Bodyweight
Holding a straight line against gravity makes the abdominals work isometrically to stop the spine sagging, rather than to move it.
| Muscle | Role | Action |
|---|---|---|
| Rectus abdominis | Primary | Flexes the trunk and braces the abdomen |
| Transversus abdominis | Primary | Compresses the abdomen and stabilises the lumbar spine |
| External oblique | Secondary | Rotates the trunk to the opposite side and bends it to the same side; compresses the abdomen |
| Internal oblique | Secondary | Rotates and laterally flexes the trunk; compresses the abdomen |
Primary muscles are the prime movers; secondary muscles assist or stabilise. The balance shifts with grip, stance, range of motion and technique.