Abdominal Wall Muscles: Compressors of the Abdominal Cavity

The abdominal wall comprises layered muscles that compress the viscera, stabilize the trunk, and assist respiration. Their coordinated action generates intra-abdominal pressure (IAP) for defecation, urination, parturition, vomiting, and heavy lifting.

Primary Compressors (Anterolateral Wall)

Muscle	Origin	Insertion	Action
Rectus abdominis	Pubic symphysis/crest	Xiphoid process, costal cartilages 5–7	Trunk flexion, compresses abdomen, posterior pelvic tilt
External oblique	Ribs 5–12	Iliac crest, linea alba (via aponeurosis)	Compresses abdomen, ipsilateral flexion, contralateral rotation
Internal oblique	Thoracolumbar fascia, iliac crest, inguinal ligament	Ribs 10–12, linea alba	Compresses abdomen, ipsilateral flexion/rotation
Transversus abdominis (TrA)	Ribs 7–12, thoracolumbar fascia, iliac crest	Linea alba, pubic crest (conjoint tendon)	Deepest compressor; increases IAP, stabilizes lumbar spine (feedforward activation)

Posterior & Superior Contributors

• Erector spinae (iliocostalis, longissimus, spinalis): Extend spine; co-contract with abdominals for spinal stability (not primary compressors).
• Quadratus lumborum: Lateral flexion, fixes 12th rib (respiration), stabilizes lumbar spine.
• Diaphragm: Dome-shaped; contraction ↓ dome → ↑ thoracic volume (inspiration). Simultaneously ↑ IAP (piston effect) → assists abdominal compression during Valsalva.
• Pelvic floor (levator ani, coccygeus): Inferior closure of abdominal canister; co-contracts with TrA/diaphragm for continence and IAP regulation.

Clinical Relevance

• Core stability: TrA feedforward activation precedes limb movement; delayed in low back pain.
• Diastasis recti: Linea alba widening; managed with TrA training, hypopressives.
• Hernias: Weakness in aponeurotic layers (inguinal, umbilical, incisional).
• Respiratory training: Diaphragmatic breathing + TrA activation (e.g., “dead bug,” plank progressions).

The abdominal cylinder (diaphragm roof, pelvic floor base, muscular walls) functions as a hydraulic amplifier—essential for spinal unloading and visceral function.