What action is primarily performed by the external oblique muscles?

The external oblique muscles play a crucial role in trunk rotation, which involves rotating the torso to one side or the other. These muscles are located on the sides of the abdomen and are important for various movements that require the upper body to twist. When one side of the external oblique contracts, it pulls on the ribcage and helps facilitate rotation of the trunk towards the opposite side, contributing significantly to movements like swinging during sports or performing exercises that involve twisting motions.

Trunk rotation is essential not only for athletic performance but also for everyday activities like reaching across the body or turning to look behind. In contrast, the other actions listed—hip extension, spinal flexion, and shoulder abduction—primarily involve different muscle groups. For instance, hip extension is mainly performed by the gluteus maximus and hamstring muscles, spinal flexion is primarily achieved by the rectus abdominis and other core muscles, and shoulder abduction involves the deltoid and supraspinatus muscles. Thus, trunk rotation is the specific and primary action associated with the external obliques, affirming the choice made.