Chest Anatomy: Understanding the Space Between Muscles and the Mediastinum

What is the space in between the chest?

The "space in between the chest" refers primarily to the sternum (breastbone), which serves as the central attachment point for the pectoralis muscles, and anatomically, the mediastinum – the central compartment of the thoracic cavity housing vital organs.

Understanding the "Space" – A Muscular Perspective

When individuals inquire about the "space in between the chest," they are often referring to the visible indentation or "cleavage" that can develop between the left and right pectoralis major muscles, particularly in well-trained individuals. This appearance is shaped by several anatomical and training factors:

• The Sternal Attachment: The pectoralis major muscle, the largest and most superficial chest muscle, originates from two main heads: the clavicular head (from the collarbone) and the sternocostal head (from the sternum and rib cartilages). The sternum, or breastbone, forms the central bone of the chest wall, acting as the primary anchoring point for the sternocostal heads of both pectoralis muscles. The "space" is essentially where these two large muscle masses meet and attach to the sternum.
• Muscle Belly Development: The degree of "separation" or depth of the "line" observed is influenced by the thickness and fullness of the inner portion of the pectoralis major muscle bellies. As these muscles hypertrophy (grow in size) through resistance training, their mass can create a more pronounced delineation around the sternum.
• Genetics: Individual genetic predisposition plays a significant role in muscle belly shape, insertion points, and overall muscle fullness. Some individuals naturally have a wider or narrower sternal attachment, which can influence the perceived "gap" regardless of training intensity.
• Body Fat Percentage: A lower body fat percentage will make muscle definition, including the separation between the pectoralis muscles, more apparent. High body fat can obscure muscle detail.

The Anatomy Behind the Appearance: The Mediastinum

Beyond the muscular aesthetics, the true anatomical "space" in the center of the chest is the mediastinum. This is not an empty void but a critical anatomical region within the thoracic cavity. It is bordered by the sternum anteriorly, the vertebral column posteriorly, and the pleural sacs (containing the lungs) laterally.

The mediastinum is further subdivided and contains numerous vital structures:

• Heart: The primary organ of the circulatory system, located centrally within the mediastinum, slightly to the left.
• Great Vessels: Major blood vessels such as the aorta, pulmonary artery, vena cavae, and their branches.
• Trachea: The windpipe, extending from the larynx into the thoracic cavity, where it branches into the bronchi.
• Esophagus: The muscular tube that connects the pharynx to the stomach, running posterior to the trachea.
• Thymus Gland: An immune organ, particularly active in childhood, located in the anterior mediastinum.
• Lymph Nodes: Part of the lymphatic system, involved in immune function.
• Nerves: Including the vagus and phrenic nerves, which control various physiological functions.

The mediastinum serves as a protective compartment for these vital organs, cushioning them and allowing for their functional movements (e.g., heart contractions, lung expansion).

Training for Chest Development and Definition

While genetics dictate the ultimate shape and insertion of your pectoralis muscles, strategic training can maximize their development and enhance the appearance of the "space" between them. The goal is to build overall pec mass and strength.

• Compound Movements: Exercises that involve multiple joints and muscle groups are foundational for overall chest development.
  • Barbell Bench Press: Excellent for overall pec mass, especially the sternal head.
  • Dumbbell Bench Press: Allows for a greater range of motion and independent limb work, potentially enhancing inner pec activation.
  • Incline Press (Barbell or Dumbbell): Targets the upper chest (clavicular head) but contributes to overall pec fullness.
• Isolation Movements: These can help target specific areas and enhance definition, though they won't "create" new muscle insertions.
  • Dumbbell Flyes: Focus on the stretch and contraction of the pectoralis major.
  • Cable Crossovers: Provide constant tension throughout the movement, allowing for a strong squeeze at the peak contraction, which can emphasize the inner chest. Varying cable height can target different fibers.
  • Pec Deck Flyes: Similar to dumbbell flyes but with a fixed path, often allowing for a strong peak contraction.
• Full Range of Motion: Always strive for a full range of motion in your exercises to ensure complete muscle activation and development.
• Progressive Overload: Consistently increasing the challenge (weight, reps, sets, time under tension) over time is crucial for muscle hypertrophy.

Beyond Aesthetics: The Importance of a Strong Chest

Developing strong chest muscles extends far beyond aesthetic appeal. The pectoralis muscles play crucial roles in:

• Pushing Movements: Essential for daily activities like pushing open doors, lifting objects, and pushing oneself up.
• Breathing: The pectoralis minor, located beneath the pectoralis major, assists in elevating the ribs during forced inspiration.
• Shoulder Stability and Movement: They contribute to various shoulder joint actions, including adduction (bringing the arm towards the body), flexion (raising the arm forward), and internal rotation.
• Posture: Balanced strength between the chest and back muscles is vital for maintaining good posture and preventing imbalances that can lead to shoulder impingement or rounded shoulders.

Key Takeaways

The "space in between the chest" refers to both the visible line formed by the development of the pectoralis major muscles around the sternum and, anatomically, the vital mediastinum. While training can significantly enhance the muscular definition and fullness of the chest, the fundamental structure of the sternum and the contents of the mediastinum remain constant. A comprehensive approach to chest training, focusing on compound movements, proper form, and progressive overload, will not only contribute to a well-developed physique but also enhance functional strength and overall health.