椎间盘啊

https://www.issaonline.com/blog/index.cfm/2016/are-sit-ups-bad-for-you-the-us-military-seems-to-think-so

Sit-ups impose extremely large compression forces on the discs and vertebrae of the spine, especially in the lower back. The U.S. National Institute of Occupational Safety and Health has set the action limit for low back compression at 3300 N and repetitive loading above this level is linked with higher injury rates in workers. Compression readings that surpass this limit are imposed on the spine with each sit up repetition.