Skip to content What is the physiological engine behind our breathing, our vital capacity, and the control over our voice? The answer is the diaphragm. Although we control this muscle unconsciously (and sometimes consciously) every day, a complex world lies behind its anatomy and function.
In the article below, neurologist J.C.J. van Hemert dives deep into the science of the diaphragm. He explains how this muscle develops in the earliest stages of life, how its unique muscle fibers function, and how the diaphragm can be trained for strength and endurance.
A fascinating physiological foundation for anyone who wants to understand the power of controlled breathing to permanently overcome stuttering.
Before the diaphragm reaches its final or adult anatomical form, considerable structural replacements occur within the tissues from which it originates, particularly during the embryological stage. For instance, the central tendon (centrum tendineum), the central aponeurosis of the diaphragm, develops during the third week of embryonic life from a mass of mesodermal tissue. This is then termed the septum transversum; it is primarily located near the developing sternum and ribs.
A second component that forms the future diaphragm emerges during the seventh week. At this stage, the posterior portion of the diaphragm develops from the membranes of the thoracic and abdominal cavities. These fuse together to achieve a complete separation between the thoracic and abdominal cavities. The mesentery of the esophagus also participates in this process: specifically, muscle fibers grow from the posterior side of the septum transversum toward the esophageal mesentery, giving rise to the muscular pillars (crura) of the future diaphragm.
Between the ninth and twelfth weeks of embryological development, there is an ingrowth of muscle tissue into the septum transversum from the lateral sides—originating from the sixth and seventh thoracic myotomes. Along with this tissue, nerve fibers also grow inward, which will later form the sensory nerves (proprioception) of the peripheral portion of the diaphragm.
1 | Diaphragm
2 | Central tendon
3 | Lungs
4 | Heart
5 | Thorax
6 | Abdominal cavity
During the further development of the diaphragm, all previously mentioned components undergo a drastic positional shift. Specifically, the septum transversum begins to move downward, during which, around the fifth week, nerve fibers from the third, fourth, and fifth cervical (neck) segments of the spinal cord grow into the septum transversum alongside the heart. These nerve fibers form the future phrenic nerve (nervus phrenicus).
Due to the rapid growth of the posterior region—the dorsal side—of the embryo, a downward development of the diaphragm occurs, reaching the level of the thoracic somites during the sixth week. Around the ninth week of development, the diaphragm reaches its final position at the level of the first lumbar vertebra, where it attaches to the vertebrae and assumes its definitive form.
In its definitive form, the diaphragm exhibits a dome-shaped structure that separates the thoracic cavity from the abdominal cavity. Its shape, particularly that of the dome, varies significantly between maximal inspiration and maximal expiration. The fully developed diaphragm is a musculotendinous sheet, the margins of which consist of muscle fibers attached peripherally to the bony structure of the torso. The center consists of a central tendon (aponeurosis), which simultaneously forms the highest section of the diaphragm. It is attached superiorly to the membrane surrounding the heart (pericardium).
The muscle fibers present in the periphery are grouped into bundles attached to the sternum, the ribs, and the lumbar vertebrae. For example, the sternal part consists of two strong muscle bundles attached to the xiphoid process of the sternum. The dome-shaped structure of the diaphragm exhibits a subdivision into two sub-domes (hemidiaphragms). The costal part forms the left and right domes, originating from the inner surface of the lower six ribs and their costal cartilages. These muscle bundles interdigitate with the origins of the transversus abdominis muscle.
On the posterior side, aponeurotic bands (crura) originate from the upper three lumbar vertebrae, to which the muscular part of the diaphragm is attached. The aponeurotic bands on the left side originate from the upper two lumbar vertebrae. These two fibrous bands converge at the location where the aorta passes through the diaphragm (aortic hiatus). The length of the diaphragmatic muscle fibers varies from person to person. Furthermore, there are significant individual variations regarding the attachment points to the torso, which we will not address in further detail. Three large openings can be distinguished within the diaphragm, through which the aorta, the esophagus, and the inferior vena cava pass. Additionally, there are several smaller openings through which abdominal vessels and nerves pass.
Within the muscular portion of the diaphragm, three distinct types of muscle fibers occur: fast-contracting, easily fatiguable fibers (type IIb); fast-contracting fibers with greater resistance to fatigue (type IIa); and slow-contracting, fatigue-resistant fibers (type I). During low-intensity contraction, primarily type I fibers are recruited. As contraction progresses, type IIa fibers are also engaged, and during even stronger contractions, type IIb fibers come into action. The impulses responsible for this travel primarily via the alpha and gamma nerve pathways, but the regulation of these impulses takes place via the intercostal phrenic reflexes (via the sensory nerves of the diaphragm).
Studies have shown that the diaphragmatic muscle consists of approximately 60% type IIa fibers. Regarding inactivity and training, the muscle fibers of the diaphragm respond in a manner consistent with skeletal muscles. For instance, during endurance training, we see an increase in the percentage of type IIa muscle fibers, while the percentage of type IIb fibers decreases. Under the microscope, an increase in mitochondria and, indirectly, oxidative capacity is also observed. The reverse process can be witnessed when diaphragmatic activity diminishes.
Like all muscles in the human body, the respiratory muscles can increase in strength and endurance through training. The positive impact of diaphragmatic training has already been demonstrated in athletes and in patients with respiratory disorders, such as asthma and emphysema. The development of greater muscle strength is theoretically possible in several ways: A widely known principle in muscle physiology is that through limited elongation of a muscle (i.e., within physiological boundaries), the muscle is capable of developing greater force. With respect to the diaphragm, this implies that the higher the dome rises during exhalation, the more the muscle fiber length increases, resulting in a subsequently more powerful inspiration.
Furthermore, the muscle can also be consciously elongated by moving the ribs, to which the diaphragmatic muscle is attached, outward; this increases the length of the diaphragmatic muscle fibers. Under these conditions, we observe a physiological shift from type IIb to type IIa fibers. Additionally, there is the theoretical possibility of increasing diaphragmatic strength via inspiratory resistance training (by inhaling through a small aperture). This constitutes a dynamic-concentric resistance training to enhance the strength of the diaphragmatic muscle.
Resistance training alters a muscle. On the one hand, we see an increase in type IIb muscle fibers and a decrease in type IIa fibers (in contrast to the aforementioned possibility); on the other hand, hypertrophy of the muscle occurs—as with the previously mentioned possibility—meaning that the muscle thickens. In normal skeletal muscles, this hypertrophy is limited by impairment of the blood supply. However, this is not the case for the diaphragm, which maintains an excellent blood supply at all levels of contraction. The same applies to the heart.
The diaphragm derives half of its energy requirements from carbohydrate metabolism, primarily through the utilization of lactate, much like the myocardium. Consequently, the diaphragm possesses high resistance to anaerobic metabolic conditions (fatigue)—again, in contrast to skeletal muscles. Based on the results of experiments involving a paralyzed diaphragm, it can be stated that the ventilation-perfusion ratio is increased by enhanced diaphragmatic function. This is particularly true for the lower portions of the lungs, which constitute the best-perfused regions.
Although the mechanisms are not entirely clear, it must be concluded from experiments that respiratory muscle weakness (failure) is associated with abnormalities in lung volume and gas exchange. Furthermore, vital capacity is reduced in the presence of respiratory muscle weakness. Conversely, it stands to reason that an increase in the muscular strength of the respiratory muscles leads to a rise in vital capacity.
Due to possible reciprocal innervation of the diaphragm and the abdominal muscles, the abdominal muscles function as expiratory muscles during exhalation. Contracting the abdominal muscles helps the diaphragm to relax as much as possible. During inhalation, the abdominal muscles can also be considered accessory respiratory muscles when they relax. From this, it must be inferred that a persistently increased tone of the abdominal musculature diminishes the function of these muscles as accessory respiratory muscles. A risk of this exists when training focuses too heavily on the abdominal muscles.
Curious about how this respiratory muscle can be the physical cause of speech problems? Read Van Hemert’s clinical research on stuttering and the discovery of the faltering diaphragmatic muscle.
Embryonic Growth
The diaphragm develops between the 3rd and 12th week of pregnancy from various tissues and descends from the cervical region to the first lumbar vertebra during growth.
Anatomical Boundary
It is a dome-shaped musculotendinous sheet that separates the thoracic cavity (lungs and heart) from the abdominal cavity and contains three large openings for the aorta, esophagus, and vena cava.
Unique muscle fibers
The diaphragm consists of approximately 60% type IIa fibers (fast and fatigue-resistant). Much like the myocardium, the diaphragm—in contrast to skeletal muscles—possesses an uninterrupted, excellent blood supply and utilizes lactate for energy, ensuring immense resistance to fatigue.
Trainability
The respiratory muscle can be trained in both endurance (endurance training increases type IIa fibers) and pure strength (via inspiratory resistance training). The higher the dome rises during exhalation, or the further the ribs move outward, the greater the muscular strength during the subsequent inhalation.
Vital capacity
A more strongly trained diaphragm increases vital capacity (lung volume) and improves gas exchange, particularly in the well-perfused lower portions of the lungs.
The role of the abdominal muscles
Abdominal muscles support the diaphragm. Relaxation of the abdominal muscles aids in inhalation, while contraction assists during exhalation. However, an excessively high, constant tension (tone) on the abdominal muscles obstructs respiratory function.
Curious how these physiological insights translate into practice? Read here how this knowledge was applied in Van Hemert’s specific research on stuttering.
How the Method Works
Discover how we translate this physiological evidence into a practical speech technique. Explore our 5-step plan here.
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Scientific Validation
Read the comprehensive overview of all medical and scientific research regarding stuttering.
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Aftercare & Securing Results
Medical evidence is one thing, but how do we sustain it? Learn how our aftercare program prevents a stuttering relapse.
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Most people who stutter receive the same advice: you have to learn to live with it. At Del Ferro, we have taken a very different approach for nearly 50 years.
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Nearly everyone who stutters is told the same thing: you just have to learn to live with it. At Del Ferro, we look at this very differently.
After all, the key to fluent speech does not lie in your head, but in a specific physical action that traditional speech therapy often overlooks.
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