What is the primary action of the transversus abdominis?

The transversus abdominis plays a crucial role in core stability and the overall functioning of the abdominal region. Its primary action is to compress the abdominal cavity, which helps to stabilize the pelvis and lumbar spine during various movements. This compression effectively increases intra-abdominal pressure, providing support to the spine while allowing for smooth and coordinated movement of the trunk and limbs.

In contrast, the other options pertain to actions related to the upper and lower limbs. Elbow extension and flexion are actions performed by muscles that act on the arm, such as the triceps and biceps respectively, and extending the hip involves muscles like the gluteus maximus and hamstrings, which are unrelated to the function of the transversus abdominis. Therefore, understanding these distinctions highlights the specific role that the transversus abdominis plays in core stability and abdominal function.