The oblique muscles
The oblique muscles make up two of the three layers that create our abdominal walls. The third layer is the popular transverse abdominus muscle.
What do the names of the external and internal oblique muscles, mean?
Internal means inside and oblique comes from the Latin word obliquus, which means a slanting orientation. So, the name internal obliques refers to the location and shape of the muscle. The slanting direction refers to the direction of the fibers of this muscle. The fiber direction of the internal obliques is in the direction of reaching across your body. As in, if you place your hand on the opposite side of your abdomen, your fingers represent the direction of the fibers.
Like the name of the internal obliques, the name external obliques also refers to the location and shape of the muscle. External means outside and oblique, as we said, means a slanting orientation, which refers to the muscle fiber direction. In contrast to the internal obliques, the fiber direction of these muscles is like the direction of reaching into your pockets.
Where do the external and internal oblique muscles attach?
The internal obliques originate on the inguinal ligament, which is a ligament that runs from the anterior iliac spine to the pubic bone. Additionally they originate on the anterior iliac crest. The external obliques, however, originate on the lower eight ribs.
The internal obliques insert onto the costal cartilages of the lower four ribs and the abdominal aponeurosis, which is a superficial sheet of connective tissue over the abdomen. Additionally, they also insert on the linea alba, which is a fibrous band of connective tissue that runs from the xiphoid process to the pubic symphysis. However, the external obliques insert onto the abdominal aponeurosis, the linea alba, the iliac crest, and the pubic bone.
What actions do the external and internal oblique muscles do?
In the case of these abdominal muscles, stabilization is a key function. The internal obliques can function bilaterally, which means both sides working together. Bilaterally they flex the trunk and compress its contents. They can also function unilaterally, which means one-sided. Unilaterally, they laterally flex the trunk and rotate it to the same side. Like the internal obliques, the external obliques function bilaterally to flex the trunk and compress its contents. However, they function unilaterally to laterally flex the trunk and rotate the trunk to the opposite side.
Poses where the external and internal oblique muscles contract
If we look at a posture such as navasana, we see that all of the abdominals will be doing an isometric contraction in order to help stabilize the trunk. This helps keep it rigid so that the hip flexors can do their work.
As we twist right in marichyasana C, the left side of the external oblique contracts with the right side of the internal obliques (they work together as synergists) when twisting.
Poses where the external and internal oblique muscles are lengthened
In a posture such as urdhva dhanurasana, we can see a general lengthening of all of the abdominal muscles. This is because the rib cage and the pelvis are required to move further apart, generally making more distance between origins and insertions.
If we look at the twisting motion of marichyasana C, as we twist to the right, we are going to find the right side of the external obliques lengthening with the left side of the internal obliques.