Final answer:
The external oblique engages after the arm moves, working together with the opposite side's internal oblique for trunk rotation. It is one of the three layers of abdominal muscles, with fibers that extend in a diagonal manner similar to sliding hands into pockets.
Step-by-step explanation:
The external oblique is a superficial abdominal muscle with fibers that extend inferiorly and medially, similar to the motion of sliding one's hand into a pocket. When the arm moves, such as in the rotation of the trunk, different abdominal muscles work together to facilitate this movement. Before the arm moves, muscles like the internal oblique and transverse abdominis are involved. After the arm moves, the external oblique comes into play, often working in concert with the opposite side's internal oblique to rotate the trunk. These muscles are part of the three layers of abdominal muscles that protect the internal organs and provide movement and support to the trunk. The transversus abdominis works like a belt, stabilizing the lower back and compressing the abdominal contents, while the internal oblique muscles are sandwiched between the external oblique and transverse abdominis, aiding in trunk rotation and lateral flexion.