When you try to bend the middle and upper part of the spine, the lower part of the lumbar spine must remain stable and form a stable platform. In order to achieve this goal, the hip flexor, hip extensor and lower back should be contracted. Because the upper part of iliopsoas muscle is attached to the lumbar spine, its contraction will pull the lumbar spine, which may lead to back pain. This makes you think it's low back pain.

To solve this problem, you need to keep the lumbar spine stable. The practice is to keep the pelvis backward, and the abdominal muscles such as the lower part of rectus abdominis contract, increasing abdominal pressure and forming a backward thrust on the lumbar spine.