, when performing a squat there is a need for a valid support of the abdominal muscles. If this contraction were not there, the chest would collapse onto the lower body as the abdomen would not be able to support the compression.
For what has been said previously, the stronger the intra-abdominal pressure, the more efficient it is possible to be and, therefore, to be stronger.
Unfortunately, the capacity of the human body has a limit: at a certain point the intra-abdominal pressure is such that the rigidity of the muscle tissue is not sufficient. At this point the lining of the superficial muscle bundles is torn and it can also occur that the viscera are pushed by the same pressure into the torn muscle tissue.
When the viscera are engaged in this protrusion it is called an abdominal hernia. In general, this injury occurs when the deep muscles are not strong enough to support the more superficial muscles which are therefore affected. Once again we are talking about the abdominal transversus. This muscle is the deepest of the 4 abdominal muscles and is by far the one with the most toned contraction.
This means that good strength in this muscle gives both aesthetic feedback and benefits in training and health. Since the traverse muscle is the first to respond to intra-abdominal pressure, it therefore plays a primary role in the formation of the latter.
If this muscle is strong, the pressure cannot overcome the resistance of the muscle tissue. This muscle is the most predisposed to keeping the viscera in its place, both in physiological and stress conditions. A weakness of this leads to an overload of the more superficial abdominal muscles with unpleasant outcomes.