The erector spinae, also known as the sacrospinalis, is a complex muscle group often misunderstood due to its extensive coverage and deep location within the back. Originating from the sacrum's posterior surface and extending to the back of the skull, it fills the deep groove between the spinous processes and the rib angles. From the sacrum to the occiput, it forms a pair of powerful muscles responsible for extending the spine. While commonly referred to as "lower back" muscles, the erector spinae actually runs the entire length of the spine. Its upper fibers lie beneath the latissimus dorsi and trapezius muscles, making them invisible to the naked eye.

The bulky muscle fibers of the erector spinae in the upper lumbar region are divided into three columns: the iliocostalis (lateral column), the longissimus (middle column), and the spinalis (medial column).

The number of muscle fibers required varies significantly with different body postures. When the lumbar region is relaxed, even though the back muscles are passively stretched, the extent is minor, and slight muscle contraction is sufficient to maintain spinal curvature. In contrast, when standing straight, the back muscles actively contract more extensively, and the electrical activity of the muscles (measured as electromyography or EMG) is greater than in a relaxed posture. Changes in arm position also significantly affect the spinal curvature, with increased muscle activity in positions that demand more support, such as leaning forward compared to unsupported or arm-supported positions. Leg movements have a minimal impact on spinal curvature and muscle activity, showing no significant differences in EMG readings.