Flexion and extension of the trunk refer to movements of the spine in the sagittal plane. Trunk flexion: Bending forward, decreasing the angle between the thorax and pelvis. This primarily involves the rectus abdominis, external and internal obliques, and psoas major. Trunk extension: Straightening or arching the back backward, increasing the angle between the thorax and pelvis. This is primarily driven by the erector spinae (iliocostalis, longissimus, spinalis) and multifidus. These movements o