Final answer:
The muscle group that works in opposition to the rectus abdominis is the external obliques.
Step-by-step explanation:
The muscle group that works in opposition to the rectus abdominis is the external obliques.
Located on the flanks of the body, medial to the rectus abdominis, the external oblique muscles form the superficial layer of the abdominal wall. They are responsible for rotating and flexing the trunk, as well as providing stability to the spine. When the external obliques contract, they work in opposition to the rectus abdominis, which helps to maintain balance and prevent excessive forward bending.