The External Intercostals Elevate The Rib Cage During

The external intercostals, a group of muscles nestled between our ribs, play a pivotal role in the mechanics of breathing, specifically in elevating the rib cage during inhalation. Their contribution to this essential physiological process is both significant and elegantly designed. This article delves into the anatomy, function, and clinical relevance of the external intercostal muscles, providing a comprehensive understanding of their role in respiration and overall health.

Anatomy of the External Intercostal Muscles

The external intercostals are one of three layers of intercostal muscles, the others being the internal and innermost intercostals. Understanding their anatomical arrangement is crucial to appreciating their function.

Location: As their name suggests, the intercostal muscles reside between the ribs. The external intercostals specifically span from the tubercles of the ribs posteriorly to the costochondral junctions anteriorly (where the ribs meet the cartilage). They are present in all intercostal spaces except for the uppermost spaces near the vertebral column.
Fiber Direction: The fibers of the external intercostals run in an oblique direction, similar to that of hands placed in pockets – downwards and forwards. This specific orientation is key to their function in elevating the rib cage.
Attachments: Each external intercostal muscle originates on the inferior border of the rib above and inserts onto the superior border of the rib below. This attachment pattern allows them to act as a bridge between adjacent ribs.
Layers: The external intercostals are the most superficial layer of the intercostal muscles. Beneath them lie the internal intercostals, whose fibers run in a perpendicular direction, and then the innermost intercostals.
Neurovascular Bundle: The intercostal neurovascular bundle (containing the intercostal nerve, artery, and vein) runs along the costal groove on the inferior border of each rib, deep to the internal intercostal muscles but superficial to the innermost intercostals.

The Mechanics of Rib Cage Elevation During Inhalation

The primary function of the external intercostals is to assist in inspiration, the act of drawing air into the lungs. They achieve this by elevating the rib cage, which increases the volume of the thoracic cavity. Here’s a step-by-step breakdown of the process:

Muscle Contraction: When stimulated by the intercostal nerves, the external intercostal muscles contract.
Rib Cage Elevation: Due to their oblique fiber orientation and attachments, the contraction of the external intercostals pulls the rib below upwards and outwards. This is often described as a "bucket-handle" motion, where the ribs swing upwards and outwards, increasing both the lateral and anterior-posterior diameters of the thorax.
Thoracic Cavity Expansion: As the ribs elevate, the thoracic cavity expands, increasing its volume.
Pressure Gradient: This increase in volume leads to a decrease in pressure within the thoracic cavity (intrapleural pressure). The pressure inside the lungs (intrapulmonary pressure) also decreases, becoming lower than the atmospheric pressure.
Airflow into Lungs: Because air naturally flows from areas of high pressure to areas of low pressure, air rushes into the lungs to equalize the pressure difference. This is how inhalation occurs.

The diaphragm, a large dome-shaped muscle located at the base of the thoracic cavity, is the primary muscle of inspiration. However, the external intercostals play a crucial supporting role, especially during forced or deep breathing.

Synergistic and Antagonistic Muscles

Understanding the interplay between different muscles involved in respiration provides a more complete picture of the process.

Synergistic Muscles: The external intercostals work synergistically with the diaphragm to achieve inspiration. Other muscles, such as the scalenes and sternocleidomastoid, may also assist in elevating the rib cage during forced inspiration.
Antagonistic Muscles: The internal intercostals, particularly the internal intercostales, are considered the primary antagonists to the external intercostals. While the role of the internal intercostals is debated, they are generally believed to assist in expiration (breathing out) by depressing the rib cage, decreasing the volume of the thoracic cavity. Other muscles, like the rectus abdominis and oblique muscles, also contribute to forced expiration.

Factors Affecting External Intercostal Function

Several factors can impact the function of the external intercostal muscles, affecting breathing efficiency and overall respiratory health.

Age: As we age, the elasticity of the rib cage and the strength of the respiratory muscles can decrease, potentially impacting the effectiveness of the external intercostals.
Posture: Poor posture can restrict rib cage movement and limit the ability of the external intercostals to function optimally.
Obesity: Excess weight, particularly around the abdomen, can put pressure on the diaphragm and restrict rib cage expansion, hindering the action of the external intercostals.
Respiratory Diseases: Conditions like asthma, chronic obstructive pulmonary disease (COPD), and pneumonia can impair lung function and increase the workload on the respiratory muscles, including the external intercostals.
Musculoskeletal Conditions: Conditions affecting the ribs, spine, or surrounding muscles, such as scoliosis, kyphosis, or intercostal muscle strain, can directly impact the function of the external intercostals.
Neurological Disorders: Neurological conditions like spinal cord injury or muscular dystrophy can affect the nerves that control the external intercostal muscles, leading to weakness or paralysis.

Clinical Significance and Related Conditions

The external intercostal muscles are clinically relevant in various respiratory conditions and injuries.

Intercostal Muscle Strain: This is a common injury, often caused by sudden twisting or overstretching. Symptoms include pain, tenderness, and difficulty breathing. Treatment typically involves rest, ice, compression, and pain relief medication.
Rib Fractures: Fractured ribs can cause significant pain and limit rib cage movement, impacting the function of the external intercostals. Pain management and respiratory support are crucial in these cases.
Pleurisy: Inflammation of the pleura (the lining of the lungs and chest cavity) can cause sharp chest pain, especially during breathing. This pain can limit rib cage movement and affect the function of the external intercostals.
Pneumothorax: This condition occurs when air leaks into the space between the lung and the chest wall, causing the lung to collapse. The resulting pressure on the chest wall can impair the function of the external intercostals.
Intercostal Neuralgia: Damage or irritation to the intercostal nerves can cause chronic pain along the ribs, potentially affecting the function of the intercostal muscles.
Surgical Procedures: Thoracic surgeries, such as lung resections or chest wall reconstructions, can directly impact the intercostal muscles and their function.
Respiratory Muscle Weakness: Conditions causing generalized muscle weakness, like muscular dystrophy or Guillain-Barré syndrome, can affect the strength of the external intercostals and impair breathing.

Assessment and Diagnostic Techniques

Assessing the function of the external intercostal muscles is important in diagnosing and managing respiratory conditions. Several methods can be used:

Physical Examination: A doctor can assess rib cage movement and palpate the intercostal muscles to identify areas of tenderness or muscle spasm. They can also observe the patient's breathing pattern to detect any signs of respiratory distress or abnormal muscle use.
Pulmonary Function Tests (PFTs): These tests measure lung volumes and airflow rates, providing information about the overall respiratory function. While PFTs don't directly measure the function of the external intercostals, they can help identify respiratory impairments that may be related to muscle weakness or dysfunction.
Chest X-ray: This imaging technique can help identify rib fractures, lung collapse, or other abnormalities that may be affecting the function of the external intercostals.
Computed Tomography (CT) Scan: A CT scan provides more detailed images of the chest, allowing for a more precise assessment of the intercostal muscles and surrounding structures.
Electromyography (EMG): This test measures the electrical activity of muscles, and can be used to assess the function of the intercostal muscles in cases of suspected nerve damage or muscle weakness.
Inspiratory Muscle Strength Testing: This involves measuring maximal inspiratory pressure (MIP) to assess the strength of the inspiratory muscles, including the diaphragm and external intercostals.

Strengthening and Rehabilitation Exercises

Strengthening the external intercostal muscles can improve breathing efficiency and overall respiratory health, particularly for individuals with respiratory conditions or muscle weakness. Here are some exercises that can be beneficial:

Diaphragmatic Breathing: This technique focuses on using the diaphragm as the primary muscle of inspiration, while minimizing the use of the accessory muscles (including the external intercostals). While it may seem counterintuitive to minimize their use, strengthening the diaphragm allows it to take on a greater workload, reducing the strain on the intercostals and allowing them to function more efficiently.
Segmental Breathing: This involves consciously directing air into specific areas of the lungs, such as the upper chest (to target the external intercostals) or the lower chest (to engage the diaphragm). This can help improve localized lung expansion and strengthen specific respiratory muscles.
Pursed-Lip Breathing: This technique involves breathing in through the nose and exhaling slowly through pursed lips. This helps to slow down the breathing rate and maintain open airways, making breathing easier and more efficient.
Resistance Training: Using devices like inspiratory muscle trainers (IMTs) can provide resistance during inhalation, strengthening the inspiratory muscles, including the external intercostals.
Yoga and Pilates: These practices incorporate breathing exercises and postures that can improve rib cage mobility and strengthen the respiratory muscles.
Stretching: Gentle stretching of the chest and intercostal muscles can help improve flexibility and reduce muscle tension, allowing for more efficient breathing.

It is important to consult with a healthcare professional or respiratory therapist before starting any new exercise program, especially if you have a respiratory condition or other health concerns.

Research and Future Directions

Research continues to explore the intricacies of the external intercostal muscles and their role in respiratory health. Areas of ongoing investigation include:

The precise contribution of the external intercostals versus the internal intercostals during different breathing patterns: Sophisticated imaging and electromyography techniques are being used to further elucidate the specific roles of these muscle groups during quiet breathing, forced breathing, and various respiratory maneuvers.
The effects of aging and disease on the structure and function of the intercostal muscles: Understanding how these muscles change with age and in the presence of respiratory conditions can help inform targeted interventions.
The development of new therapies to strengthen and rehabilitate the intercostal muscles: Researchers are exploring novel approaches, such as neuromuscular electrical stimulation and targeted exercise programs, to improve respiratory muscle function.
The role of the intercostal muscles in non-respiratory functions: Emerging research suggests that the intercostal muscles may also play a role in postural control and spinal stability.

Conclusion

The external intercostal muscles are essential components of the respiratory system, playing a crucial role in elevating the rib cage during inhalation. Their unique anatomical arrangement, fiber orientation, and synergistic action with the diaphragm enable efficient expansion of the thoracic cavity, facilitating airflow into the lungs. Understanding the function of these muscles, as well as the factors that can affect their performance, is vital for maintaining respiratory health and managing various clinical conditions. Through targeted exercises and rehabilitation strategies, it is possible to strengthen the external intercostals and improve breathing efficiency, enhancing overall well-being. As research continues to unravel the complexities of these muscles, we can expect further advancements in our understanding of their role in respiration and potential therapeutic interventions.