11.4 Axial Muscles of the Abdominal Wall, and Thorax

The abdominal wall, thoracic, and pelvic floor muscles form a dynamic system crucial for core stability. These muscle groups work together to protect organs, support posture, and enable movement in daily activities and exercise.

Understanding how these muscles function is key to grasping their roles in breathing, organ support, and body mechanics. Their coordinated actions impact everything from maintaining continence to powering athletic performance and preventing injuries.

Abdominal Wall Muscles

Structure of abdominal muscles

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• External obliques
  • Outermost layer of abdominal muscles runs inferiorly and medially from lower ribs to iliac crest and linea alba (midline raphe)
  • Compresses and supports abdominal viscera (organs), flexes and rotates trunk (sit-ups, twisting motions)
• Internal obliques
  • Middle layer of abdominal muscles runs perpendicular to external obliques, superiorly and medially from iliac crest to lower ribs and linea alba
  • Compresses and supports abdominal viscera, flexes and rotates trunk in opposite direction of external obliques (stabilizes spine during rotational movements)
• Transversus abdominis
  • Innermost layer of abdominal muscles runs horizontally around the abdomen, attaching to lower ribs, iliac crest, and pubic bone
  • Compresses and supports abdominal viscera, stabilizes the spine and pelvis (acts like a corset)
• Rectus abdominis
  • Paired, vertical muscles on the anterior abdominal wall separated by the linea alba, attaches to pubic crest and xiphoid process
  • Flexes the trunk (crunches), compresses abdominal viscera (increases intra-abdominal pressure)
• These muscles collectively form the abdominal wall, which provides protection and support for internal organs

Thoracic Muscles and Breathing

Breathing mechanics of thoracic muscles

• Diaphragm
  • Dome-shaped muscle separating thoracic and abdominal cavities, attaches to lower ribs, sternum, and lumbar vertebrae
  • Contraction flattens the diaphragm, increasing vertical dimension of thoracic cavity (inhalation)
  • Relaxation allows the diaphragm to rise, decreasing vertical dimension of thoracic cavity (exhalation)
• Intercostal muscles
  • External intercostals
    • Fibers run inferiorly and anteriorly from rib to rib
    • Contraction elevates the ribs, increasing anteroposterior and transverse dimensions of thoracic cavity (inhalation)
  • Internal intercostals
    • Fibers run inferiorly and posteriorly from rib to rib
    • Contraction depresses the ribs, decreasing anteroposterior and transverse dimensions of thoracic cavity (exhalation)
• Breathing mechanism
  1. Inhalation: Diaphragm contracts and flattens, external intercostals contract and elevate ribs, increasing thoracic cavity size and decreasing pressure, causing air to flow into lungs
  2. Exhalation: Diaphragm relaxes and rises, internal intercostals contract and depress ribs, decreasing thoracic cavity size and increasing pressure, causing air to flow out of lungs
• These muscles are collectively known as respiratory muscles and play a crucial role in the mechanics of breathing within the thorax

Pelvic Floor and Perineal Muscles

Pelvic vs perineal muscle functions

• Pelvic floor muscles
  • Levator ani
    • Broad, thin muscle forming the floor of the pelvic cavity, attaches to pubic bone, ischial spine, and coccyx
    • Supports pelvic organs (bladder, uterus, rectum), resists intra-abdominal pressure (prevents prolapse)
  • Coccygeus
    • Triangular muscle extending from ischial spine to coccyx
    • Supports the posterior pelvic floor, assists in stabilizing the sacrum and coccyx
• Perineal muscles
  • Superficial perineal muscles
    • Bulbospongiosus, ischiocavernosus, and superficial transverse perineal muscles
    • Support and control external genitalia (erection, ejaculation) and urethral sphincter (urinary continence)
  • Deep perineal muscles
    • External anal sphincter and external urethral sphincter
    • Control opening and closing of anus (fecal continence) and urethra (urinary continence)
• Roles in excretory and reproductive functions
  • Pelvic floor muscles
    • Maintain continence by supporting bladder and rectum, prevent prolapse
    • Aid in childbirth by relaxing and stretching during delivery, support uterus during pregnancy
  • Perineal muscles
    • Control voluntary urination and defecation by regulating external urethral and anal sphincters
    • Support and control external genitalia during sexual function (erection, ejaculation, orgasm)
• The pelvic diaphragm, formed by these muscles, plays a crucial role in maintaining pelvic organ position and function

Core Stability and Functional Integration

• Abdominal wall muscles, thoracic muscles, and pelvic floor muscles work together to provide core stability
• This integrated system helps maintain posture, support the spine, and facilitate efficient movement
• Core stability is essential for everyday activities, athletic performance, and injury prevention