Synergies among the abdominal muscles depend on the trunk flexion task:

Palpate the anterior costal margins while someone performs:

1. a sit-up (open chain; sternum moving toward relatively stable pubis)

2. a reverse trunk curl (closed chain trunk movement as, for example, when a person is supine with the hips and knees in flexion, then flexes the lumbar spine, approximating the pubis and LE toward the relatively stable ribcage)

While the rectus abdominus is active for both of these trunk flexion tasks, one or the other of the abdominal oblique muscles predominates, depending on the task. You can determine which of the abdominal obliques is more active by observing the person's infrasternal angle (the angle formed by the anterior costal margins).
	Detailed diagrams of these fan-shaped muscles, their lines of application, and their attachments to the ribcage are available in Kendall, McCreary, & Provance (1993, p.150). The angle increases or widens when the internal obliques, acting bilaterally, are more active than the external obliques.
	The angle decreases or narrows when the external oblique's activity predominates.

Sit-up (open chain)

The sit-up produces a synergy between the rectus abdominus and the internal oblique. We observe that "the ribs flare outward and the infrasternal angle is increased. These movements are compatible with the action of the internal oblique" (Kendall, McCreary, and Provance, 1993, p. 152).

Trunk curl (closed chain)

Tasks that produce or control movement of the pelvis on the stable ribcage, like a closed-chain trunk curl, a posterior tilting of the pelvis, or "leg lowering" (Kendall, McCreary, & Provance, 1993, pp.154-156) involve a synergy between the rectus abdominus and the external oblique. The external oblique attaches to the ribcage at an angle that produces a telltale narrowing of the infrasternal angle.

---

Reference: Kendall, F.P., McCreary, E.K., & Provance, P.G. (1993). Muscles: Testing and function (4th ed.). Baltimore: Williams & Wilkins.

---

Last updated 2-20-01 ©Dave Thompson PT
return to PHTH 7243 / OCTH 7242 lecture schedule