Which of the following muscle fibers run mainly horizontally?

The transversus abdominus is the correct answer because it is a deep abdominal muscle that runs primarily horizontally across the body. This orientation is significant because the transversus abdominus plays a crucial role in stabilizing the pelvis and lumbar spine, providing increased core stability and support for the abdominal wall. Its horizontal fibers help create a "corset-like" effect around the abdomen, which is essential for maintaining proper posture and assisting in movements that require core strength.

In contrast, the other muscles mentioned have different fiber orientations. The rectus abdominus runs vertically, working to flex the spine and support movements like sitting up or performing crunches. The internal abdominal oblique has fibers that run diagonally, oriented from the lower ribs toward the pelvis, which assists in trunk rotation and lateral flexion. The external abdominal oblique features fibers running in a downward and forward diagonal direction, also contributing to rotation and side bending of the torso. Understanding the fiber orientation and function of these muscles is vital for effective Pilates training and in promoting proper movement mechanics.