Abdominals (Midsection)
Sources and review status
Review pending — this text hasn’t been signed off by a reviewer yet.
Historical reference: its only source is the original reference manual, and some of it may be out of date. Not used for AI answers until re-sourced.
See this lesson’s sources0% read · 2 min left
Four muscles — rectus abdominis, external oblique, internal oblique, transverse abdominis — that flex, rotate, and laterally flex the trunk and compress the abdomen.
Detail
The abdominals (Ellison, 1991) are: i) Rectus Abdominis ii) External Oblique iii) Internal Oblique iv) Transverse Abdominis
The walls of the abdominal cavity have no bones to support them, so three layers of muscle make up for the lack of strength. The abdominal muscles function as expiratory muscles as well as trunk flexors and rotators. Their actions:
- One-side (unilateral) contraction of both external and internal obliques causes lateral flexion of the trunk.
- Contraction of an external oblique and the opposite internal oblique causes trunk rotation.
- Bilateral contraction of both internal/external obliques compresses the abdominal cavity (exhalation, defecation, urination).
- Trunk flexion is produced by the rectus abdominis and the internal and external obliques.
ROM from the abdominals chart: the muscles flex the spine through a full ROM of 70 degrees (smaller-ROM and compression exercises are used for posture/spinal stabilization), laterally bend the torso ~30 degrees, and rotate the torso ~40 degrees with hips fixed. Note there is no "upper" or "lower" abdominal muscle. Exercises: crunches, side bends.
Training framing: resistance training strengthens the abdominals and provides functional stability to the midsection — but ab exercises alone will not reveal them under a layer of fat. This is the spot-reduction principle (Spot Reduction (Myth)) applied specifically to ab training: if fat covers the midsection, diet and cardiovascular exercise are what's needed, not more ab work. Hip-flexor involvement should also be reduced to a minimum during abdominal training (Iliopsoas, Lumbar Lordosis & Sit-Up Lower-Back Caution).
Key facts
- 4 abdominal muscles: rectus abdominis, external oblique, internal oblique, transverse abdominis.
- 3 muscle layers compensate for the lack of bony support.
- Unilateral obliques → lateral flexion; opposite external+internal obliques → rotation; bilateral obliques → compression; rectus + obliques → trunk flexion.
- ROM: spinal flexion 70°, lateral bend ~30°, rotation ~40°.
- No separate "upper"/"lower" abdominal muscle.
- Resistance training strengthens/stabilizes the midsection, but does not spot-reduce fat covering it — diet and cardio are what's needed for visibility.
- Minimize hip-flexor involvement during ab training.
Connections
- Rectus Abdominis / External Oblique / Internal Oblique / Transverse Abdominis — the four muscles.
- Flexion — trunk flexion.
- Antigravity Musculature — abdominals as reflex antigravity muscles.
- Crunches / Side Bends — training exercises.
- Spot Reduction (Myth) — why ab exercises alone won't reveal the abs.
- Iliopsoas, Lumbar Lordosis & Sit-Up Lower-Back Caution — minimize hip-flexor involvement.
- Atrophic Weakening Muscle Groups — the abdominals as one of the muscle groups prone to atrophic weakening.
- Abdominal Work — Common Misconceptions (Gym & Class Setting) — common myths about training this group (potbellies, rep count, sit-ups).
Sources
Exercise guides
Keep this lesson, or ask how it applies to you.
Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.