Internal Oblique
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A flat abdominal sandwiched between the external obliques and transversus; its fibers run at right angles to the external oblique and it twists the trunk to the same side.
Detail
The internal oblique is sandwiched between the external obliques and the transversus. Its muscle fibers run upward at right angles to the external obliques. It will twist the body toward the right if only the right side contracts. Together with the transversus abdominis (the inner abdominal muscles), the internal oblique helps the lower back: the greater the tone in these inner abdominal muscles, the greater the stability in the lower back, because they fix the thoracolumbar fascia that holds the psoas, quadratus lumborum, and erector spinae in place.
Key facts
- Sandwiched between the external obliques and transversus.
- Fibers run at right angles to the external oblique.
- One side only: twists the body toward the same side (right contraction → twist right).
- With the transversus, stabilizes the lower back via the thoracolumbar fascia.
Connections
- Abdominals (Midsection) — group it belongs to.
- External Oblique — opposite fiber direction; paired in rotation.
- Transverse Abdominis — co-stabilizes the lower back.
- Erector Spinae (Sacrospinalis) — held in place by the fascia the obliques fix.
- Abdominal Work — Common Misconceptions (Gym & Class Setting) — the "sit-ups make the back strong" myth this mechanism actually explains.
Sources
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Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.