The main antagonistic muscle of rectus abdominis muscle is
The rectus abdominus muscle reflexes the trunk/lumbar spine, therefore it is a antagonist muscles should extend the spine. The erector spinae muscles are a group of muscles that perform this functions.These can be divided up it into individual muscles. The erector spinae muscles lies on the each side of the veebral column.and extends along the lumbar, thoraxic and cervical sections of the spine.It straightens the back and provide side to side rotation. Ref - researhgate.net
Psoas major
Latissimus dorsi
Quadratus lumborum
Erector spinae
Abdomen and pelvis